Minor changes to types

* Add chunk to constructor

* Add test

* Bump version

.gitignore

@@ -4,3 +4,14 @@ dist
 sample_output
 yarn-error.log
 TODO.md
+
+
+#VIM
+## Swap
+[._]*.s[a-v][a-z]
+!*.svg  # comment out if you don't need vector files
+[._]*.sw[a-p]
+[._]s[a-rt-v][a-z]
+[._]ss[a-gi-z]
+[._]sw[a-p]
+*.orig

package.json

@@ -1,6 +1,6 @@
 {
-    "name": "mhysa",
-    "version": "1.0.2",
+    "name": "@jogogo/mhysa",
+    "version": "2.0.0-alpha.4",
     "description": "Streams and event emitter utils for Node.js",
     "keywords": [
         "promise",
@@ -11,40 +11,55 @@
     "author": {
         "name": "Wenzil"
     },
+    "contributors": [
+        {
+            "name": "jerry",
+            "email": "jerry@jogogo.co"
+        },
+        {
+            "name": "lewis",
+            "email": "lewis@jogogo.co"
+        }
+    ],
     "license": "MIT",
     "main": "dist/index.js",
     "types": "dist/index.d.ts",
     "files": [
         "dist"
     ],
+    "publishConfig": {
+        "registry": "https://npm.dev.jogogo.co/"
+    },
     "repository": {
-        "url": "git@github.com:Wenzil/Mhysa.git",
+        "url": "git@github.com:Jogogoplay/mhysa.git",
         "type": "git"
     },
     "scripts": {
         "test": "ava",
         "lint": "tslint -p tsconfig.json",
         "validate:tslint": "tslint-config-prettier-check ./tslint.json",
-        "prepublishOnly": "yarn lint && yarn test && yarn tsc"
+        "prepublishOnly": "yarn lint && yarn test && yarn tsc -d"
     },
     "dependencies": {},
     "devDependencies": {
         "@types/chai": "^4.1.7",
-        "@types/node": "^10.12.10",
-        "@types/typescript": "^2.0.0",
-        "ava": "^1.0.0-rc.2",
+        "@types/node": "^12.12.15",
+        "@types/sinon": "^7.0.13",
+        "ava": "^2.4.0",
         "chai": "^4.2.0",
         "mhysa": "./",
         "prettier": "^1.14.3",
-        "ts-node": "^7.0.1",
+        "sinon": "^7.4.2",
+        "ts-node": "^8.3.0",
         "tslint": "^5.11.0",
         "tslint-config-prettier": "^1.16.0",
         "tslint-plugin-prettier": "^2.0.1",
-        "typescript": "^3.1.6"
+        "typescript": "^3.5.3"
     },
     "ava": {
         "files": [
-            "src/**/*.spec.ts"
+            "tests/*.spec.ts",
+            "tests/utils/*.spec.ts"
         ],
         "sources": [
             "src/**/*.ts"

src/functions/accumulator.ts

@@ -0,0 +1,180 @@
+import { Transform, TransformOptions } from "stream";
+
+export enum FlushStrategy {
+    rolling = "rolling",
+    sliding = "sliding",
+}
+
+export type AccumulatorByIteratee<T> = (event: T, bufferChunk: T) => boolean;
+
+function _accumulator<T>(
+    accumulateBy: (data: T, buffer: T[], stream: Transform) => void,
+    shouldFlush: boolean = true,
+    options: TransformOptions = {},
+) {
+    const buffer: T[] = [];
+    return new Transform({
+        ...options,
+        transform(data: T, encoding, callback) {
+            accumulateBy(data, buffer, this);
+            callback();
+        },
+        flush(callback) {
+            if (shouldFlush) {
+                this.push(buffer);
+            }
+            callback();
+        },
+    });
+}
+
+function _sliding<T>(
+    windowLength: number,
+    key?: string,
+): (event: T, buffer: T[], stream: Transform) => void {
+    return (event: T, buffer: T[], stream: Transform) => {
+        if (key) {
+            let index = 0;
+            if (event[key] === undefined) {
+                stream.emit(
+                    "error",
+                    new Error(
+                        `Key is missing in event: (${key}, ${JSON.stringify(
+                            event,
+                        )})`,
+                    ),
+                );
+                stream.resume();
+                return;
+            }
+            while (
+                index < buffer.length &&
+                buffer[index][key] + windowLength <= event[key]
+            ) {
+                index++;
+            }
+            buffer.splice(0, index);
+        } else if (buffer.length === windowLength) {
+            buffer.shift();
+        }
+        buffer.push(event);
+        stream.push([...buffer]);
+    };
+}
+
+function _slidingByFunction<T>(
+    iteratee: AccumulatorByIteratee<T>,
+): (event: T, buffer: T[], stream: Transform) => void {
+    return (event: T, buffer: T[], stream: Transform) => {
+        let index = 0;
+        while (index < buffer.length && iteratee(event, buffer[index])) {
+            index++;
+        }
+        buffer.splice(0, index);
+        buffer.push(event);
+        stream.push([...buffer]);
+    };
+}
+
+function _rollingByFunction<T>(
+    iteratee: AccumulatorByIteratee<T>,
+): (event: T, buffer: T[], stream: Transform) => void {
+    return (event: T, buffer: T[], stream: Transform) => {
+        if (iteratee) {
+            if (buffer.length > 0 && iteratee(event, buffer[0])) {
+                stream.push(buffer.slice(0));
+                buffer.length = 0;
+            }
+        }
+        buffer.push(event);
+    };
+}
+
+function _rolling<T>(
+    windowLength: number,
+    key?: string,
+): (event: T, buffer: T[], stream: Transform) => void {
+    return (event: T, buffer: T[], stream: Transform) => {
+        if (key) {
+            if (event[key] === undefined) {
+                stream.emit(
+                    "error",
+                    new Error(
+                        `Key is missing in event: (${key}, ${JSON.stringify(
+                            event,
+                        )})`,
+                    ),
+                );
+                stream.resume();
+                return;
+            } else if (
+                buffer.length > 0 &&
+                buffer[0][key] + windowLength <= event[key]
+            ) {
+                stream.push(buffer.slice(0));
+                buffer.length = 0;
+            }
+        } else if (buffer.length === windowLength) {
+            stream.push(buffer.slice(0));
+            buffer.length = 0;
+        }
+        buffer.push(event);
+    };
+}
+
+export function accumulator(
+    flushStrategy: FlushStrategy,
+    batchSize: number,
+    keyBy?: string,
+    options?: TransformOptions,
+): Transform {
+    switch (flushStrategy) {
+        case FlushStrategy.sliding:
+            return sliding(batchSize, keyBy, options);
+        case FlushStrategy.rolling:
+            return rolling(batchSize, keyBy, options);
+    }
+}
+
+export function accumulatorBy<T>(
+    flushStrategy: FlushStrategy,
+    iteratee: AccumulatorByIteratee<T>,
+    options?: TransformOptions,
+): Transform {
+    switch (flushStrategy) {
+        case FlushStrategy.sliding:
+            return slidingBy(iteratee, options);
+        case FlushStrategy.rolling:
+            return rollingBy(iteratee, options);
+    }
+}
+
+function sliding(
+    windowLength: number,
+    key?: string,
+    options?: TransformOptions,
+): Transform {
+    return _accumulator(_sliding(windowLength, key), false, options);
+}
+
+function slidingBy<T>(
+    iteratee: AccumulatorByIteratee<T>,
+    options?: TransformOptions,
+): Transform {
+    return _accumulator(_slidingByFunction(iteratee), false, options);
+}
+
+function rolling(
+    windowLength: number,
+    key?: string,
+    options?: TransformOptions,
+): Transform {
+    return _accumulator(_rolling(windowLength, key), true, options);
+}
+
+function rollingBy<T>(
+    iteratee: AccumulatorByIteratee<T>,
+    options?: TransformOptions,
+): Transform {
+    return _accumulator(_rollingByFunction(iteratee), true, options);
+}

src/functions/baseDefinitions.ts

@@ -1,12 +1,3 @@
-export interface ThroughOptions {
-    objectMode?: boolean;
-}
-
-export interface TransformOptions {
-    readableObjectMode?: boolean;
-    writableObjectMode?: boolean;
-}
-
 export interface WithEncoding {
     encoding: string;
 }
@@ -21,4 +12,3 @@ export type JsonValue = JsonPrimitive | JsonPrimitive[];
 export interface JsonParseOptions {
     pretty: boolean;
 }
-

src/functions/batch.ts

@@ -0,0 +1,40 @@
+import { Transform, TransformOptions } from "stream";
+
+export function batch(
+    batchSize: number = 1000,
+    maxBatchAge: number = 500,
+    options: TransformOptions = {},
+): Transform {
+    let buffer: any[] = [];
+    let timer: NodeJS.Timer | null = null;
+    const sendChunk = (self: Transform) => {
+        if (timer) {
+            clearTimeout(timer);
+        }
+        timer = null;
+        if (buffer.length > 0) {
+            self.push(buffer);
+        }
+        buffer = [];
+    };
+    return new Transform({
+        ...options,
+        transform(chunk, encoding, callback) {
+            buffer.push(chunk);
+            if (buffer.length === batchSize) {
+                sendChunk(this);
+            } else {
+                if (timer === null) {
+                    timer = setInterval(() => {
+                        sendChunk(this);
+                    }, maxBatchAge);
+                }
+            }
+            callback();
+        },
+        flush(callback) {
+            sendChunk(this);
+            callback();
+        },
+    });
+}

src/functions/child.ts

@@ -0,0 +1,11 @@
+import { ChildProcess } from "child_process";
+import { duplex } from "./duplex";
+
+export function child(childProcess: ChildProcess) {
+    if (childProcess.stdin === null) {
+        throw new Error("childProcess.stdin is null");
+    } else if (childProcess.stdout === null) {
+        throw new Error("childProcess.stdout is null");
+    }
+    return duplex(childProcess.stdin, childProcess.stdout);
+}

src/functions/collect.ts

@@ -0,0 +1,18 @@
+import { Transform, TransformOptions } from "stream";
+
+export function collect(options: TransformOptions = {}): Transform {
+    const collected: any[] = [];
+    return new Transform({
+        ...options,
+        transform(data, encoding, callback) {
+            collected.push(data);
+            callback();
+        },
+        flush(callback) {
+            this.push(
+                options.objectMode ? collected : Buffer.concat(collected),
+            );
+            callback();
+        },
+    });
+}

src/functions/compose.ts

@@ -0,0 +1,88 @@
+import { AllStreams, isReadable } from "../helpers";
+import { PassThrough, pipeline, TransformOptions, Transform } from "stream";
+
+export function compose(
+    streams: Array<
+        NodeJS.ReadableStream | NodeJS.ReadWriteStream | NodeJS.WritableStream
+    >,
+    errorCallback?: (err: any) => void,
+    options?: TransformOptions,
+): Compose {
+    if (streams.length < 2) {
+        throw new Error("At least two streams are required to compose");
+    }
+
+    return new Compose(streams, errorCallback, options);
+}
+
+enum EventSubscription {
+    Last = 0,
+    First,
+    All,
+    Self,
+}
+
+export class Compose extends Transform {
+    private first: AllStreams;
+    private last: AllStreams;
+    private streams: AllStreams[];
+    private inputStream: ReadableStream;
+
+    constructor(
+        streams: AllStreams[],
+        errorCallback?: (err: any) => void,
+        options?: TransformOptions,
+    ) {
+        super(options);
+        this.first = new PassThrough(options);
+        this.last = streams[streams.length - 1];
+        this.streams = streams;
+        pipeline(
+            [this.first, ...streams],
+            errorCallback ||
+                ((error: any) => {
+                    if (error) {
+                        this.emit("error", error);
+                    }
+                }),
+        );
+
+        if (isReadable(this.last)) {
+            (this.last as NodeJS.ReadWriteStream).pipe(
+                new Transform({
+                    ...options,
+                    transform: (d: any, encoding, cb) => {
+                        this.push(d);
+                        cb();
+                    },
+                }),
+            );
+        }
+    }
+
+    public _transform(chunk: any, encoding: string, cb: any) {
+        (this.first as NodeJS.WritableStream).write(chunk, encoding, cb);
+    }
+
+    public _flush(cb: any) {
+        if (isReadable(this.first)) {
+            (this.first as any).push(null);
+        }
+        this.last.once("end", () => {
+            cb();
+        });
+    }
+
+    public _destroy(error: any, cb: (error?: any) => void) {
+        this.streams.forEach(s => (s as any).destroy());
+        cb(error);
+    }
+
+    public bubble(...events: string[]) {
+        this.streams.forEach(s => {
+            events.forEach(e => {
+                s.on(e, (...args) => super.emit(e, ...args));
+            });
+        });
+    }
+}

src/functions/concat.ts

@@ -0,0 +1,37 @@
+import { Readable } from "stream";
+
+export function concat(...streams: NodeJS.ReadableStream[]): Readable {
+    let isStarted = false;
+    let currentStreamIndex = 0;
+    const startCurrentStream = () => {
+        if (currentStreamIndex >= streams.length) {
+            wrapper.push(null);
+        } else {
+            streams[currentStreamIndex]
+                .on("data", chunk => {
+                    if (!wrapper.push(chunk)) {
+                        streams[currentStreamIndex].pause();
+                    }
+                })
+                .on("error", err => wrapper.emit("error", err))
+                .on("end", () => {
+                    currentStreamIndex++;
+                    startCurrentStream();
+                });
+        }
+    };
+
+    const wrapper = new Readable({
+        objectMode: true,
+        read() {
+            if (!isStarted) {
+                isStarted = true;
+                startCurrentStream();
+            }
+            if (currentStreamIndex < streams.length) {
+                streams[currentStreamIndex].resume();
+            }
+        },
+    });
+    return wrapper;
+}

src/functions/demux.ts

@@ -0,0 +1,131 @@
+import { DuplexOptions, Duplex, Transform } from "stream";
+
+import { isReadable } from "../helpers";
+
+type DemuxStreams = NodeJS.WritableStream | NodeJS.ReadWriteStream;
+
+export interface DemuxOptions extends DuplexOptions {
+    remultiplex?: boolean;
+}
+
+export function demux(
+    pipelineConstructor: (
+        destKey?: string,
+        chunk?: any,
+    ) => DemuxStreams | DemuxStreams[],
+    demuxBy: string | ((chunk: any) => string),
+    options?: DemuxOptions,
+): Duplex {
+    return new Demux(pipelineConstructor, demuxBy, options);
+}
+
+class Demux extends Duplex {
+    private streamsByKey: {
+        [key: string]: DemuxStreams[];
+    };
+    private demuxer: (chunk: any) => string;
+    private pipelineConstructor: (
+        destKey?: string,
+        chunk?: any,
+    ) => DemuxStreams[];
+    private remultiplex: boolean;
+    private transform: Transform;
+    constructor(
+        pipelineConstructor: (
+            destKey?: string,
+            chunk?: any,
+        ) => DemuxStreams | DemuxStreams[],
+        demuxBy: string | ((chunk: any) => string),
+        options: DemuxOptions = {},
+    ) {
+        super(options);
+        this.demuxer =
+            typeof demuxBy === "string" ? chunk => chunk[demuxBy] : demuxBy;
+        this.pipelineConstructor = (destKey: string, chunk?: any) => {
+            const pipeline = pipelineConstructor(destKey, chunk);
+            return Array.isArray(pipeline) ? pipeline : [pipeline];
+        };
+        this.remultiplex =
+            options.remultiplex === undefined ? true : options.remultiplex;
+        this.streamsByKey = {};
+        this.transform = new Transform({
+            ...options,
+            transform: (d, _, cb) => {
+                this.push(d);
+                cb(null);
+            },
+        });
+
+        this.on("unpipe", () => this._flush());
+    }
+
+    // tslint:disable-next-line
+    public _read(size: number) {}
+
+    public async _write(chunk: any, encoding: any, cb: any) {
+        const destKey = this.demuxer(chunk);
+        if (this.streamsByKey[destKey] === undefined) {
+            const newPipelines = this.pipelineConstructor(destKey, chunk);
+            this.streamsByKey[destKey] = newPipelines;
+
+            newPipelines.forEach(newPipeline => {
+                if (this.remultiplex && isReadable(newPipeline)) {
+                    (newPipeline as NodeJS.ReadWriteStream).pipe(
+                        this.transform,
+                    );
+                } else if (this.remultiplex) {
+                    console.error(
+                        `Pipeline construct for ${destKey} does not implement readable interface`,
+                    );
+                }
+            });
+        }
+        const pipelines = this.streamsByKey[destKey];
+        const pendingDrains: Array<Promise<any>> = [];
+
+        pipelines.forEach(pipeline => {
+            if (!pipeline.write(chunk, encoding)) {
+                pendingDrains.push(
+                    new Promise(resolve => {
+                        pipeline.once("drain", () => {
+                            resolve();
+                        });
+                    }),
+                );
+            }
+        });
+        await Promise.all(pendingDrains);
+        cb();
+    }
+
+    public _flush() {
+        const pipelines: DemuxStreams[] = [].concat.apply(
+            [],
+            Object.values(this.streamsByKey),
+        );
+        const flushPromises: Array<Promise<void>> = [];
+        pipelines.forEach(pipeline => {
+            flushPromises.push(
+                new Promise(resolve => {
+                    pipeline.once("end", () => {
+                        resolve();
+                    });
+                }),
+            );
+        });
+        pipelines.forEach(pipeline => pipeline.end());
+        Promise.all(flushPromises).then(() => {
+            this.push(null);
+            this.emit("end");
+        });
+    }
+
+    public _destroy(error: any, cb: (error?: any) => void) {
+        const pipelines: DemuxStreams[] = [].concat.apply(
+            [],
+            Object.values(this.streamsByKey),
+        );
+        pipelines.forEach(p => (p as any).destroy());
+        cb(error);
+    }
+}

src/functions/duplex.ts

@@ -0,0 +1,31 @@
+import { Duplex } from "stream";
+
+export function duplex(
+    writable: NodeJS.WritableStream,
+    readable: NodeJS.ReadableStream,
+) {
+    const wrapper = new Duplex({
+        readableObjectMode: true,
+        writableObjectMode: true,
+        read() {
+            readable.resume();
+        },
+        write(chunk, encoding, callback) {
+            return writable.write(chunk, encoding, callback);
+        },
+        final(callback) {
+            writable.end(callback);
+        },
+    });
+    readable
+        .on("data", chunk => {
+            if (!wrapper.push(chunk)) {
+                readable.pause();
+            }
+        })
+        .on("error", err => wrapper.emit("error", err))
+        .on("end", () => wrapper.push(null));
+    writable.on("drain", () => wrapper.emit("drain"));
+    writable.on("error", err => wrapper.emit("error", err));
+    return wrapper;
+}

src/functions/filter.ts

@@ -0,0 +1,20 @@
+import { Transform, TransformOptions } from "stream";
+
+export function filter<T>(
+    predicate:
+        | ((chunk: T, encoding: string) => boolean)
+        | ((chunk: T, encoding: string) => Promise<boolean>),
+    options?: TransformOptions,
+) {
+    return new Transform({
+        ...options,
+        async transform(chunk: T, encoding?: any, callback?: any) {
+            const result = await predicate(chunk, encoding);
+            if (result === true) {
+                callback(null, chunk);
+            } else {
+                callback();
+            }
+        },
+    });
+}

src/functions/flatMap.ts

@@ -0,0 +1,16 @@
+import { Transform, TransformOptions } from "stream";
+
+export function flatMap<T, R>(
+    mapper:
+        | ((chunk: T, encoding: string) => R[])
+        | ((chunk: T, encoding: string) => Promise<R[]>),
+    options?: TransformOptions,
+): Transform {
+    return new Transform({
+        ...options,
+        async transform(chunk: T, encoding, callback) {
+            (await mapper(chunk, encoding)).forEach(c => this.push(c));
+            callback();
+        },
+    });
+}

src/functions/fromArray.ts

@@ -0,0 +1,16 @@
+import { Readable } from "stream";
+
+export function fromArray(array: any[]): Readable {
+    let cursor = 0;
+    return new Readable({
+        objectMode: true,
+        read() {
+            if (cursor < array.length) {
+                this.push(array[cursor]);
+                cursor++;
+            } else {
+                this.push(null);
+            }
+        },
+    });
+}

src/functions/functions.ts

@@ -1,604 +0,0 @@
-import { Transform, Readable, Writable, Duplex } from "stream";
-import { performance } from "perf_hooks";
-import { ChildProcess } from "child_process";
-import { StringDecoder } from "string_decoder";
-
-import {
-    TransformOptions,
-    ThroughOptions,
-    WithEncoding,
-    SerializationFormats,
-    JsonValue,
-    JsonParseOptions,
-} from "./definitions";
-import { sleep } from "../helpers";
-
-/**
- * Convert an array into a Readable stream of its elements
- * @param array Array of elements to stream
- */
-export function fromArray(array: any[]): NodeJS.ReadableStream {
-    let cursor = 0;
-    return new Readable({
-        objectMode: true,
-        read() {
-            if (cursor < array.length) {
-                this.push(array[cursor]);
-                cursor++;
-            } else {
-                this.push(null);
-            }
-        },
-    });
-}
-
-/**
- * Return a ReadWrite stream that maps streamed chunks
- * @param mapper Mapper function, mapping each (chunk, encoding) to a new chunk (or a promise of such)
- * @param options
- * @param options.readableObjectMode Whether this stream should behave as a readable stream of objects
- * @param options.writableObjectMode Whether this stream should behave as a writable stream of objects
- */
-export function map<T, R>(
-    mapper: (chunk: T, encoding: string) => R,
-    options: TransformOptions = {
-        readableObjectMode: true,
-        writableObjectMode: true,
-    },
-): NodeJS.ReadWriteStream {
-    return new Transform({
-        ...options,
-        async transform(chunk: T, encoding, callback) {
-            try {
-                const mapped = await mapper(chunk, encoding);
-                this.push(mapped);
-                callback();
-            } catch (err) {
-                callback(err);
-            }
-        },
-    });
-}
-
-/**
- * Return a ReadWrite stream that flat maps streamed chunks
- * @param mapper Mapper function, mapping each (chunk, encoding) to an array of new chunks (or a promise of such)
- * @param options
- * @param options.readableObjectMode Whether this stream should behave as a readable stream of objects
- * @param options.writableObjectMode Whether this stream should behave as a writable stream of objects
- */
-export function flatMap<T, R>(
-    mapper:
-        | ((chunk: T, encoding: string) => R[])
-        | ((chunk: T, encoding: string) => Promise<R[]>),
-    options: TransformOptions = {
-        readableObjectMode: true,
-        writableObjectMode: true,
-    },
-): NodeJS.ReadWriteStream {
-    return new Transform({
-        ...options,
-        async transform(chunk: T, encoding, callback) {
-            let isPromise = false;
-            try {
-                const mapped = mapper(chunk, encoding);
-                isPromise = mapped instanceof Promise;
-                (await mapped).forEach(c => this.push(c));
-                callback();
-            } catch (err) {
-                if (isPromise) {
-                    // Calling the callback asynchronously with an error wouldn't emit the error, so emit directly
-                    this.emit("error", err);
-                    callback();
-                } else {
-                    callback(err);
-                }
-            }
-        },
-    });
-}
-
-/**
- * Return a ReadWrite stream that filters out streamed chunks for which the predicate does not hold
- * @param predicate Predicate with which to filter scream chunks
- * @param options
- * @param options.objectMode Whether this stream should behave as a stream of objects
- */
-export function filter<T>(
-    predicate:
-        | ((chunk: T, encoding: string) => boolean)
-        | ((chunk: T, encoding: string) => Promise<boolean>),
-    options: ThroughOptions = {
-        objectMode: true,
-    },
-) {
-    return new Transform({
-        readableObjectMode: options.objectMode,
-        writableObjectMode: options.objectMode,
-        async transform(chunk: T, encoding, callback) {
-            let isPromise = false;
-            try {
-                const result = predicate(chunk, encoding);
-                isPromise = result instanceof Promise;
-                if (!!(await result)) {
-                    callback(undefined, chunk);
-                } else {
-                    callback();
-                }
-            } catch (err) {
-                if (isPromise) {
-                    // Calling the callback asynchronously with an error wouldn't emit the error, so emit directly
-                    this.emit("error", err);
-                    callback();
-                } else {
-                    callback(err);
-                }
-            }
-        },
-    });
-}
-
-/**
- * Return a ReadWrite stream that reduces streamed chunks down to a single value and yield that
- * value
- * @param iteratee Reducer function to apply on each streamed chunk
- * @param initialValue Initial value
- * @param options
- * @param options.readableObjectMode Whether this stream should behave as a readable stream of objects
- * @param options.writableObjectMode Whether this stream should behave as a writable stream of objects
- */
-export function reduce<T, R>(
-    iteratee:
-        | ((previousValue: R, chunk: T, encoding: string) => R)
-        | ((previousValue: R, chunk: T, encoding: string) => Promise<R>),
-    initialValue: R,
-    options: TransformOptions = {
-        readableObjectMode: true,
-        writableObjectMode: true,
-    },
-) {
-    let value = initialValue;
-    return new Transform({
-        readableObjectMode: options.readableObjectMode,
-        writableObjectMode: options.writableObjectMode,
-        async transform(chunk: T, encoding, callback) {
-            let isPromise = false;
-            try {
-                const result = iteratee(value, chunk, encoding);
-                isPromise = result instanceof Promise;
-                value = await result;
-                callback();
-            } catch (err) {
-                if (isPromise) {
-                    // Calling the callback asynchronously with an error wouldn't emit the error, so emit directly
-                    this.emit("error", err);
-                    callback();
-                } else {
-                    callback(err);
-                }
-            }
-        },
-        flush(callback) {
-            // Best effort attempt at yielding the final value (will throw if e.g. yielding an object and
-            // downstream doesn't expect objects)
-            try {
-                callback(undefined, value);
-            } catch (err) {
-                try {
-                    this.emit("error", err);
-                } catch {
-                    // Best effort was made
-                }
-            }
-        },
-    });
-}
-
-/**
- * Return a ReadWrite stream that splits streamed chunks using the given separator
- * @param separator Separator to split by, defaulting to "\n"
- * @param options
- * @param options.encoding Encoding written chunks are assumed to use
- */
-export function split(
-    separator: string | RegExp = "\n",
-    options: WithEncoding = { encoding: "utf8" },
-): NodeJS.ReadWriteStream {
-    let buffered = "";
-    const decoder = new StringDecoder(options.encoding);
-
-    return new Transform({
-        readableObjectMode: true,
-        transform(chunk: Buffer, encoding, callback) {
-            const asString = decoder.write(chunk);
-            const splitted = asString.split(separator);
-            if (splitted.length > 1) {
-                splitted[0] = buffered.concat(splitted[0]);
-                buffered = "";
-            }
-            buffered += splitted[splitted.length - 1];
-            splitted.slice(0, -1).forEach((part: string) => this.push(part));
-            callback();
-        },
-        flush(callback) {
-            callback(undefined, buffered + decoder.end());
-        },
-    });
-}
-
-/**
- * Return a ReadWrite stream that joins streamed chunks using the given separator
- * @param separator Separator to join with
- * @param options
- * @param options.encoding Encoding written chunks are assumed to use
- */
-export function join(
-    separator: string,
-    options: WithEncoding = { encoding: "utf8" },
-): NodeJS.ReadWriteStream {
-    let isFirstChunk = true;
-    const decoder = new StringDecoder(options.encoding);
-    return new Transform({
-        readableObjectMode: true,
-        async transform(chunk: Buffer, encoding, callback) {
-            const asString = decoder.write(chunk);
-            // Take care not to break up multi-byte characters spanning multiple chunks
-            if (asString !== "" || chunk.length === 0) {
-                if (!isFirstChunk) {
-                    this.push(separator);
-                }
-                this.push(asString);
-                isFirstChunk = false;
-            }
-            callback();
-        },
-    });
-}
-
-/**
- * Return a ReadWrite stream that replaces occurrences of the given string or regular expression  in
- * the streamed chunks with the specified replacement string
- * @param searchValue Search string to use
- * @param replaceValue Replacement string to use
- * @param options
- * @param options.encoding Encoding written chunks are assumed to use
- */
-export function replace(
-    searchValue: string | RegExp,
-    replaceValue: string,
-    options: WithEncoding = { encoding: "utf8" },
-): NodeJS.ReadWriteStream {
-    const decoder = new StringDecoder(options.encoding);
-    return new Transform({
-        readableObjectMode: true,
-        transform(chunk: Buffer, encoding, callback) {
-            const asString = decoder.write(chunk);
-            // Take care not to break up multi-byte characters spanning multiple chunks
-            if (asString !== "" || chunk.length === 0) {
-                callback(
-                    undefined,
-                    asString.replace(searchValue, replaceValue),
-                );
-            } else {
-                callback();
-            }
-        },
-    });
-}
-
-/**
- * Return a ReadWrite stream that parses the streamed chunks as JSON. Each streamed chunk
- * must be a fully defined JSON string.
- * @param format Format of serialized data, only utf8 supported.
- */
-export function parse(
-    format: SerializationFormats = SerializationFormats.utf8,
-): NodeJS.ReadWriteStream {
-    const decoder = new StringDecoder(format);
-    return new Transform({
-        readableObjectMode: true,
-        writableObjectMode: true,
-        async transform(chunk: Buffer, encoding, callback) {
-            try {
-                const asString = decoder.write(chunk);
-                // Using await causes parsing errors to be emitted
-                callback(undefined, await JSON.parse(asString));
-            } catch (err) {
-                callback(err);
-            }
-        },
-    });
-}
-
-/**
- * Return a ReadWrite stream that stringifies the streamed chunks to JSON
- */
-export function stringify(
-    options: JsonParseOptions = { pretty: false },
-): NodeJS.ReadWriteStream {
-    return new Transform({
-        readableObjectMode: true,
-        writableObjectMode: true,
-        transform(chunk: JsonValue, encoding, callback) {
-            callback(
-                undefined,
-                options.pretty
-                    ? JSON.stringify(chunk, null, 2)
-                    : JSON.stringify(chunk),
-            );
-        },
-    });
-}
-
-/**
- * Return a ReadWrite stream that collects streamed chunks into an array or buffer
- * @param options
- * @param options.objectMode Whether this stream should behave as a stream of objects
- */
-export function collect(
-    options: ThroughOptions = { objectMode: false },
-): NodeJS.ReadWriteStream {
-    const collected: any[] = [];
-    return new Transform({
-        readableObjectMode: options.objectMode,
-        writableObjectMode: options.objectMode,
-        transform(data, encoding, callback) {
-            collected.push(data);
-            callback();
-        },
-        flush(callback) {
-            this.push(
-                options.objectMode ? collected : Buffer.concat(collected),
-            );
-            callback();
-        },
-    });
-}
-
-/**
- * Return a Readable stream of readable streams concatenated together
- * @param streams Readable streams to concatenate
- */
-export function concat(
-    ...streams: NodeJS.ReadableStream[]
-): NodeJS.ReadableStream {
-    let isStarted = false;
-    let currentStreamIndex = 0;
-    const startCurrentStream = () => {
-        if (currentStreamIndex >= streams.length) {
-            wrapper.push(null);
-        } else {
-            streams[currentStreamIndex]
-                .on("data", chunk => {
-                    if (!wrapper.push(chunk)) {
-                        streams[currentStreamIndex].pause();
-                    }
-                })
-                .on("error", err => wrapper.emit("error", err))
-                .on("end", () => {
-                    currentStreamIndex++;
-                    startCurrentStream();
-                });
-        }
-    };
-
-    const wrapper = new Readable({
-        objectMode: true,
-        read() {
-            if (!isStarted) {
-                isStarted = true;
-                startCurrentStream();
-            }
-            if (currentStreamIndex < streams.length) {
-                streams[currentStreamIndex].resume();
-            }
-        },
-    });
-    return wrapper;
-}
-
-/**
- * Return a Readable stream of readable streams merged together in chunk arrival order
- * @param streams Readable streams to merge
- */
-export function merge(
-    ...streams: NodeJS.ReadableStream[]
-): NodeJS.ReadableStream {
-    let isStarted = false;
-    let streamEndedCount = 0;
-    return new Readable({
-        objectMode: true,
-        read() {
-            if (streamEndedCount >= streams.length) {
-                this.push(null);
-            } else if (!isStarted) {
-                isStarted = true;
-                streams.forEach(stream =>
-                    stream
-                        .on("data", chunk => {
-                            if (!this.push(chunk)) {
-                                streams.forEach(s => s.pause());
-                            }
-                        })
-                        .on("error", err => this.emit("error", err))
-                        .on("end", () => {
-                            streamEndedCount++;
-                            if (streamEndedCount === streams.length) {
-                                this.push(null);
-                            }
-                        }),
-                );
-            } else {
-                streams.forEach(s => s.resume());
-            }
-        },
-    });
-}
-
-/**
- * Return a Duplex stream from a writable stream that is assumed to somehow, when written to,
- * cause the given readable stream to yield chunks
- * @param writable Writable stream assumed to cause the readable stream to yield chunks when written to
- * @param readable Readable stream assumed to yield chunks when the writable stream is written to
- */
-export function duplex(writable: Writable, readable: Readable) {
-    const wrapper = new Duplex({
-        readableObjectMode: true,
-        writableObjectMode: true,
-        read() {
-            readable.resume();
-        },
-        write(chunk, encoding, callback) {
-            return writable.write(chunk, encoding, callback);
-        },
-        final(callback) {
-            writable.end(callback);
-        },
-    });
-    readable
-        .on("data", chunk => {
-            if (!wrapper.push(chunk)) {
-                readable.pause();
-            }
-        })
-        .on("error", err => wrapper.emit("error", err))
-        .on("end", () => wrapper.push(null));
-    writable.on("drain", () => wrapper.emit("drain"));
-    writable.on("error", err => wrapper.emit("error", err));
-    return wrapper;
-}
-
-/**
- * Return a Duplex stream from a child process' stdin and stdout
- * @param childProcess Child process from which to create duplex stream
- */
-export function child(childProcess: ChildProcess) {
-    return duplex(childProcess.stdin, childProcess.stdout);
-}
-
-/**
- * Return a Promise resolving to the last streamed chunk of the given readable stream, after it has
- * ended
- * @param readable Readable stream to wait on
- */
-export function last<T>(readable: Readable): Promise<T | null> {
-    let lastChunk: T | null = null;
-    return new Promise((resolve, reject) => {
-        readable
-            .on("data", chunk => (lastChunk = chunk))
-            .on("end", () => resolve(lastChunk));
-    });
-}
-
-/**
- * Stores chunks of data internally in array and batches when batchSize is reached.
- *
- * @param batchSize Size of the batches
- * @param maxBatchAge Max lifetime of a batch
- */
-export function batch(batchSize: number = 1000, maxBatchAge: number = 500) {
-    let buffer: any[] = [];
-    let timer: NodeJS.Timer | null = null;
-    let sendChunk = (self: Transform) => {
-        timer && clearTimeout(timer);
-        timer = null;
-        self.push(buffer);
-        buffer = [];
-    };
-    return new Transform({
-        objectMode: true,
-        transform(chunk, encoding, callback) {
-            buffer.push(chunk);
-            if (buffer.length === batchSize) {
-                sendChunk(this);
-            } else {
-                if (timer === null) {
-                    timer = setInterval(() => {
-                        sendChunk(this);
-                    }, maxBatchAge);
-                }
-            }
-            callback();
-        },
-        flush(callback) {
-            console.error("flushing");
-            sendChunk(this);
-            callback();
-        },
-    });
-}
-
-/**
- * Unbatches and sends individual chunks of data
- */
-export function unbatch() {
-    return new Transform({
-        objectMode: true,
-        transform(data, encoding, callback) {
-            for (const d of data) {
-                this.push(d);
-            }
-            callback();
-        },
-    });
-}
-
-/**
- * Limits date of data transferred into stream.
- * @param targetRate Desired rate in ms
- * @param period Period to sleep for when rate is above or equal to targetRate
- */
-export function rate(targetRate: number = 50, period: number = 2) {
-    const deltaMS = ((1 / targetRate) * 1000) / period; // Skip half a period
-    let total = 0;
-    const start = performance.now();
-    return new Transform({
-        objectMode: true,
-        async transform(data, encoding, callback) {
-            const currentRate = (total / (performance.now() - start)) * 1000;
-            if (targetRate && currentRate > targetRate) {
-                await sleep(deltaMS);
-            }
-            total += 1;
-            callback(undefined, data);
-        },
-    });
-}
-
-/**
- * Limits number of parallel processes in flight.
- * @param parallel Max number of parallel processes.
- * @param func Function to execute on each data chunk
- * @param pause Amount of time to pause processing when max number of parallel processes are executing.
- */
-export function parallelMap<T, R>(
-    mapper: (data: T) => R,
-    parallel: number = 10,
-    sleepTime: number = 5,
-) {
-    let inflight = 0;
-    return new Transform({
-        objectMode: true,
-        async transform(data, encoding, callback) {
-            while (parallel <= inflight) {
-                await sleep(sleepTime);
-            }
-            inflight += 1;
-            callback();
-            try {
-                const res = await mapper(data);
-                this.push(res);
-            } catch (e) {
-                this.emit(e);
-            } finally {
-                inflight -= 1;
-            }
-        },
-        async flush(callback) {
-            while (inflight > 0) {
-                await sleep(sleepTime);
-            }
-            callback();
-        },
-    });
-}

src/functions/index.ts

@@ -1,21 +1,59 @@
-import { Readable, Writable } from "stream";
-import { ChildProcess } from "child_process";
-import * as baseFunctions from "./functions";
-
 import {
-    ThroughOptions,
+    Transform,
     TransformOptions,
-    WithEncoding,
-    JsonParseOptions,
-} from "./definitions";
+    WritableOptions,
+    ReadableOptions,
+} from "stream";
+import { accumulator, accumulatorBy } from "./accumulator";
+import { batch } from "./batch";
+import { child } from "./child";
+import { collect } from "./collect";
+import { concat } from "./concat";
+import { duplex } from "./duplex";
+import { filter } from "./filter";
+import { flatMap } from "./flatMap";
+import { fromArray } from "./fromArray";
+import { join } from "./join";
+import { last } from "./last";
+import { map } from "./map";
+import { merge } from "./merge";
+import { parallelMap } from "./parallelMap";
+import { parse } from "./parse";
+import { rate } from "./rate";
+import { reduce } from "./reduce";
+import { replace } from "./replace";
+import { split } from "./split";
+import { stringify } from "./stringify";
+import { unbatch } from "./unbatch";
+import { compose } from "./compose";
+import { demux } from "./demux";
 
+export default function mhysa(defaultOptions?: TransformOptions) {
+    function withDefaultOptions<T extends any[], R>(
+        n: number,
+        fn: (...args: T) => R,
+    ): (...args: T) => R {
+        return (...args) => {
+            const options = {
+                ...defaultOptions,
+                ...((args[n] || {}) as TransformOptions | {}),
+            };
+            const provided = args.slice(0, n);
+            const nextArgs = [
+                ...provided,
+                ...Array(n - provided.length).fill(undefined),
+                options,
+            ] as T;
+            return fn(...nextArgs) as R;
+        };
+    }
+
+    return {
         /**
          * Convert an array into a Readable stream of its elements
          * @param array Array of elements to stream
          */
-export function fromArray(array: any[]): NodeJS.ReadableStream {
-    return baseFunctions.fromArray(array);
-}
+        fromArray,
 
         /**
          * Return a ReadWrite stream that maps streamed chunks
@@ -24,12 +62,7 @@ export function fromArray(array: any[]): NodeJS.ReadableStream {
          * @param options.readableObjectMode? Whether this stream should behave as a readable stream of objects
          * @param options.writableObjectMode? Whether this stream should behave as a writable stream of objects
          */
-export function map<T, R>(
-    mapper: (chunk: T, encoding?: string) => R,
-    options?: TransformOptions,
-): NodeJS.ReadWriteStream {
-    return baseFunctions.map(mapper, options);
-}
+        map: withDefaultOptions(1, map),
 
         /**
          * Return a ReadWrite stream that flat maps streamed chunks
@@ -38,14 +71,7 @@ export function map<T, R>(
          * @param options.readableObjectMode? Whether this stream should behave as a readable stream of objects
          * @param options.writableObjectMode? Whether this stream should behave as a writable stream of objects
          */
-export function flatMap<T, R>(
-    mapper:
-        | ((chunk: T, encoding: string) => R[])
-        | ((chunk: T, encoding: string) => Promise<R[]>),
-    options?: TransformOptions,
-): NodeJS.ReadWriteStream {
-    return baseFunctions.flatMap(mapper, options);
-}
+        flatMap: withDefaultOptions(1, flatMap),
 
         /**
          * Return a ReadWrite stream that filters out streamed chunks for which the predicate does not hold
@@ -53,14 +79,7 @@ export function flatMap<T, R>(
          * @param options?
          * @param options.objectMode? Whether this stream should behave as a stream of objects.
          */
-export function filter<T>(
-    mapper:
-        | ((chunk: T, encoding: string) => boolean)
-        | ((chunk: T, encoding: string) => Promise<boolean>),
-    options?: ThroughOptions,
-): NodeJS.ReadWriteStream {
-    return baseFunctions.filter(mapper, options);
-}
+        filter: withDefaultOptions(1, filter),
 
         /**
          * Return a ReadWrite stream that reduces streamed chunks down to a single value and yield that
@@ -71,15 +90,7 @@ export function filter<T>(
          * @param options.readableObjectMode? Whether this stream should behave as a readable stream of objects
          * @param options.writableObjectMode? Whether this stream should behave as a writable stream of objects
          */
-export function reduce<T, R>(
-    iteratee:
-        | ((previousValue: R, chunk: T, encoding: string) => R)
-        | ((previousValue: R, chunk: T, encoding: string) => Promise<R>),
-    initialValue: R,
-    options?: TransformOptions,
-): NodeJS.ReadWriteStream {
-    return baseFunctions.reduce(iteratee, initialValue, options);
-}
+        reduce: withDefaultOptions(2, reduce),
 
         /**
          * Return a ReadWrite stream that splits streamed chunks using the given separator
@@ -87,12 +98,7 @@ export function reduce<T, R>(
          * @param options? Defaults to encoding: utf8
          * @param options.encoding? Encoding written chunks are assumed to use
          */
-export function split(
-    separator?: string | RegExp,
-    options?: WithEncoding,
-): NodeJS.ReadWriteStream {
-    return baseFunctions.split(separator, options);
-}
+        split,
 
         /**
          * Return a ReadWrite stream that joins streamed chunks using the given separator
@@ -100,12 +106,7 @@ export function split(
          * @param options? Defaults to encoding: utf8
          * @param options.encoding? Encoding written chunks are assumed to use
          */
-export function join(
-    separator: string,
-    options?: WithEncoding,
-): NodeJS.ReadWriteStream {
-    return baseFunctions.join(separator, options);
-}
+        join: withDefaultOptions(1, join),
 
         /**
          * Return a ReadWrite stream that replaces occurrences of the given string or regular expression  in
@@ -115,21 +116,17 @@ export function join(
          * @param options? Defaults to encoding: utf8
          * @param options.encoding Encoding written chunks are assumed to use
          */
-export function replace(
-    searchValue: string | RegExp,
-    replaceValue: string,
-    options?: WithEncoding,
-): NodeJS.ReadWriteStream {
-    return baseFunctions.replace(searchValue, replaceValue, options);
-}
+        replace,
 
         /**
          * Return a ReadWrite stream that parses the streamed chunks as JSON. Each streamed chunk
          * must be a fully defined JSON string in utf8.
+         * @param format: @type SerializationFormats defaults SerializationFormats.utf8
+         * @param emitError: @type boolean Whether or not to emit an error when
+         * failing to parse. An error will automatically close the stream.
+         * Defaults to true.
          */
-export function parse(): NodeJS.ReadWriteStream {
-    return baseFunctions.parse();
-}
+        parse,
 
         /**
          * Return a ReadWrite stream that stringifies the streamed chunks to JSON
@@ -137,38 +134,26 @@ export function parse(): NodeJS.ReadWriteStream {
          * @param options.pretty If true, whitespace is inserted into the stringified chunks.
          *
          */
-export function stringify(options?: JsonParseOptions): NodeJS.ReadWriteStream {
-    return baseFunctions.stringify(options);
-}
+        stringify,
 
         /**
          * Return a ReadWrite stream that collects streamed chunks into an array or buffer
          * @param options?
          * @param options.objectMode? Whether this stream should behave as a stream of objects
          */
-export function collect(options?: ThroughOptions): NodeJS.ReadWriteStream {
-    return baseFunctions.collect(options);
-}
+        collect: withDefaultOptions(0, collect),
 
         /**
          * Return a Readable stream of readable streams concatenated together
          * @param streams Readable streams to concatenate
          */
-export function concat(
-    ...streams: NodeJS.ReadableStream[]
-): NodeJS.ReadableStream {
-    return baseFunctions.concat(...streams);
-}
+        concat,
 
         /**
          * Return a Readable stream of readable streams concatenated together
          * @param streams Readable streams to merge
          */
-export function merge(
-    ...streams: NodeJS.ReadableStream[]
-): NodeJS.ReadableStream {
-    return baseFunctions.merge(...streams);
-}
+        merge,
 
         /**
          * Return a Duplex stream from a writable stream that is assumed to somehow, when written to,
@@ -176,66 +161,107 @@ export function merge(
          * @param writable Writable stream assumed to cause the readable stream to yield chunks when written to
          * @param readable Readable stream assumed to yield chunks when the writable stream is written to
          */
-export function duplex(
-    writable: Writable,
-    readable: Readable,
-): NodeJS.ReadWriteStream {
-    return baseFunctions.duplex(writable, readable);
-}
+        duplex,
 
         /**
          * Return a Duplex stream from a child process' stdin and stdout
          * @param childProcess Child process from which to create duplex stream
          */
-export function child(childProcess: ChildProcess): NodeJS.ReadWriteStream {
-    return baseFunctions.child(childProcess);
-}
+        child,
 
         /**
          * Return a Promise resolving to the last streamed chunk of the given readable stream, after it has
          * ended
          * @param readable Readable stream to wait on
          */
-export function last<T>(readable: Readable): Promise<T | null> {
-    return baseFunctions.last(readable);
-}
+        last,
 
         /**
          * Stores chunks of data internally in array and batches when batchSize is reached.
          * @param batchSize Size of the batches, defaults to 1000.
          * @param maxBatchAge? Max lifetime of a batch, defaults to 500
- */
-export function batch(batchSize: number, maxBatchAge?: number): NodeJS.ReadWriteStream {
-    return baseFunctions.batch(batchSize, maxBatchAge);
-}
-
-/**
- * Unbatches and sends individual chunks of data
- */
-export function unbatch(): NodeJS.ReadWriteStream {
-    return baseFunctions.unbatch();
-}
-
-/**
- * Limits date of data transferred into stream.
          * @param options?
- * @param targetRate? Desired rate in ms
- * @param period? Period to sleep for when rate is above or equal to targetRate
+         * @param options.objectMode? Whether this stream should behave as a stream of objects
          */
-export function rate(targetRate?: number, period?: number): NodeJS.ReadWriteStream {
-    return baseFunctions.rate(targetRate, period);
-}
+        batch: withDefaultOptions(2, batch),
+
+        /**
+         * Unbatches and sends individual chunks of data.
+         * @param options?
+         * @param options.objectMode? Whether this stream should behave as a stream of objects
+         */
+        unbatch: withDefaultOptions(0, unbatch),
+
+        /**
+         * Limits rate of data transferred into stream.
+         * @param targetRate? Desired rate in ms.
+         * @param period? Period to sleep for when rate is above or equal to targetRate.
+         * @param options?
+         */
+        rate: withDefaultOptions(2, rate),
 
         /**
          * Limits number of parallel processes in flight.
          * @param parallel Max number of parallel processes.
- * @param func Function to execute on each data chunk
+         * @param func Function to execute on each data chunk.
          * @param pause Amount of time to pause processing when max number of parallel processes are executing.
          */
-export function parallelMap<T, R>(
-    mapper: (chunk: T) => R,
-    parallel?: number,
-    sleepTime?: number,
-) {
-    return baseFunctions.parallelMap(mapper, parallel, sleepTime);
+        parallelMap: withDefaultOptions(3, parallelMap),
+
+        /**
+         * Accummulates and sends batches of data. Each chunk that flows into the stream is checked against items
+         * in the buffer. How the buffer is mutated is based on 1 of 2 possible buffering strategies:
+         * 	1. Sliding
+         * 		- If the buffer is larger than the batchSize, the front of the buffer is popped to maintain
+         * 		the batchSize. When no key is provided, the batchSize is effectively the buffer length. When
+         * 		a key is provided, the batchSize is based on the value at that key. For example, given a key
+         * 		of `timestamp` and a batchSize of 3000, each item in the buffer will be guaranteed to be
+         * 		within 3000 timestamp units from the first element. This means that with a key, multiple elements
+         * 		may be spliced off the front of the buffer. The buffer is then pushed into the stream.
+         * 	2. Rolling
+         * 		- If the buffer is larger than the batchSize, the buffer is cleared and pushed into the stream.
+         * 		When no key is provided, the batchSize is the buffer length. When a key is provided, the batchSize
+         * 		is based on the value at that key. For example, given a key of `timestamp` and a batchSize of 3000,
+         * 		each item in the buffer will be guaranteed to be within 3000 timestamp units from the first element.
+         * @param flushStrategy Buffering strategy to use.
+         * @param batchSize Size of the batch (in units of buffer length or value at key).
+         * @param keyBy Key to determine if element fits into buffer or items need to be cleared from buffer.
+         * @param options Transform stream options
+         */
+        accumulator: withDefaultOptions(3, accumulator),
+
+        /**
+         * Accummulates and sends batches of data. Each chunk that flows into the stream is checked against items
+         * in the buffer. How the buffer is mutated is based on 1 of 2 possible buffering strategies:
+         * 	1. Sliding
+         * 		- If the iteratee returns false, the front of the buffer is popped until iteratee returns true. The
+         * 		item is pushed into the buffer and buffer is pushed into stream.
+         * 	2. Rolling
+         * 		- If the iteratee returns false, the buffer is cleared and pushed into stream. The item is
+         * 		then pushed into the buffer.
+         * @param flushStrategy Buffering strategy to use.
+         * @param iteratee Function applied to buffer when a chunk of data enters stream to determine if element fits into
+         * or items need to be cleared from buffer.
+         * @param options Transform stream options
+         */
+        accumulatorBy: withDefaultOptions(2, accumulatorBy),
+
+        /**
+         * Composes multiple streams together. Writing occurs on first stream, piping occurs from last stream.
+         * @param streams Array of streams to compose. Minimum of two.
+         * @param errorCallback a function that handles any error coming out of the pipeline
+         * @param options Transform stream options
+         */
+        compose: withDefaultOptions(2, compose),
+
+        /**
+         * Composes multiple streams together. Writing occurs on first stream, piping occurs from last stream.
+         * @param construct Constructor for new output source. Should return a Writable or ReadWrite stream.
+         * @param demuxBy
+         * @param demuxBy.key? Key to fetch value from source chunks to demultiplex source.
+         * @param demuxBy.keyBy? Function to fetch value from source chunks to demultiplex source.
+         * @param options Writable stream options
+         */
+        demux: withDefaultOptions(2, demux),
+    };
 }

src/functions/join.ts

@@ -0,0 +1,26 @@
+import { Transform, TransformOptions } from "stream";
+import { StringDecoder } from "string_decoder";
+import { WithEncoding } from "./baseDefinitions";
+
+export function join(
+    separator: string,
+    options: WithEncoding & TransformOptions = { encoding: "utf8" },
+): Transform {
+    let isFirstChunk = true;
+    const decoder = new StringDecoder(options.encoding);
+    return new Transform({
+        readableObjectMode: true,
+        async transform(chunk: Buffer, encoding, callback) {
+            const asString = decoder.write(chunk);
+            // Take care not to break up multi-byte characters spanning multiple chunks
+            if (asString !== "" || chunk.length === 0) {
+                if (!isFirstChunk) {
+                    this.push(separator);
+                }
+                this.push(asString);
+                isFirstChunk = false;
+            }
+            callback();
+        },
+    });
+}

src/functions/last.ts

@@ -0,0 +1,8 @@
+export function last<T>(readable: NodeJS.ReadableStream): Promise<T | null> {
+    let lastChunk: T | null = null;
+    return new Promise((resolve, _) => {
+        readable
+            .on("data", chunk => (lastChunk = chunk))
+            .on("end", () => resolve(lastChunk));
+    });
+}

src/functions/map.ts

@@ -0,0 +1,13 @@
+import { Transform, TransformOptions } from "stream";
+
+export function map<T, R>(
+    mapper: (chunk: T, encoding: string) => R,
+    options: TransformOptions = { objectMode: true },
+): Transform {
+    return new Transform({
+        ...options,
+        async transform(chunk: T, encoding, callback) {
+            callback(null, await mapper(chunk, encoding));
+        },
+    });
+}

src/functions/merge.ts

@@ -0,0 +1,33 @@
+import { Readable } from "stream";
+
+export function merge(...streams: Readable[]): Readable {
+    let isStarted = false;
+    let streamEndedCount = 0;
+    return new Readable({
+        objectMode: true,
+        read() {
+            if (streamEndedCount >= streams.length) {
+                this.push(null);
+            } else if (!isStarted) {
+                isStarted = true;
+                streams.forEach(stream =>
+                    stream
+                        .on("data", chunk => {
+                            if (!this.push(chunk)) {
+                                streams.forEach(s => s.pause());
+                            }
+                        })
+                        .on("error", err => this.emit("error", err))
+                        .on("end", () => {
+                            streamEndedCount++;
+                            if (streamEndedCount === streams.length) {
+                                this.push(null);
+                            }
+                        }),
+                );
+            } else {
+                streams.forEach(s => s.resume());
+            }
+        },
+    });
+}

src/functions/parallelMap.ts

@@ -0,0 +1,30 @@
+import { Transform, TransformOptions } from "stream";
+import { sleep } from "../helpers";
+
+export function parallelMap<T, R>(
+    mapper: (data: T) => R,
+    parallel: number = 10,
+    sleepTime: number = 1,
+    options?: TransformOptions,
+) {
+    let inflight = 0;
+    return new Transform({
+        ...options,
+        async transform(data, encoding, callback) {
+            while (parallel <= inflight) {
+                await sleep(sleepTime);
+            }
+            inflight += 1;
+            callback();
+            const res = await mapper(data);
+            this.push(res);
+            inflight -= 1;
+        },
+        async flush(callback) {
+            while (inflight > 0) {
+                await sleep(sleepTime);
+            }
+            callback();
+        },
+    });
+}

src/functions/parse.ts

@@ -0,0 +1,27 @@
+import { Transform } from "stream";
+import { StringDecoder } from "string_decoder";
+import { SerializationFormats } from "./baseDefinitions";
+
+export function parse(
+    format: SerializationFormats = SerializationFormats.utf8,
+    emitError: boolean = true,
+): Transform {
+    const decoder = new StringDecoder(format);
+    return new Transform({
+        readableObjectMode: true,
+        writableObjectMode: true,
+        async transform(chunk: Buffer, encoding, callback) {
+            try {
+                const asString = decoder.write(chunk);
+                // Using await causes parsing errors to be emitted
+                callback(null, await JSON.parse(asString));
+            } catch (err) {
+                if (emitError) {
+                    callback(err);
+                } else {
+                    callback();
+                }
+            }
+        },
+    });
+}

src/functions/rate.ts

@@ -0,0 +1,24 @@
+import { Transform, TransformOptions } from "stream";
+import { performance } from "perf_hooks";
+import { sleep } from "../helpers";
+
+export function rate(
+    targetRate: number = 50,
+    period: number = 1,
+    options?: TransformOptions,
+): Transform {
+    const deltaMS = ((1 / targetRate) * 1000) / period; // Skip a full period
+    let total = 0;
+    const start = performance.now();
+    return new Transform({
+        ...options,
+        async transform(data, encoding, callback) {
+            const currentRate = (total / (performance.now() - start)) * 1000;
+            if (targetRate && currentRate > targetRate) {
+                await sleep(deltaMS);
+            }
+            total += 1;
+            callback(undefined, data);
+        },
+    });
+}

src/functions/reduce.ts

@@ -0,0 +1,31 @@
+import { Transform, TransformOptions } from "stream";
+
+export function reduce<T, R>(
+    iteratee:
+        | ((previousValue: R, chunk: T, encoding: string) => R)
+        | ((previousValue: R, chunk: T, encoding: string) => Promise<R>),
+    initialValue: R,
+    options?: TransformOptions,
+) {
+    let value = initialValue;
+    return new Transform({
+        ...options,
+        async transform(chunk: T, encoding, callback) {
+            value = await iteratee(value, chunk, encoding);
+            callback();
+        },
+        flush(callback) {
+            // Best effort attempt at yielding the final value (will throw if e.g. yielding an object and
+            // downstream doesn't expect objects)
+            try {
+                callback(undefined, value);
+            } catch (err) {
+                try {
+                    this.emit("error", err);
+                } catch {
+                    // Best effort was made
+                }
+            }
+        },
+    });
+}

src/functions/replace.ts

@@ -0,0 +1,25 @@
+import { Transform } from "stream";
+import { StringDecoder } from "string_decoder";
+import { WithEncoding } from "./baseDefinitions";
+export function replace(
+    searchValue: string | RegExp,
+    replaceValue: string,
+    options: WithEncoding = { encoding: "utf8" },
+): Transform {
+    const decoder = new StringDecoder(options.encoding);
+    return new Transform({
+        readableObjectMode: true,
+        transform(chunk: Buffer, encoding, callback) {
+            const asString = decoder.write(chunk);
+            // Take care not to break up multi-byte characters spanning multiple chunks
+            if (asString !== "" || chunk.length === 0) {
+                callback(
+                    undefined,
+                    asString.replace(searchValue, replaceValue),
+                );
+            } else {
+                callback();
+            }
+        },
+    });
+}

src/functions/split.ts

@@ -0,0 +1,29 @@
+import { Transform } from "stream";
+import { StringDecoder } from "string_decoder";
+import { WithEncoding } from "./baseDefinitions";
+
+export function split(
+    separator: string | RegExp = "\n",
+    options: WithEncoding = { encoding: "utf8" },
+): Transform {
+    let buffered = "";
+    const decoder = new StringDecoder(options.encoding);
+
+    return new Transform({
+        readableObjectMode: true,
+        transform(chunk: Buffer, encoding, callback) {
+            const asString = decoder.write(chunk);
+            const splitted = asString.split(separator);
+            if (splitted.length > 1) {
+                splitted[0] = buffered.concat(splitted[0]);
+                buffered = "";
+            }
+            buffered += splitted[splitted.length - 1];
+            splitted.slice(0, -1).forEach((part: string) => this.push(part));
+            callback();
+        },
+        flush(callback) {
+            callback(undefined, buffered + decoder.end());
+        },
+    });
+}

src/functions/stringify.ts

@@ -0,0 +1,19 @@
+import { Transform } from "stream";
+import { JsonValue, JsonParseOptions } from "./baseDefinitions";
+
+export function stringify(
+    options: JsonParseOptions = { pretty: false },
+): Transform {
+    return new Transform({
+        readableObjectMode: true,
+        writableObjectMode: true,
+        transform(chunk: JsonValue, encoding, callback) {
+            callback(
+                undefined,
+                options.pretty
+                    ? JSON.stringify(chunk, null, 2)
+                    : JSON.stringify(chunk),
+            );
+        },
+    });
+}

src/functions/unbatch.ts

@@ -0,0 +1,13 @@
+import { Transform, TransformOptions } from "stream";
+
+export function unbatch(options?: TransformOptions) {
+    return new Transform({
+        ...options,
+        transform(data, encoding, callback) {
+            for (const d of data) {
+                this.push(d);
+            }
+            callback();
+        },
+    });
+}

src/helpers.ts

@@ -1,3 +1,17 @@
 export async function sleep(time: number): Promise<{} | null> {
     return time > 0 ? new Promise(resolve => setTimeout(resolve, time)) : null;
 }
+
+export type AllStreams =
+    | NodeJS.ReadableStream
+    | NodeJS.ReadWriteStream
+    | NodeJS.WritableStream;
+
+export function isReadable(
+    stream: AllStreams,
+): stream is NodeJS.WritableStream {
+    return (
+        (stream as NodeJS.ReadableStream).pipe !== undefined &&
+        (stream as any).readable === true
+    );
+}

src/index.ts

@@ -1,22 +1,6 @@
-export {
-    fromArray,
-    map,
-    flatMap,
-    filter,
-    reduce,
-    split,
-    join,
-    replace,
-    parse,
-    stringify,
-    collect,
-    concat,
-    merge,
-    duplex,
-    child,
-    last,
-    batch,
-    unbatch,
-    rate,
-    parallelMap,
-} from "./functions";
+import mhysa from "./functions";
+import * as _utils from "./utils";
+export default mhysa;
+
+// @TODO fix this with proper import export
+export const utils = { ..._utils };

src/utils/collected.ts

@@ -0,0 +1,12 @@
+import { Transform } from "stream";
+
+export function collected(stream: Transform): any {
+    return new Promise((resolve, reject) => {
+        stream.once("data", d => {
+            resolve(d);
+        });
+        stream.once("error", e => {
+            reject(e);
+        });
+    });
+}

src/utils/index.ts

@@ -0,0 +1 @@
+export { collected } from "./collected";

tests/accumulator.spec.ts

@@ -0,0 +1,557 @@
+import test from "ava";
+import { expect } from "chai";
+import { Readable } from "stream";
+import mhysa from "../src";
+import { FlushStrategy } from "../src/functions/accumulator";
+import { performance } from "perf_hooks";
+const { accumulator, accumulatorBy } = mhysa({ objectMode: true });
+
+test.cb("accumulator() rolling", t => {
+    t.plan(3);
+    let chunkIndex = 0;
+    interface TestObject {
+        ts: number;
+        key: string;
+    }
+    const source = new Readable({ objectMode: true });
+    const firstFlush = [{ ts: 0, key: "a" }, { ts: 1, key: "b" }];
+    const secondFlush = [{ ts: 2, key: "d" }, { ts: 3, key: "e" }];
+    const thirdFlush = [{ ts: 4, key: "f" }];
+    const flushes = [firstFlush, secondFlush, thirdFlush];
+
+    source
+        .pipe(
+            accumulator(FlushStrategy.rolling, 2, undefined, {
+                objectMode: true,
+            }),
+        )
+        .on("data", (flush: TestObject[]) => {
+            t.deepEqual(flush, flushes[chunkIndex]);
+            chunkIndex++;
+        })
+        .on("error", (e: any) => {
+            t.end(e);
+        })
+        .on("end", t.end);
+    [...firstFlush, ...secondFlush, ...thirdFlush].forEach(item => {
+        source.push(item);
+    });
+    source.push(null);
+});
+
+test.cb("accumulator() rolling with key", t => {
+    t.plan(2);
+    let chunkIndex = 0;
+    interface TestObject {
+        ts: number;
+        key: string;
+    }
+    const source = new Readable({ objectMode: true });
+    const firstFlush = [
+        { ts: 0, key: "a" },
+        { ts: 1, key: "b" },
+        { ts: 2, key: "c" },
+        { ts: 2, key: "d" },
+    ];
+    const secondFlush = [{ ts: 3, key: "e" }];
+    const flushes = [firstFlush, secondFlush];
+
+    source
+        .pipe(accumulator(FlushStrategy.rolling, 3, "ts", { objectMode: true }))
+        .on("data", (flush: TestObject[]) => {
+            t.deepEqual(flush, flushes[chunkIndex]);
+            chunkIndex++;
+        })
+        .on("error", (e: any) => {
+            t.end(e);
+        })
+        .on("end", t.end);
+    [...firstFlush, ...secondFlush].forEach(item => {
+        source.push(item);
+    });
+    source.push(null);
+});
+
+test.cb(
+    "accumulator() rolling should emit error and ignore chunk when its missing key",
+    t => {
+        t.plan(2);
+        let index = 0;
+        interface TestObject {
+            ts: number;
+            key: string;
+        }
+        const source = new Readable({ objectMode: true });
+        const accumulatorStream = accumulator(
+            FlushStrategy.rolling,
+            3,
+            "nonExistingKey",
+            { objectMode: true },
+        );
+        const input = [{ ts: 0, key: "a" }, { ts: 1, key: "b" }];
+
+        source
+            .pipe(accumulatorStream)
+            .on("data", (flush: TestObject[]) => {
+                // No valid data output
+                expect(flush).to.deep.equal([]);
+            })
+            .on("error", (err: any) => {
+                source.pipe(accumulatorStream);
+                accumulatorStream.resume();
+                expect(err.message).to.equal(
+                    `Key is missing in event: (nonExistingKey, ${JSON.stringify(
+                        input[index],
+                    )})`,
+                );
+                index++;
+                t.pass();
+            })
+            .on("end", t.end);
+        input.forEach(item => {
+            source.push(item);
+        });
+        source.push(null);
+    },
+);
+
+test.cb(
+    "accumulator() rolling should emit error, ignore chunk when key is missing and continue processing chunks correctly",
+    t => {
+        t.plan(3);
+        let chunkIndex = 0;
+        interface TestObject {
+            ts: number;
+            key: string;
+        }
+        const source = new Readable({ objectMode: true });
+        const accumulatorStream = accumulator(FlushStrategy.rolling, 3, "ts", {
+            objectMode: true,
+        });
+        const input = [
+            { ts: 0, key: "a" },
+            { ts: 1, key: "b" },
+            { ts: 2, key: "c" },
+            { key: "d" },
+            { ts: 3, key: "e" },
+        ];
+        const firstFlush = [
+            { ts: 0, key: "a" },
+            { ts: 1, key: "b" },
+            { ts: 2, key: "c" },
+        ];
+        const secondFlush = [{ ts: 3, key: "e" }];
+        const flushes = [firstFlush, secondFlush];
+
+        source
+            .pipe(accumulatorStream)
+            .on("data", (flush: TestObject[]) => {
+                t.deepEqual(flush, flushes[chunkIndex]);
+                chunkIndex++;
+            })
+            .on("error", (err: any) => {
+                source.pipe(accumulatorStream);
+                accumulatorStream.resume();
+                expect(err.message).to.equal(
+                    `Key is missing in event: (ts, ${JSON.stringify(
+                        input[3],
+                    )})`,
+                );
+                t.pass();
+            })
+            .on("end", t.end);
+        input.forEach(item => {
+            source.push(item);
+        });
+        source.push(null);
+    },
+);
+
+test.cb("accumulator() sliding", t => {
+    t.plan(4);
+    let chunkIndex = 0;
+    interface TestObject {
+        ts: number;
+        key: string;
+    }
+    const source = new Readable({ objectMode: true });
+    const input = [
+        { ts: 0, key: "a" },
+        { ts: 1, key: "b" },
+        { ts: 2, key: "c" },
+        { ts: 4, key: "d" },
+    ];
+    const firstFlush = [{ ts: 0, key: "a" }];
+    const secondFlush = [{ ts: 0, key: "a" }, { ts: 1, key: "b" }];
+    const thirdFlush = [
+        { ts: 0, key: "a" },
+        { ts: 1, key: "b" },
+        { ts: 2, key: "c" },
+    ];
+    const fourthFlush = [
+        { ts: 1, key: "b" },
+        { ts: 2, key: "c" },
+        { ts: 4, key: "d" },
+    ];
+
+    const flushes = [firstFlush, secondFlush, thirdFlush, fourthFlush];
+    source
+        .pipe(
+            accumulator(FlushStrategy.sliding, 3, undefined, {
+                objectMode: true,
+            }),
+        )
+        .on("data", (flush: TestObject[]) => {
+            t.deepEqual(flush, flushes[chunkIndex]);
+            chunkIndex++;
+        })
+        .on("error", (e: any) => {
+            t.end(e);
+        })
+        .on("end", t.end);
+    input.forEach(item => {
+        source.push(item);
+    });
+    source.push(null);
+});
+
+test.cb("accumulator() sliding with key", t => {
+    t.plan(6);
+    let chunkIndex = 0;
+    interface TestObject {
+        ts: number;
+        key: string;
+    }
+    const source = new Readable({ objectMode: true });
+    const input = [
+        { ts: 0, key: "a" },
+        { ts: 1, key: "b" },
+        { ts: 2, key: "c" },
+        { ts: 3, key: "d" },
+        { ts: 5, key: "f" },
+        { ts: 6, key: "g" },
+    ];
+    const firstFlush = [{ ts: 0, key: "a" }];
+    const secondFlush = [{ ts: 0, key: "a" }, { ts: 1, key: "b" }];
+    const thirdFlush = [
+        { ts: 0, key: "a" },
+        { ts: 1, key: "b" },
+        { ts: 2, key: "c" },
+    ];
+    const fourthFlush = [
+        { ts: 1, key: "b" },
+        { ts: 2, key: "c" },
+        { ts: 3, key: "d" },
+    ];
+    const fifthFlush = [{ ts: 3, key: "d" }, { ts: 5, key: "f" }];
+    const sixthFlush = [{ ts: 5, key: "f" }, { ts: 6, key: "g" }];
+
+    const flushes = [
+        firstFlush,
+        secondFlush,
+        thirdFlush,
+        fourthFlush,
+        fifthFlush,
+        sixthFlush,
+    ];
+    source
+        .pipe(accumulator(FlushStrategy.sliding, 3, "ts", { objectMode: true }))
+        .on("data", (flush: TestObject[]) => {
+            t.deepEqual(flush, flushes[chunkIndex]);
+            chunkIndex++;
+        })
+        .on("error", (e: any) => {
+            t.end(e);
+        })
+        .on("end", t.end);
+    input.forEach(item => {
+        source.push(item);
+    });
+    source.push(null);
+});
+
+test.cb(
+    "accumulator() sliding should emit error and ignore chunk when key is missing",
+    t => {
+        t.plan(2);
+        let index = 0;
+        interface TestObject {
+            ts: number;
+            key: string;
+        }
+        const source = new Readable({ objectMode: true });
+        const accumulatorStream = accumulator(
+            FlushStrategy.sliding,
+            3,
+            "nonExistingKey",
+            { objectMode: true },
+        );
+        const input = [{ ts: 0, key: "a" }, { ts: 1, key: "b" }];
+
+        source
+            .pipe(accumulatorStream)
+            .on("data", (flush: TestObject[]) => {
+                expect(flush).to.deep.equal([]);
+            })
+            .on("error", (err: any) => {
+                source.pipe(accumulatorStream);
+                accumulatorStream.resume();
+                expect(err.message).to.equal(
+                    `Key is missing in event: (nonExistingKey, ${JSON.stringify(
+                        input[index],
+                    )})`,
+                );
+                index++;
+                t.pass();
+            })
+            .on("end", t.end);
+        input.forEach(item => {
+            source.push(item);
+        });
+        source.push(null);
+    },
+);
+
+test.cb(
+    "accumulator() sliding should emit error, ignore chunk when key is missing and continue processing chunks correctly",
+    t => {
+        t.plan(6);
+        let chunkIndex = 0;
+        interface TestObject {
+            ts: number;
+            key: string;
+        }
+        const source = new Readable({ objectMode: true });
+        const accumulatorStream = accumulator(FlushStrategy.sliding, 3, "ts", {
+            objectMode: true,
+        });
+        const input = [
+            { ts: 0, key: "a" },
+            { key: "b" },
+            { ts: 2, key: "c" },
+            { ts: 3, key: "d" },
+            { ts: 5, key: "f" },
+            { ts: 6, key: "g" },
+        ];
+        const firstFlush = [{ ts: 0, key: "a" }];
+        const secondFlush = [{ ts: 0, key: "a" }, { ts: 2, key: "c" }];
+        const thirdFlush = [{ ts: 2, key: "c" }, { ts: 3, key: "d" }];
+        const fourthFlush = [{ ts: 3, key: "d" }, { ts: 5, key: "f" }];
+        const fifthFlush = [{ ts: 5, key: "f" }, { ts: 6, key: "g" }];
+
+        const flushes = [
+            firstFlush,
+            secondFlush,
+            thirdFlush,
+            fourthFlush,
+            fifthFlush,
+        ];
+        source
+            .pipe(accumulatorStream)
+            .on("data", (flush: TestObject[]) => {
+                t.deepEqual(flush, flushes[chunkIndex]);
+                chunkIndex++;
+            })
+            .on("error", (err: any) => {
+                source.pipe(accumulatorStream);
+                accumulatorStream.resume();
+                expect(err.message).to.equal(
+                    `Key is missing in event: (ts, ${JSON.stringify(
+                        input[1],
+                    )})`,
+                );
+                t.pass();
+            })
+            .on("end", t.end);
+        input.forEach(item => {
+            source.push(item);
+        });
+        source.push(null);
+    },
+);
+
+test.cb("accumulatorBy() rolling", t => {
+    t.plan(2);
+    let chunkIndex = 0;
+    interface TestObject {
+        ts: number;
+        key: string;
+    }
+    const source = new Readable({ objectMode: true });
+    const firstFlush = [
+        { ts: 0, key: "a" },
+        { ts: 1, key: "b" },
+        { ts: 2, key: "c" },
+        { ts: 2, key: "d" },
+    ];
+    const secondFlush = [{ ts: 3, key: "e" }];
+    const flushes = [firstFlush, secondFlush];
+
+    source
+        .pipe(
+            accumulatorBy(
+                FlushStrategy.rolling,
+                (event: TestObject, bufferChunk: TestObject) => {
+                    return bufferChunk.ts + 3 <= event.ts;
+                },
+                { objectMode: true },
+            ),
+        )
+        .on("data", (flush: TestObject[]) => {
+            t.deepEqual(flush, flushes[chunkIndex]);
+            chunkIndex++;
+        })
+        .on("error", (e: any) => {
+            t.end(e);
+        })
+        .on("end", t.end);
+    [...firstFlush, ...secondFlush].forEach(item => {
+        source.push(item);
+    });
+    source.push(null);
+});
+
+test.cb.skip(
+    "accumulatorBy() rolling should emit error when key iteratee throws",
+    t => {
+        t.plan(2);
+        interface TestObject {
+            ts: number;
+            key: string;
+        }
+        const source = new Readable({ objectMode: true });
+        const input = [
+            { ts: 0, key: "a" },
+            { ts: 1, key: "b" },
+            { ts: 2, key: "c" },
+        ];
+        const accumulaterStream = accumulatorBy(
+            FlushStrategy.rolling,
+            (event: TestObject, bufferChunk: TestObject) => {
+                if (event.key !== "a") {
+                    throw new Error("Failed mapping");
+                }
+                return bufferChunk.ts + 3 <= event.ts;
+            },
+            { objectMode: true },
+        );
+        source
+            .pipe(accumulaterStream)
+            .on("error", (err: any) => {
+                source.pipe(accumulaterStream);
+                accumulaterStream.resume();
+                expect(err.message).to.equal("Failed mapping");
+                t.pass();
+            })
+            .on("end", t.end);
+
+        input.forEach(item => {
+            source.push(item);
+        });
+        source.push(null);
+    },
+);
+
+test.cb("accumulatorBy() sliding", t => {
+    t.plan(6);
+    let chunkIndex = 0;
+    interface TestObject {
+        ts: number;
+        key: string;
+    }
+    const source = new Readable({ objectMode: true });
+    const input = [
+        { ts: 0, key: "a" },
+        { ts: 1, key: "b" },
+        { ts: 2, key: "c" },
+        { ts: 3, key: "d" },
+        { ts: 5, key: "f" },
+        { ts: 6, key: "g" },
+    ];
+    const firstFlush = [{ ts: 0, key: "a" }];
+    const secondFlush = [{ ts: 0, key: "a" }, { ts: 1, key: "b" }];
+    const thirdFlush = [
+        { ts: 0, key: "a" },
+        { ts: 1, key: "b" },
+        { ts: 2, key: "c" },
+    ];
+    const fourthFlush = [
+        { ts: 1, key: "b" },
+        { ts: 2, key: "c" },
+        { ts: 3, key: "d" },
+    ];
+    const fifthFlush = [{ ts: 3, key: "d" }, { ts: 5, key: "f" }];
+    const sixthFlush = [{ ts: 5, key: "f" }, { ts: 6, key: "g" }];
+
+    const flushes = [
+        firstFlush,
+        secondFlush,
+        thirdFlush,
+        fourthFlush,
+        fifthFlush,
+        sixthFlush,
+    ];
+    source
+        .pipe(
+            accumulatorBy(
+                FlushStrategy.sliding,
+                (event: TestObject, bufferChunk: TestObject) => {
+                    return bufferChunk.ts + 3 <= event.ts ? true : false;
+                },
+                { objectMode: true },
+            ),
+        )
+        .on("data", (flush: TestObject[]) => {
+            t.deepEqual(flush, flushes[chunkIndex]);
+            chunkIndex++;
+        })
+        .on("error", (e: any) => {
+            t.end(e);
+        })
+        .on("end", t.end);
+    input.forEach(item => {
+        source.push(item);
+    });
+    source.push(null);
+});
+
+test.cb.skip(
+    "accumulatorBy() sliding should emit error when key iteratee throws",
+    t => {
+        t.plan(2);
+        interface TestObject {
+            ts: number;
+            key: string;
+        }
+        const source = new Readable({ objectMode: true });
+        const input = [
+            { ts: 0, key: "a" },
+            { ts: 1, key: "b" },
+            { ts: 2, key: "c" },
+        ];
+        const accumulaterStream = accumulatorBy(
+            FlushStrategy.sliding,
+            (event: TestObject, bufferChunk: TestObject) => {
+                if (event.key !== "a") {
+                    throw new Error("Failed mapping");
+                }
+                return bufferChunk.ts + 3 <= event.ts ? true : false;
+            },
+            { objectMode: true },
+        );
+        source
+            .pipe(accumulaterStream)
+            .on("error", (err: any) => {
+                source.pipe(accumulaterStream);
+                accumulaterStream.resume();
+                expect(err.message).to.equal("Failed mapping");
+                t.pass();
+            })
+            .on("end", t.end);
+
+        input.forEach(item => {
+            source.push(item);
+        });
+        source.push(null);
+    },
+);

tests/batch.spec.ts

@@ -0,0 +1,84 @@
+import { Readable } from "stream";
+import test from "ava";
+import { expect } from "chai";
+import mhysa from "../src";
+const { batch, map, fromArray } = mhysa({ objectMode: true });
+
+test.cb("batch() batches chunks together", t => {
+    t.plan(3);
+    const source = new Readable({ objectMode: true });
+    const expectedElements = [["a", "b", "c"], ["d", "e", "f"], ["g"]];
+    let i = 0;
+    source
+        .pipe(batch(3))
+        .on("data", (element: string[]) => {
+            t.deepEqual(element, expectedElements[i]);
+            i++;
+        })
+        .on("error", t.end)
+        .on("end", t.end);
+
+    source.push("a");
+    source.push("b");
+    source.push("c");
+    source.push("d");
+    source.push("e");
+    source.push("f");
+    source.push("g");
+    source.push(null);
+});
+
+test.cb("batch() yields a batch after the timeout", t => {
+    t.plan(3);
+    const source = new Readable({
+        objectMode: true,
+        read(size: number) {
+            return;
+        },
+    });
+    const expectedElements = [["a", "b"], ["c"], ["d"]];
+    let i = 0;
+    source
+        .pipe(batch(3))
+        .on("data", (element: string[]) => {
+            t.deepEqual(element, expectedElements[i]);
+            i++;
+        })
+        .on("error", t.fail)
+        .on("end", t.end);
+
+    source.push("a");
+    source.push("b");
+    setTimeout(() => {
+        source.push("c");
+    }, 600);
+    setTimeout(() => {
+        source.push("d");
+        source.push(null);
+    }, 600 * 2);
+});
+
+test.cb(
+    "batch() yields all input data even when the last element(s) dont make a full batch",
+    t => {
+        const data = [1, 2, 3, 4, 5, 6, 7];
+
+        fromArray([...data])
+            .pipe(batch(3))
+            .pipe(
+                map(d => {
+                    t.deepEqual(
+                        d,
+                        [data.shift(), data.shift(), data.shift()].filter(
+                            x => !!x,
+                        ),
+                    );
+                }),
+            )
+            .on("error", t.fail)
+            .on("finish", () => {
+                t.is(data.length, 0);
+                t.end();
+            });
+    },
+);

tests/child.spec.ts

@@ -0,0 +1,29 @@
+import * as cp from "child_process";
+import { Readable } from "stream";
+import test from "ava";
+import { expect } from "chai";
+import mhysa from "../src";
+const { child } = mhysa();
+
+test.cb(
+    "child() allows easily writing to child process stdin and reading from its stdout",
+    t => {
+        t.plan(1);
+        const source = new Readable();
+        const catProcess = cp.exec("cat");
+        let out = "";
+        source
+            .pipe(child(catProcess))
+            .on("data", chunk => (out += chunk))
+            .on("error", t.end)
+            .on("end", () => {
+                expect(out).to.equal("abcdef");
+                t.pass();
+                t.end();
+            });
+        source.push("ab");
+        source.push("cd");
+        source.push("ef");
+        source.push(null);
+    },
+);

tests/collect.spec.ts

@@ -0,0 +1,133 @@
+import { Readable } from "stream";
+import test from "ava";
+import { expect } from "chai";
+import mhysa from "../src";
+const { collect } = mhysa();
+
+test.cb(
+    "collect() collects streamed elements into an array (object, flowing mode)",
+    t => {
+        t.plan(1);
+        const source = new Readable({ objectMode: true });
+
+        source
+            .pipe(collect({ objectMode: true }))
+            .on("data", collected => {
+                expect(collected).to.deep.equal(["a", "b", "c"]);
+                t.pass();
+            })
+            .on("error", t.end)
+            .on("end", t.end);
+
+        source.push("a");
+        source.push("b");
+        source.push("c");
+        source.push(null);
+    },
+);
+
+test.cb(
+    "collect() collects streamed elements into an array (object, paused mode)",
+    t => {
+        t.plan(1);
+        const source = new Readable({ objectMode: true });
+        const collector = source.pipe(collect({ objectMode: true }));
+
+        collector
+            .on("readable", () => {
+                let collected = collector.read();
+                while (collected !== null) {
+                    expect(collected).to.deep.equal(["a", "b", "c"]);
+                    t.pass();
+                    collected = collector.read();
+                }
+            })
+            .on("error", t.end)
+            .on("end", t.end);
+
+        source.push("a");
+        source.push("b");
+        source.push("c");
+        source.push(null);
+    },
+);
+
+test.cb(
+    "collect() collects streamed bytes into a buffer (non-object, flowing mode)",
+    t => {
+        t.plan(1);
+        const source = new Readable({ objectMode: false });
+
+        source
+            .pipe(collect())
+            .on("data", collected => {
+                expect(collected).to.deep.equal(Buffer.from("abc"));
+                t.pass();
+            })
+            .on("error", t.end)
+            .on("end", t.end);
+
+        source.push("a");
+        source.push("b");
+        source.push("c");
+        source.push(null);
+    },
+);
+
+test.cb(
+    "collect() collects streamed bytes into a buffer (non-object, paused mode)",
+    t => {
+        t.plan(1);
+        const source = new Readable({ objectMode: false });
+        const collector = source.pipe(collect({ objectMode: false }));
+        collector
+            .on("readable", () => {
+                let collected = collector.read();
+                while (collected !== null) {
+                    expect(collected).to.deep.equal(Buffer.from("abc"));
+                    t.pass();
+                    collected = collector.read();
+                }
+            })
+            .on("error", t.end)
+            .on("end", t.end);
+
+        source.push("a");
+        source.push("b");
+        source.push("c");
+        source.push(null);
+    },
+);
+
+test.cb(
+    "collect() emits an empty array if the source was empty (object mode)",
+    t => {
+        t.plan(1);
+        const source = new Readable({ objectMode: true });
+        const collector = source.pipe(collect({ objectMode: true }));
+        collector
+            .on("data", collected => {
+                expect(collected).to.deep.equal([]);
+                t.pass();
+            })
+            .on("error", t.end)
+            .on("end", t.end);
+
+        source.push(null);
+    },
+);
+
+test.cb(
+    "collect() emits nothing if the source was empty (non-object mode)",
+    t => {
+        t.plan(0);
+        const source = new Readable({ objectMode: false });
+        const collector = source.pipe(collect({ objectMode: false }));
+        collector
+            .on("data", () => t.fail())
+            .on("error", t.end)
+            .on("end", t.end);
+
+        source.push(null);
+    },
+);

tests/compose.spec.ts

@@ -0,0 +1,580 @@
+import * as test from "ava";
+import { expect } from "chai";
+import { sleep } from "../src/helpers";
+import { Readable, Writable } from "stream";
+import mhysa from "../src";
+import { performance } from "perf_hooks";
+const { compose, map, fromArray } = mhysa({ objectMode: true });
+
+test.cb("compose() chains two streams together in the correct order", t => {
+    t.plan(3);
+    interface Chunk {
+        visited: number[];
+        key: string;
+    }
+
+    let i = 0;
+    const first = map((chunk: Chunk) => {
+        chunk.visited.push(1);
+        return chunk;
+    });
+    const second = map((chunk: Chunk) => {
+        chunk.visited.push(2);
+        return chunk;
+    });
+
+    const composed = compose([first, second]);
+
+    composed.on("data", data => {
+        expect(data).to.deep.equal(result[i]);
+        t.pass();
+        i++;
+        if (i === 3) {
+            t.end();
+        }
+    });
+
+    const input = [
+        { key: "a", visited: [] },
+        { key: "b", visited: [] },
+        { key: "c", visited: [] },
+    ];
+    const result = [
+        { key: "a", visited: [1, 2] },
+        { key: "b", visited: [1, 2] },
+        { key: "c", visited: [1, 2] },
+    ];
+
+    input.forEach(item => composed.write(item));
+});
+
+test.cb("piping compose() maintains correct order", t => {
+    t.plan(3);
+    interface Chunk {
+        visited: number[];
+        key: string;
+    }
+    let i = 0;
+    const first = map((chunk: Chunk) => {
+        chunk.visited.push(1);
+        return chunk;
+    });
+    const second = map((chunk: Chunk) => {
+        chunk.visited.push(2);
+        return chunk;
+    });
+
+    const composed = compose([first, second]);
+    const third = map((chunk: Chunk) => {
+        chunk.visited.push(3);
+        return chunk;
+    });
+
+    composed.pipe(third).on("data", data => {
+        expect(data).to.deep.equal(result[i]);
+        t.pass();
+        i++;
+        if (i === 3) {
+            t.end();
+        }
+    });
+
+    composed.on("error", err => {
+        t.end(err);
+    });
+
+    const input = [
+        { key: "a", visited: [] },
+        { key: "b", visited: [] },
+        { key: "c", visited: [] },
+    ];
+    const result = [
+        { key: "a", visited: [1, 2, 3] },
+        { key: "b", visited: [1, 2, 3] },
+        { key: "c", visited: [1, 2, 3] },
+    ];
+
+    input.forEach(item => composed.write(item));
+});
+
+test("compose() writable length should be less than highWaterMark when handing writes", async t => {
+    t.plan(2);
+    return new Promise(async (resolve, reject) => {
+        interface Chunk {
+            key: string;
+            mapped: number[];
+        }
+        const first = map(async (chunk: Chunk) => {
+            chunk.mapped.push(1);
+            return chunk;
+        });
+
+        const second = map(async (chunk: Chunk) => {
+            chunk.mapped.push(2);
+            return chunk;
+        });
+
+        const composed = compose(
+            [first, second],
+            undefined,
+            {
+                highWaterMark: 2,
+            },
+        );
+        composed.on("error", err => {
+            reject();
+        });
+
+        composed.on("drain", () => {
+            t.pass();
+            expect(composed._writableState.length).to.be.equal(0);
+        });
+
+        composed.on("data", (chunk: Chunk) => {
+            if (chunk.key === "e") {
+                resolve();
+            }
+        });
+
+        const input = [
+            { key: "a", mapped: [] },
+            { key: "b", mapped: [] },
+            { key: "c", mapped: [] },
+            { key: "d", mapped: [] },
+            { key: "e", mapped: [] },
+        ];
+
+        fromArray(input).pipe(composed);
+    });
+});
+
+test("compose() should emit drain event ~rate * highWaterMark ms for every write that causes backpressure", async t => {
+    t.plan(2);
+    const _rate = 100;
+    const highWaterMark = 2;
+    return new Promise(async (resolve, reject) => {
+        interface Chunk {
+            key: string;
+            mapped: number[];
+        }
+        const first = map(async (chunk: Chunk) => {
+            await sleep(_rate);
+            chunk.mapped.push(1);
+            return chunk;
+        });
+
+        const second = map(async (chunk: Chunk) => {
+            chunk.mapped.push(2);
+            return chunk;
+        });
+
+        const composed = compose(
+            [first, second],
+            undefined,
+            {
+                highWaterMark,
+            },
+        );
+        composed.on("error", err => {
+            reject();
+        });
+
+        composed.on("drain", () => {
+            t.pass();
+            expect(composed._writableState.length).to.be.equal(0);
+        });
+
+        composed.on("data", (chunk: Chunk) => {
+            t.deepEqual(chunk.mapped, [1, 2]);
+        });
+
+        composed.on("finish", () => resolve());
+
+        const input = [
+            { key: "a", mapped: [] },
+            { key: "b", mapped: [] },
+            { key: "c", mapped: [] },
+            { key: "d", mapped: [] },
+            { key: "e", mapped: [] },
+        ];
+        fromArray(input).pipe(composed);
+    });
+});
+
+test.cb(
+    "compose() should emit drain event after 500 ms when writing 5 items that take 100ms to process with a highWaterMark of 5 ",
+    t => {
+        t.plan(6);
+        const _rate = 100;
+        interface Chunk {
+            key: string;
+            mapped: number[];
+        }
+        const first = map(async (chunk: Chunk) => {
+            await sleep(_rate);
+            chunk.mapped.push(1);
+            return chunk;
+        });
+
+        const second = map(async (chunk: Chunk) => {
+            chunk.mapped.push(2);
+            return chunk;
+        });
+
+        const composed = compose(
+            [first, second],
+            undefined,
+            {
+                highWaterMark: 5,
+            },
+        );
+
+        composed.on("error", err => {
+            t.end(err);
+        });
+
+        composed.on("drain", () => {
+            expect(composed._writableState.length).to.be.equal(0);
+            t.pass();
+        });
+
+        composed.on("data", (chunk: Chunk) => {
+            t.pass();
+            if (chunk.key === "e") {
+                t.end();
+            }
+        });
+
+        const input = [
+            { key: "a", mapped: [] },
+            { key: "b", mapped: [] },
+            { key: "c", mapped: [] },
+            { key: "d", mapped: [] },
+            { key: "e", mapped: [] },
+        ];
+        input.forEach(item => {
+            composed.write(item);
+        });
+    },
+);
+
+test.cb(
+    "compose() should emit drain event immediately when second stream is bottleneck",
+    t => {
+        t.plan(6);
+        const _rate = 200;
+        interface Chunk {
+            key: string;
+            mapped: number[];
+        }
+        const first = map((chunk: Chunk) => {
+            chunk.mapped.push(1);
+            return chunk;
+        });
+
+        const second = map(
+            async (chunk: Chunk) => {
+                pendingReads--;
+                await sleep(_rate);
+                expect(second._writableState.length).to.be.equal(1);
+                expect(first._readableState.length).to.equal(pendingReads);
+                chunk.mapped.push(2);
+                return chunk;
+            },
+            { highWaterMark: 1 },
+        );
+
+        const composed = compose(
+            [first, second],
+            undefined,
+            {
+                highWaterMark: 5,
+            },
+        );
+        composed.on("error", err => {
+            t.end(err);
+        });
+
+        composed.on("drain", () => {
+            expect(composed._writableState.length).to.be.equal(0);
+            expect(performance.now() - start).to.be.lessThan(_rate);
+            t.pass();
+        });
+
+        composed.on("data", (chunk: Chunk) => {
+            expect(composed._writableState.length).to.be.equal(0);
+            t.pass();
+            if (chunk.key === "e") {
+                t.end();
+            }
+        });
+
+        const input = [
+            { key: "a", mapped: [] },
+            { key: "b", mapped: [] },
+            { key: "c", mapped: [] },
+            { key: "d", mapped: [] },
+            { key: "e", mapped: [] },
+        ];
+        let pendingReads = input.length;
+
+        input.forEach(item => {
+            composed.write(item);
+        });
+
+        const start = performance.now();
+    },
+);
+
+test.cb(
+    "compose() should emit drain event and first should contain up to highWaterMark items in readable state when second is bottleneck",
+    t => {
+        t.plan(6);
+        interface Chunk {
+            index: number;
+            mapped: string[];
+        }
+        const first = map(
+            async (chunk: Chunk) => {
+                expect(first._readableState.length).to.be.at.most(2);
+                chunk.mapped.push("first");
+                return chunk;
+            },
+            {
+                highWaterMark: 2,
+            },
+        );
+
+        const second = map(
+            async (chunk: Chunk) => {
+                expect(second._writableState.length).to.be.equal(1);
+                await sleep(100);
+                chunk.mapped.push("second");
+                return chunk;
+            },
+            { highWaterMark: 2 },
+        );
+
+        const composed = compose(
+            [first, second],
+            undefined,
+            {
+                highWaterMark: 5,
+            },
+        );
+        composed.on("error", err => {
+            t.end(err);
+        });
+
+        composed.on("data", (chunk: Chunk) => {
+            expect(chunk.mapped.length).to.equal(2);
+            expect(chunk.mapped).to.deep.equal(["first", "second"]);
+            t.pass();
+            if (chunk.index === 5) {
+                t.end();
+            }
+        });
+
+        composed.on("drain", () => {
+            expect(composed._writableState.length).to.be.equal(0);
+            t.pass();
+        });
+
+        const input = [
+            { index: 1, mapped: [] },
+            { index: 2, mapped: [] },
+            { index: 3, mapped: [] },
+            { index: 4, mapped: [] },
+            { index: 5, mapped: [] },
+        ];
+
+        input.forEach(item => {
+            composed.write(item);
+        });
+    },
+);
+
+test.cb(
+    "compose() should not emit drain event writing 5 items to compose with a highWaterMark of 6",
+    t => {
+        t.plan(5);
+        const _rate = 100;
+        interface Chunk {
+            key: string;
+            mapped: number[];
+        }
+        const first = map(async (chunk: Chunk) => {
+            await sleep(_rate);
+            chunk.mapped.push(1);
+            return chunk;
+        });
+
+        const second = map(async (chunk: Chunk) => {
+            chunk.mapped.push(2);
+            return chunk;
+        });
+
+        const composed = compose(
+            [first, second],
+            undefined,
+            {
+                highWaterMark: 6,
+            },
+        );
+
+        composed.on("error", err => {
+            t.end(err);
+        });
+
+        composed.on("drain", () => {
+            t.end(new Error("Drain should not be emitted"));
+        });
+
+        composed.on("data", (chunk: Chunk) => {
+            t.pass();
+            if (chunk.key === "e") {
+                t.end();
+            }
+        });
+
+        const input = [
+            { key: "a", mapped: [] },
+            { key: "b", mapped: [] },
+            { key: "c", mapped: [] },
+            { key: "d", mapped: [] },
+            { key: "e", mapped: [] },
+        ];
+
+        input.forEach(item => {
+            composed.write(item);
+        });
+    },
+);
+
+test.cb("compose() should be 'destroyable'", t => {
+    t.plan(3);
+    const _sleep = 100;
+    interface Chunk {
+        key: string;
+        mapped: number[];
+    }
+
+    const first = map(async (chunk: Chunk) => {
+        await sleep(_sleep);
+        chunk.mapped.push(1);
+        return chunk;
+    });
+
+    const second = map(async (chunk: Chunk) => {
+        chunk.mapped.push(2);
+        return chunk;
+    });
+
+    const composed = compose(
+        [first, second],
+        (err: any) => {
+            t.pass();
+        },
+    );
+
+    const fakeSource = new Readable({
+        objectMode: true,
+        read() {
+            return;
+        },
+    });
+
+    const fakeSink = new Writable({
+        objectMode: true,
+        write(data, enc, cb) {
+            const cur = input.shift();
+            t.is(cur.key, data.key);
+            t.deepEqual(cur.mapped, [1, 2]);
+            if (cur.key === "a") {
+                composed.destroy();
+            }
+            cb();
+        },
+    });
+
+    composed.on("close", t.end);
+    fakeSource.pipe(composed).pipe(fakeSink);
+
+    const input = [
+        { key: "a", mapped: [] },
+        { key: "b", mapped: [] },
+        { key: "c", mapped: [] },
+        { key: "d", mapped: [] },
+        { key: "e", mapped: [] },
+    ];
+    fakeSource.push(input[0]);
+    fakeSource.push(input[1]);
+    fakeSource.push(input[2]);
+    fakeSource.push(input[3]);
+    fakeSource.push(input[4]);
+});
+
+test.cb("compose() `finish` and `end` propagates", t => {
+    interface Chunk {
+        key: string;
+        mapped: number[];
+    }
+
+    t.plan(8);
+    const first = map(async (chunk: Chunk) => {
+        chunk.mapped.push(1);
+        return chunk;
+    });
+
+    const second = map(async (chunk: Chunk) => {
+        chunk.mapped.push(2);
+        return chunk;
+    });
+
+    const composed = compose(
+        [first, second],
+        undefined,
+        {
+            highWaterMark: 3,
+        },
+    );
+
+    const fakeSource = new Readable({
+        objectMode: true,
+        read() {
+            return;
+        },
+    });
+    const sink = map((d: Chunk) => {
+        const curr = input.shift();
+        t.is(curr.key, d.key);
+        t.deepEqual(d.mapped, [1, 2]);
+    });
+
+    fakeSource.pipe(composed).pipe(sink);
+
+    fakeSource.on("end", () => {
+        t.pass();
+    });
+    composed.on("finish", () => {
+        t.pass();
+    });
+    composed.on("end", () => {
+        t.pass();
+        t.end();
+    });
+    sink.on("finish", () => {
+        t.pass();
+    });
+
+    const input = [
+        { key: "a", mapped: [] },
+        { key: "b", mapped: [] },
+        { key: "c", mapped: [] },
+        { key: "d", mapped: [] },
+        { key: "e", mapped: [] },
+    ];
+    fakeSource.push(input[0]);
+    fakeSource.push(input[1]);
+    fakeSource.push(null);
+});

tests/concat.spec.ts

@@ -0,0 +1,181 @@
+import { Readable } from "stream";
+import test from "ava";
+import { expect } from "chai";
+import mhysa from "../src";
+const { concat, collect } = mhysa();
+
+test.cb(
+    "concat() concatenates multiple readable streams (object, flowing mode)",
+    t => {
+        t.plan(6);
+        const source1 = new Readable({ objectMode: true });
+        const source2 = new Readable({ objectMode: true });
+        const expectedElements = ["a", "b", "c", "d", "e", "f"];
+        let i = 0;
+        concat(source1, source2)
+            .on("data", (element: string) => {
+                expect(element).to.equal(expectedElements[i]);
+                t.pass();
+                i++;
+            })
+            .on("error", t.end)
+            .on("end", t.end);
+
+        source1.push("a");
+        source2.push("d");
+        source1.push("b");
+        source2.push("e");
+        source1.push("c");
+        source2.push("f");
+        source2.push(null);
+        source1.push(null);
+    },
+);
+
+test.cb(
+    "concat() concatenates multiple readable streams (object, paused mode)",
+    t => {
+        t.plan(6);
+        const source1 = new Readable({ objectMode: true });
+        const source2 = new Readable({ objectMode: true });
+        const expectedElements = ["a", "b", "c", "d", "e", "f"];
+        let i = 0;
+        const concatenation = concat(source1, source2)
+            .on("readable", () => {
+                let element = concatenation.read();
+                while (element !== null) {
+                    expect(element).to.equal(expectedElements[i]);
+                    t.pass();
+                    i++;
+                    element = concatenation.read();
+                }
+            })
+            .on("error", t.end)
+            .on("end", t.end);
+
+        source1.push("a");
+        source2.push("d");
+        source1.push("b");
+        source2.push("e");
+        source1.push("c");
+        source2.push("f");
+        source2.push(null);
+        source1.push(null);
+    },
+);
+
+test.cb(
+    "concat() concatenates multiple readable streams (non-object, flowing mode)",
+    t => {
+        t.plan(6);
+        const source1 = new Readable({ objectMode: false });
+        const source2 = new Readable({ objectMode: false });
+        const expectedElements = ["a", "b", "c", "d", "e", "f"];
+        let i = 0;
+        concat(source1, source2)
+            .on("data", (element: string) => {
+                expect(element).to.deep.equal(Buffer.from(expectedElements[i]));
+                t.pass();
+                i++;
+            })
+            .on("error", t.end)
+            .on("end", t.end);
+
+        source1.push("a");
+        source2.push("d");
+        source1.push("b");
+        source2.push("e");
+        source1.push("c");
+        source2.push("f");
+        source2.push(null);
+        source1.push(null);
+    },
+);
+
+test.cb(
+    "concat() concatenates multiple readable streams (non-object, paused mode)",
+    t => {
+        t.plan(6);
+        const source1 = new Readable({ objectMode: false, read: () => ({}) });
+        const source2 = new Readable({ objectMode: false, read: () => ({}) });
+        const expectedElements = ["a", "b", "c", "d", "e", "f"];
+        let i = 0;
+        const concatenation = concat(source1, source2)
+            .on("readable", () => {
+                let element = concatenation.read();
+                while (element !== null) {
+                    expect(element).to.deep.equal(
+                        Buffer.from(expectedElements[i]),
+                    );
+                    t.pass();
+                    i++;
+                    element = concatenation.read();
+                }
+            })
+            .on("error", t.end)
+            .on("end", t.end);
+
+        source1.push("a");
+        setTimeout(() => source2.push("d"), 10);
+        setTimeout(() => source1.push("b"), 20);
+        setTimeout(() => source2.push("e"), 30);
+        setTimeout(() => source1.push("c"), 40);
+        setTimeout(() => source2.push("f"), 50);
+        setTimeout(() => source2.push(null), 60);
+        setTimeout(() => source1.push(null), 70);
+    },
+);
+
+test.cb("concat() concatenates a single readable stream (object mode)", t => {
+    t.plan(3);
+    const source = new Readable({ objectMode: true });
+    const expectedElements = ["a", "b", "c", "d", "e", "f"];
+    let i = 0;
+    concat(source)
+        .on("data", (element: string) => {
+            expect(element).to.equal(expectedElements[i]);
+            t.pass();
+            i++;
+        })
+        .on("error", t.end)
+        .on("end", t.end);
+
+    source.push("a");
+    source.push("b");
+    source.push("c");
+    source.push(null);
+});
+
+test.cb(
+    "concat() concatenates a single readable stream (non-object mode)",
+    t => {
+        t.plan(3);
+        const source = new Readable({ objectMode: false });
+        const expectedElements = ["a", "b", "c", "d", "e", "f"];
+        let i = 0;
+        concat(source)
+            .on("data", (element: string) => {
+                expect(element).to.deep.equal(Buffer.from(expectedElements[i]));
+                t.pass();
+                i++;
+            })
+            .on("error", t.end)
+            .on("end", t.end);
+
+        source.push("a");
+        source.push("b");
+        source.push("c");
+        source.push(null);
+    },
+);
+
+test.cb("concat() concatenates empty list of readable streams", t => {
+    t.plan(0);
+    concat()
+        .pipe(collect())
+        .on("data", _ => {
+            t.fail();
+        })
+        .on("error", t.end)
+        .on("end", t.end);
+});

tests/defaultOptions.spec.ts

@@ -0,0 +1,21 @@
+import { Readable } from "stream";
+import test from "ava";
+import mhysa from "../src";
+
+const withDefaultOptions = mhysa({ objectMode: true });
+const withoutOptions = mhysa();
+
+test("Mhysa instances can have default options", t => {
+    let batch = withDefaultOptions.batch();
+    t.true(batch._readableState.objectMode);
+    t.true(batch._writableState.objectMode);
+    batch = withDefaultOptions.batch(3);
+    t.true(batch._readableState.objectMode);
+    t.true(batch._writableState.objectMode);
+    batch = withDefaultOptions.batch(3, 1);
+    t.true(batch._readableState.objectMode);
+    t.true(batch._writableState.objectMode);
+    batch = withDefaultOptions.batch(3, 1, { objectMode: false });
+    t.false(batch._readableState.objectMode);
+    t.false(batch._writableState.objectMode);
+});

tests/demux.spec.ts

@@ -0,0 +1,864 @@
+import test from "ava";
+import { expect } from "chai";
+import mhysa from "../src";
+import { Writable, Readable } from "stream";
+const sinon = require("sinon");
+const { sleep } = require("../src/helpers");
+import { performance } from "perf_hooks";
+const { demux, map, fromArray } = mhysa({ objectMode: true });
+
+interface Test {
+    key: string;
+    visited: number[];
+}
+
+test.cb("demux() constructor should be called once per key", t => {
+    t.plan(1);
+    const input = [
+        { key: "a", visited: [] },
+        { key: "b", visited: [] },
+        { key: "a", visited: [] },
+        { key: "c", visited: [] },
+        { key: "a", visited: [] },
+        { key: "b", visited: [] },
+    ];
+    const construct = sinon.spy((destKey: string) => {
+        const dest = map((chunk: Test) => {
+            chunk.visited.push(1);
+            return chunk;
+        });
+
+        return dest;
+    });
+
+    const demuxed = demux(construct, "key", {});
+
+    demuxed.on("finish", () => {
+        expect(construct.withArgs("a").callCount).to.equal(1);
+        expect(construct.withArgs("b").callCount).to.equal(1);
+        expect(construct.withArgs("c").callCount).to.equal(1);
+        t.pass();
+        t.end();
+    });
+
+    fromArray(input).pipe(demuxed);
+});
+
+test.cb("demux() item written passed in constructor", t => {
+    t.plan(4);
+    const input = [
+        { key: "a", visited: [] },
+        { key: "b", visited: [] },
+        { key: "c", visited: [] },
+    ];
+    const construct = sinon.spy((destKey: string, item: any) => {
+        expect(item).to.deep.equal({ key: destKey, visited: [] });
+        t.pass();
+        const dest = map((chunk: Test) => {
+            chunk.visited.push(1);
+            return chunk;
+        });
+
+        return dest;
+    });
+
+    const demuxed = demux(construct, "key", {});
+
+    demuxed.on("finish", () => {
+        t.pass();
+        t.end();
+    });
+
+    fromArray(input).pipe(demuxed);
+});
+
+test.cb("demux() should send input through correct pipeline", t => {
+    t.plan(6);
+    const input = [
+        { key: "a", visited: [] },
+        { key: "b", visited: [] },
+        { key: "a", visited: [] },
+        { key: "c", visited: [] },
+        { key: "a", visited: [] },
+        { key: "b", visited: [] },
+    ];
+    const pipelineSpies = {};
+    const construct = (destKey: string) => {
+        const mapper = sinon.spy((chunk: Test) => {
+            return { ...chunk, visited: [1] };
+        });
+        const dest = map(mapper);
+        pipelineSpies[destKey] = mapper;
+
+        return dest;
+    };
+
+    const demuxed = demux(construct, "key", {});
+
+    demuxed.on("finish", () => {
+        pipelineSpies["a"].getCalls().forEach(call => {
+            expect(call.args[0].key).to.equal("a");
+            t.pass();
+        });
+        pipelineSpies["b"].getCalls().forEach(call => {
+            expect(call.args[0].key).to.equal("b");
+            t.pass();
+        });
+        pipelineSpies["c"].getCalls().forEach(call => {
+            expect(call.args[0].key).to.equal("c");
+            t.pass();
+        });
+        t.end();
+    });
+
+    fromArray(input).pipe(demuxed);
+});
+
+test.cb("demux() constructor should be called once per key using keyBy", t => {
+    t.plan(1);
+    const input = [
+        { key: "a", visited: [] },
+        { key: "b", visited: [] },
+        { key: "a", visited: [] },
+        { key: "c", visited: [] },
+        { key: "a", visited: [] },
+        { key: "b", visited: [] },
+    ];
+
+    const construct = sinon.spy((destKey: string) => {
+        const dest = map((chunk: Test) => {
+            chunk.visited.push(1);
+            return chunk;
+        });
+
+        return dest;
+    });
+
+    const demuxed = demux(construct, item => item.key, {});
+
+    demuxed.on("finish", () => {
+        expect(construct.withArgs("a").callCount).to.equal(1);
+        expect(construct.withArgs("b").callCount).to.equal(1);
+        expect(construct.withArgs("c").callCount).to.equal(1);
+        t.pass();
+        t.end();
+    });
+
+    fromArray(input).pipe(demuxed);
+});
+
+test.cb("demux() should send input through correct pipeline using keyBy", t => {
+    t.plan(6);
+    const input = [
+        { key: "a", visited: [] },
+        { key: "b", visited: [] },
+        { key: "a", visited: [] },
+        { key: "c", visited: [] },
+        { key: "a", visited: [] },
+        { key: "b", visited: [] },
+    ];
+    const pipelineSpies = {};
+    const construct = (destKey: string) => {
+        const mapper = sinon.spy((chunk: Test) => {
+            return { ...chunk, visited: [1] };
+        });
+        const dest = map(mapper);
+        pipelineSpies[destKey] = mapper;
+
+        return dest;
+    };
+
+    const demuxed = demux(construct, item => item.key, {});
+
+    demuxed.on("finish", () => {
+        pipelineSpies["a"].getCalls().forEach(call => {
+            expect(call.args[0].key).to.equal("a");
+            t.pass();
+        });
+        pipelineSpies["b"].getCalls().forEach(call => {
+            expect(call.args[0].key).to.equal("b");
+            t.pass();
+        });
+        pipelineSpies["c"].getCalls().forEach(call => {
+            expect(call.args[0].key).to.equal("c");
+            t.pass();
+        });
+        t.end();
+    });
+
+    fromArray(input).pipe(demuxed);
+});
+
+test("demux() write should return false and emit drain if more than highWaterMark items are buffered", t => {
+    return new Promise(async (resolve, reject) => {
+        t.plan(7);
+        interface Chunk {
+            key: string;
+            mapped: number[];
+        }
+        const input: Chunk[] = [
+            { key: "a", mapped: [] },
+            { key: "a", mapped: [] },
+            { key: "a", mapped: [] },
+            { key: "a", mapped: [] },
+            { key: "a", mapped: [] },
+            { key: "a", mapped: [] },
+        ];
+        let pendingReads = input.length;
+        const highWaterMark = 5;
+        const slowProcessorSpeed = 25;
+        const construct = (destKey: string) => {
+            const first = map(
+                async (chunk: Chunk) => {
+                    await sleep(slowProcessorSpeed);
+                    return { ...chunk, mapped: [1] };
+                },
+                { highWaterMark: 1 },
+            );
+
+            first.on("data", chunk => {
+                expect(chunk.mapped).to.deep.equal([1]);
+                pendingReads--;
+                if (pendingReads === 0) {
+                    resolve();
+                }
+                t.pass();
+            });
+
+            return first;
+        };
+
+        const _demux = demux(construct, "key", {
+            highWaterMark,
+        });
+
+        _demux.on("error", err => {
+            reject();
+        });
+
+        for (const item of input) {
+            const res = _demux.write(item);
+            expect(_demux._writableState.length).to.be.at.most(highWaterMark);
+            if (!res) {
+                await new Promise((resolv, rej) => {
+                    _demux.once("drain", () => {
+                        expect(_demux._writableState.length).to.be.equal(0);
+                        t.pass();
+                        resolv();
+                    });
+                });
+            }
+        }
+    });
+});
+
+test("demux() should emit one drain event after slowProcessorSpeed * highWaterMark ms when first stream is bottleneck", t => {
+    return new Promise(async (resolve, reject) => {
+        t.plan(7);
+        interface Chunk {
+            key: string;
+            mapped: number[];
+        }
+        const input: Chunk[] = [
+            { key: "a", mapped: [] },
+            { key: "a", mapped: [] },
+            { key: "a", mapped: [] },
+            { key: "a", mapped: [] },
+            { key: "a", mapped: [] },
+            { key: "a", mapped: [] },
+        ];
+        let pendingReads = input.length;
+        const highWaterMark = 5;
+        const slowProcessorSpeed = 25;
+
+        const construct = (destKey: string) => {
+            const first = map(
+                async (chunk: Chunk) => {
+                    await sleep(slowProcessorSpeed);
+                    chunk.mapped.push(1);
+                    return chunk;
+                },
+                { highWaterMark: 1 },
+            );
+
+            first.on("data", () => {
+                t.pass();
+                pendingReads--;
+                if (pendingReads === 0) {
+                    resolve();
+                }
+            });
+            return first;
+        };
+        const _demux = demux(construct, "key", {
+            highWaterMark,
+        });
+        _demux.on("error", err => {
+            reject();
+        });
+
+        const start = performance.now();
+        for (const item of input) {
+            const res = _demux.write(item);
+            if (!res) {
+                await new Promise((resolv, rej) => {
+                    // This event should be received after all items in demux are processed
+                    _demux.once("drain", () => {
+                        expect(performance.now() - start).to.be.greaterThan(
+                            slowProcessorSpeed * highWaterMark,
+                        );
+                        t.pass();
+                        resolv();
+                    });
+                });
+            }
+        }
+    });
+});
+
+test("demux() should emit one drain event when writing 6 items with highWaterMark of 5", t => {
+    return new Promise(async (resolve, reject) => {
+        t.plan(1);
+        interface Chunk {
+            key: string;
+            mapped: number[];
+        }
+        const highWaterMark = 5;
+        const input = [
+            { key: "a", mapped: [] },
+            { key: "a", mapped: [] },
+            { key: "a", mapped: [] },
+            { key: "a", mapped: [] },
+            { key: "a", mapped: [] },
+            { key: "a", mapped: [] },
+        ];
+        let pendingReads = input.length;
+        const construct = (destKey: string) => {
+            const first = map(
+                async (chunk: Chunk) => {
+                    await sleep(50);
+                    chunk.mapped.push(2);
+                    return chunk;
+                },
+                { highWaterMark: 1 },
+            );
+
+            first.on("data", () => {
+                pendingReads--;
+                if (pendingReads === 0) {
+                    resolve();
+                }
+            });
+            return first;
+        };
+        const _demux = demux(construct, "key", {
+            highWaterMark: 5,
+        });
+
+        _demux.on("error", err => {
+            reject();
+        });
+
+        for (const item of input) {
+            const res = _demux.write(item);
+            expect(_demux._writableState.length).to.be.at.most(highWaterMark);
+            if (!res) {
+                await new Promise(_resolve => {
+                    _demux.once("drain", () => {
+                        _resolve();
+                        expect(_demux._writableState.length).to.be.equal(0);
+                        t.pass();
+                    });
+                });
+            }
+        }
+    });
+});
+
+test.cb(
+    "demux() should emit drain event when second stream is bottleneck after (highWaterMark - 2) * slowProcessorSpeed ms",
+    t => {
+        // ie) first two items are pushed directly into first and second streams (highWaterMark - 2 remain in demux)
+        t.plan(8);
+        const slowProcessorSpeed = 100;
+        const highWaterMark = 5;
+        interface Chunk {
+            key: string;
+            mapped: number[];
+        }
+        const sink = new Writable({
+            objectMode: true,
+            write(chunk, encoding, cb) {
+                expect(chunk.mapped).to.deep.equal([1, 2]);
+                t.pass();
+                pendingReads--;
+                if (pendingReads === 0) {
+                    t.end();
+                }
+                cb();
+            },
+        });
+        const construct = (destKey: string) => {
+            const first = map(
+                (chunk: Chunk) => {
+                    chunk.mapped.push(1);
+                    return chunk;
+                },
+                { highWaterMark: 1 },
+            );
+
+            const second = map(
+                async (chunk: Chunk) => {
+                    await sleep(slowProcessorSpeed);
+                    chunk.mapped.push(2);
+                    return chunk;
+                },
+                { highWaterMark: 1 },
+            );
+
+            first.pipe(second).pipe(sink);
+            return first;
+        };
+        const _demux = demux(construct, () => "a", {
+            highWaterMark,
+        });
+        _demux.on("error", err => {
+            t.end(err);
+        });
+
+        _demux.on("drain", () => {
+            expect(_demux._writableState.length).to.be.equal(0);
+            expect(performance.now() - start).to.be.greaterThan(
+                slowProcessorSpeed * 3,
+            );
+            t.pass();
+        });
+
+        const input = [
+            { key: "a", mapped: [] },
+            { key: "b", mapped: [] },
+            { key: "c", mapped: [] },
+            { key: "d", mapped: [] },
+            { key: "e", mapped: [] },
+            { key: "f", mapped: [] },
+            { key: "g", mapped: [] },
+        ];
+        let pendingReads = input.length;
+
+        const start = performance.now();
+        fromArray(input).pipe(_demux);
+    },
+);
+
+test.cb(
+    "demux() should emit drain event when third stream is bottleneck",
+    t => {
+        // @TODO investigate why drain is emitted after slowProcessorSpeed
+        t.plan(8);
+        const slowProcessorSpeed = 100;
+        const highWaterMark = 5;
+        interface Chunk {
+            key: string;
+            mapped: number[];
+        }
+        const sink = new Writable({
+            objectMode: true,
+            write(chunk, encoding, cb) {
+                expect(chunk.mapped).to.deep.equal([1, 2, 3]);
+                t.pass();
+                pendingReads--;
+                if (pendingReads === 0) {
+                    t.end();
+                }
+                cb();
+            },
+        });
+        const construct = (destKey: string) => {
+            const first = map(
+                (chunk: Chunk) => {
+                    chunk.mapped.push(1);
+                    return chunk;
+                },
+                { highWaterMark: 1 },
+            );
+            const second = map(
+                (chunk: Chunk) => {
+                    chunk.mapped.push(2);
+                    return chunk;
+                },
+                { highWaterMark: 1 },
+            );
+
+            const third = map(
+                async (chunk: Chunk) => {
+                    await sleep(slowProcessorSpeed);
+                    chunk.mapped.push(3);
+                    return chunk;
+                },
+                { highWaterMark: 1 },
+            );
+
+            first
+                .pipe(second)
+                .pipe(third)
+                .pipe(sink);
+            return first;
+        };
+        const _demux = demux(construct, () => "a", {
+            highWaterMark,
+        });
+        _demux.on("error", err => {
+            t.end(err);
+        });
+
+        _demux.on("drain", () => {
+            expect(_demux._writableState.length).to.be.equal(0);
+            expect(performance.now() - start).to.be.greaterThan(
+                slowProcessorSpeed,
+            );
+            t.pass();
+        });
+
+        const input = [
+            { key: "a", mapped: [] },
+            { key: "b", mapped: [] },
+            { key: "c", mapped: [] },
+            { key: "d", mapped: [] },
+            { key: "e", mapped: [] },
+            { key: "f", mapped: [] },
+            { key: "g", mapped: [] },
+        ];
+        let pendingReads = input.length;
+
+        const start = performance.now();
+        fromArray(input).pipe(_demux);
+    },
+);
+
+test("demux() should be blocked by slowest pipeline", t => {
+    t.plan(1);
+    const slowProcessorSpeed = 100;
+    interface Chunk {
+        key: string;
+        mapped: number[];
+    }
+    return new Promise(async (resolve, reject) => {
+        const construct = (destKey: string) => {
+            const first = map(
+                async (chunk: Chunk) => {
+                    await sleep(slowProcessorSpeed);
+                    chunk.mapped.push(1);
+                    return chunk;
+                },
+                { highWaterMark: 1 },
+            );
+
+            return first;
+        };
+
+        const _demux = demux(construct, "key", {
+            highWaterMark: 1,
+        });
+
+        _demux.on("error", err => {
+            reject(err);
+        });
+
+        _demux.on("data", async chunk => {
+            pendingReads--;
+            if (chunk.key === "b") {
+                expect(performance.now() - start).to.be.greaterThan(
+                    slowProcessorSpeed * totalItems,
+                );
+                t.pass();
+                expect(pendingReads).to.equal(0);
+                resolve();
+            }
+        });
+
+        const input = [
+            { key: "a", mapped: [] },
+            { key: "a", mapped: [] },
+            { key: "c", mapped: [] },
+            { key: "c", mapped: [] },
+            { key: "b", mapped: [] },
+        ];
+
+        let pendingReads = input.length;
+        const totalItems = input.length;
+        const start = performance.now();
+        for (const item of input) {
+            if (!_demux.write(item)) {
+                await new Promise(_resolve => {
+                    _demux.once("drain", () => {
+                        _resolve();
+                    });
+                });
+            }
+        }
+    });
+});
+
+test.cb("Demux should remux to sink", t => {
+    t.plan(6);
+    let i = 0;
+    const input = [
+        { key: "a", visited: [] },
+        { key: "b", visited: [] },
+        { key: "a", visited: [] },
+        { key: "c", visited: [] },
+        { key: "a", visited: [] },
+        { key: "b", visited: [] },
+    ];
+    const result = [
+        { key: "a", visited: ["a"] },
+        { key: "b", visited: ["b"] },
+        { key: "a", visited: ["a"] },
+        { key: "c", visited: ["c"] },
+        { key: "a", visited: ["a"] },
+        { key: "b", visited: ["b"] },
+    ];
+    const construct = (destKey: string) => {
+        const dest = map((chunk: any) => {
+            chunk.visited.push(destKey);
+            return chunk;
+        });
+
+        return dest;
+    };
+
+    const sink = map(d => {
+        t.deepEqual(d, result[i]);
+        i++;
+        if (i === input.length) {
+            t.end();
+        }
+    });
+
+    const demuxed = demux(construct, "key", {});
+
+    fromArray(input)
+        .pipe(demuxed)
+        .pipe(sink);
+});
+
+test.cb("Demux should send data events", t => {
+    t.plan(6);
+    let i = 0;
+    const input = [
+        { key: "a", visited: [] },
+        { key: "b", visited: [] },
+        { key: "a", visited: [] },
+        { key: "c", visited: [] },
+        { key: "a", visited: [] },
+        { key: "b", visited: [] },
+    ];
+    const result = [
+        { key: "a", visited: ["a"] },
+        { key: "b", visited: ["b"] },
+        { key: "a", visited: ["a"] },
+        { key: "c", visited: ["c"] },
+        { key: "a", visited: ["a"] },
+        { key: "b", visited: ["b"] },
+    ];
+    const construct = (destKey: string) => {
+        const dest = map((chunk: any) => {
+            chunk.visited.push(destKey);
+            return chunk;
+        });
+
+        return dest;
+    };
+
+    const demuxed = demux(construct, "key", {});
+
+    fromArray(input).pipe(demuxed);
+
+    demuxed.on("data", d => {
+        t.deepEqual(d, result[i]);
+        i++;
+        if (i === input.length) {
+            t.end();
+        }
+    });
+});
+
+test.cb("demux() `finish` and `end` propagates", t => {
+    interface Chunk {
+        key: string;
+        mapped: number[];
+    }
+
+    t.plan(9);
+
+    const construct = (destKey: string) => {
+        const dest = map((chunk: any) => {
+            chunk.mapped.push(destKey);
+            return chunk;
+        });
+        return dest;
+    };
+
+    const _demux = demux(construct, "key", {
+        highWaterMark: 3,
+    });
+
+    const fakeSource = new Readable({
+        objectMode: true,
+        read() {
+            return;
+        },
+    });
+
+    const sink = map((d: any) => {
+        const curr = input.shift();
+        t.is(curr.key, d.key);
+        t.deepEqual(d.mapped, [d.key]);
+    });
+
+    fakeSource.pipe(_demux).pipe(sink);
+
+    fakeSource.on("end", () => {
+        t.pass();
+    });
+    _demux.on("finish", () => {
+        t.pass();
+    });
+    _demux.on("unpipe", () => {
+        t.pass();
+    });
+    _demux.on("end", () => {
+        t.pass();
+        t.end();
+    });
+    sink.on("finish", () => {
+        t.pass();
+    });
+
+    const input = [
+        { key: "a", mapped: [] },
+        { key: "b", mapped: [] },
+        { key: "a", mapped: [] },
+        { key: "a", mapped: [] },
+        { key: "b", mapped: [] },
+    ];
+    fakeSource.push(input[0]);
+    fakeSource.push(input[1]);
+    fakeSource.push(null);
+});
+
+test.cb("demux() `unpipe` propagates", t => {
+    interface Chunk {
+        key: string;
+        mapped: number[];
+    }
+
+    t.plan(7);
+
+    const construct = (destKey: string) => {
+        const dest = map((chunk: any) => {
+            chunk.mapped.push(destKey);
+            return chunk;
+        });
+        return dest;
+    };
+
+    const _demux = demux(construct, "key", {
+        highWaterMark: 3,
+    });
+
+    const fakeSource = new Readable({
+        objectMode: true,
+        read() {
+            return;
+        },
+    });
+
+    const sink = map((d: any) => {
+        const curr = input.shift();
+        t.is(curr.key, d.key);
+        t.deepEqual(d.mapped, [d.key]);
+    });
+
+    fakeSource.pipe(_demux).pipe(sink);
+
+    _demux.on("unpipe", () => {
+        t.pass();
+    });
+
+    sink.on("unpipe", () => {
+        t.pass();
+    });
+
+    sink.on("finish", () => {
+        t.pass();
+        t.end();
+    });
+
+    const input = [
+        { key: "a", mapped: [] },
+        { key: "b", mapped: [] },
+        { key: "a", mapped: [] },
+        { key: "a", mapped: [] },
+        { key: "b", mapped: [] },
+    ];
+    fakeSource.push(input[0]);
+    fakeSource.push(input[1]);
+    fakeSource.push(null);
+});
+
+test.cb("demux() should be 'destroyable'", t => {
+    t.plan(2);
+    const _sleep = 100;
+    interface Chunk {
+        key: string;
+        mapped: string[];
+    }
+
+    const construct = (destKey: string) => {
+        const first = map(async (chunk: Chunk) => {
+            await sleep(_sleep);
+            chunk.mapped.push(destKey);
+            return chunk;
+        });
+        return first;
+    };
+
+    const _demux = demux(construct, "key");
+
+    const fakeSource = new Readable({
+        objectMode: true,
+        read() {
+            return;
+        },
+    });
+
+    const fakeSink = new Writable({
+        objectMode: true,
+        write(data, enc, cb) {
+            const cur = input.shift();
+            t.is(cur.key, data.key);
+            t.deepEqual(cur.mapped, ["a"]);
+            if (cur.key === "a") {
+                _demux.destroy();
+            }
+            cb();
+        },
+    });
+
+    _demux.on("close", t.end);
+    fakeSource.pipe(_demux).pipe(fakeSink);
+
+    const input = [
+        { key: "a", mapped: [] },
+        { key: "b", mapped: [] },
+        { key: "c", mapped: [] },
+        { key: "d", mapped: [] },
+        { key: "e", mapped: [] },
+    ];
+    fakeSource.push(input[0]);
+    fakeSource.push(input[1]);
+    fakeSource.push(input[2]);
+    fakeSource.push(input[3]);
+    fakeSource.push(input[4]);
+});

tests/duplex.spec.ts

@@ -0,0 +1,29 @@
+import * as cp from "child_process";
+import { Readable } from "stream";
+import test from "ava";
+import { expect } from "chai";
+import mhysa from "../src";
+const { duplex } = mhysa();
+
+test.cb(
+    "duplex() combines a writable and readable stream into a ReadWrite stream",
+    t => {
+        t.plan(1);
+        const source = new Readable();
+        const catProcess = cp.exec("cat");
+        let out = "";
+        source
+            .pipe(duplex(catProcess.stdin!, catProcess.stdout!))
+            .on("data", chunk => (out += chunk))
+            .on("error", t.end)
+            .on("end", () => {
+                expect(out).to.equal("abcdef");
+                t.pass();
+                t.end();
+            });
+        source.push("ab");
+        source.push("cd");
+        source.push("ef");
+        source.push(null);
+    },
+);

tests/filter.spec.ts

@@ -0,0 +1,117 @@
+import test from "ava";
+import { expect } from "chai";
+import { Readable } from "stream";
+import mhysa from "../src";
+const { filter } = mhysa();
+
+test.cb("filter() filters elements synchronously", t => {
+    t.plan(2);
+    const source = new Readable({ objectMode: true });
+    const expectedElements = ["a", "c"];
+    let i = 0;
+    source
+        .pipe(
+            filter((element: string) => element !== "b", {
+                readableObjectMode: true,
+                writableObjectMode: true,
+            }),
+        )
+        .on("data", (element: string) => {
+            expect(element).to.equal(expectedElements[i]);
+            t.pass();
+            i++;
+        })
+        .on("error", t.end)
+        .on("end", t.end);
+
+    source.push("a");
+    source.push("b");
+    source.push("c");
+    source.push(null);
+});
+
+test.cb("filter() filters elements asynchronously", t => {
+    t.plan(2);
+    const source = new Readable({ objectMode: true });
+    const expectedElements = ["a", "c"];
+    let i = 0;
+    source
+        .pipe(
+            filter(
+                async (element: string) => {
+                    await Promise.resolve();
+                    return element !== "b";
+                },
+                { readableObjectMode: true, writableObjectMode: true },
+            ),
+        )
+        .on("data", (element: string) => {
+            expect(element).to.equal(expectedElements[i]);
+            t.pass();
+            i++;
+        })
+        .on("error", t.end)
+        .on("end", t.end);
+
+    source.push("a");
+    source.push("b");
+    source.push("c");
+    source.push(null);
+});
+
+test.cb.skip("filter() emits errors during synchronous filtering", t => {
+    t.plan(2);
+    const source = new Readable({ objectMode: true });
+    source
+        .pipe(
+            filter(
+                (element: string) => {
+                    if (element !== "a") {
+                        throw new Error("Failed filtering");
+                    }
+                    return true;
+                },
+                { readableObjectMode: true, writableObjectMode: true },
+            ),
+        )
+        .resume()
+        .on("error", err => {
+            expect(err.message).to.equal("Failed filtering");
+            t.pass();
+        })
+        .on("end", t.end);
+
+    source.push("a");
+    source.push("b");
+    source.push("c");
+    source.push(null);
+});
+
+test.cb.skip("filter() emits errors during asynchronous filtering", t => {
+    t.plan(2);
+    const source = new Readable({ objectMode: true });
+    source
+        .pipe(
+            filter(
+                async (element: string) => {
+                    await Promise.resolve();
+                    if (element !== "a") {
+                        throw new Error("Failed filtering");
+                    }
+                    return true;
+                },
+                { readableObjectMode: true, writableObjectMode: true },
+            ),
+        )
+        .resume()
+        .on("error", err => {
+            expect(err.message).to.equal("Failed filtering");
+            t.pass();
+        })
+        .on("end", t.end);
+
+    source.push("a");
+    source.push("b");
+    source.push("c");
+    source.push(null);
+});

tests/flatMap.spec.ts

@@ -0,0 +1,101 @@
+import { Readable } from "stream";
+import test from "ava";
+import { expect } from "chai";
+import mhysa from "../src";
+const { flatMap } = mhysa({ objectMode: true });
+
+test.cb("flatMap() maps elements synchronously", t => {
+    t.plan(6);
+    const source = new Readable({ objectMode: true });
+    const expectedElements = ["a", "A", "b", "B", "c", "C"];
+    let i = 0;
+    source
+        .pipe(flatMap((element: string) => [element, element.toUpperCase()]))
+        .on("data", (element: string) => {
+            expect(element).to.equal(expectedElements[i]);
+            t.pass();
+            i++;
+        })
+        .on("end", t.end);
+
+    source.push("a");
+    source.push("b");
+    source.push("c");
+    source.push(null);
+});
+
+test.cb("flatMap() maps elements asynchronously", t => {
+    t.plan(6);
+    const source = new Readable({ objectMode: true });
+    const expectedElements = ["a", "A", "b", "B", "c", "C"];
+    let i = 0;
+    source
+        .pipe(
+            flatMap(async (element: string) => {
+                await Promise.resolve();
+                return [element, element.toUpperCase()];
+            }),
+        )
+        .on("data", (element: string) => {
+            expect(element).to.equal(expectedElements[i]);
+            t.pass();
+            i++;
+        })
+        .on("end", t.end);
+
+    source.push("a");
+    source.push("b");
+    source.push("c");
+    source.push(null);
+});
+
+test.cb.skip("flatMap() emits errors during synchronous mapping", t => {
+    t.plan(2);
+    const source = new Readable({ objectMode: true });
+    source
+        .pipe(
+            flatMap((element: string) => {
+                if (element !== "a") {
+                    throw new Error("Failed mapping");
+                }
+                return [element, element.toUpperCase()];
+            }),
+        )
+        .resume()
+        .on("error", err => {
+            expect(err.message).to.equal("Failed mapping");
+            t.pass();
+        })
+        .on("end", t.end);
+
+    source.push("a");
+    source.push("b");
+    source.push("c");
+    source.push(null);
+});
+
+test.cb.skip("flatMap() emits errors during asynchronous mapping", t => {
+    t.plan(2);
+    const source = new Readable({ objectMode: true });
+    source
+        .pipe(
+            flatMap(async (element: string) => {
+                await Promise.resolve();
+                if (element !== "a") {
+                    throw new Error("Failed mapping");
+                }
+                return [element, element.toUpperCase()];
+            }),
+        )
+        .resume()
+        .on("error", err => {
+            expect(err.message).to.equal("Failed mapping");
+            t.pass();
+        })
+        .on("end", t.end);
+
+    source.push("a");
+    source.push("b");
+    source.push("c");
+    source.push(null);
+});

tests/fromArray.spec.ts

@@ -0,0 +1,46 @@
+import test from "ava";
+import { expect } from "chai";
+import mhysa from "../src";
+const { fromArray } = mhysa();
+
+test.cb("fromArray() streams array elements in flowing mode", t => {
+    t.plan(3);
+    const elements = ["a", "b", "c"];
+    const stream = fromArray(elements);
+    let i = 0;
+    stream
+        .on("data", (element: string) => {
+            expect(element).to.equal(elements[i]);
+            t.pass();
+            i++;
+        })
+        .on("error", t.end)
+        .on("end", t.end);
+});
+
+test.cb("fromArray() ends immediately if there are no array elements", t => {
+    t.plan(0);
+    fromArray([])
+        .on("data", () => t.fail())
+        .on("error", t.end)
+        .on("end", t.end);
+});
+
+test.cb("fromArray() streams array elements in paused mode", t => {
+    t.plan(3);
+    const elements = ["a", "b", "c"];
+    const stream = fromArray(elements);
+    let i = 0;
+    stream
+        .on("readable", () => {
+            let element = stream.read();
+            while (element !== null) {
+                expect(element).to.equal(elements[i]);
+                t.pass();
+                i++;
+                element = stream.read();
+            }
+        })
+        .on("error", t.end)
+        .on("end", t.end);
+});

tests/join.spec.ts

@@ -0,0 +1,57 @@
+import { Readable } from "stream";
+import test from "ava";
+import { expect } from "chai";
+import mhysa from "../src";
+const { join } = mhysa();
+
+test.cb("join() joins chunks using the specified separator", t => {
+    t.plan(9);
+    const source = new Readable({ objectMode: true });
+    const expectedParts = ["ab|", "|", "c|d", "|", "|", "|", "e", "|", "|f|"];
+    let i = 0;
+    source
+        .pipe(join("|"))
+        .on("data", part => {
+            expect(part).to.equal(expectedParts[i]);
+            t.pass();
+            i++;
+        })
+        .on("error", t.end)
+        .on("end", t.end);
+
+    source.push("ab|");
+    source.push("c|d");
+    source.push("|");
+    source.push("e");
+    source.push("|f|");
+    source.push(null);
+});
+
+test.cb(
+    "join() joins chunks using the specified separator without breaking up multi-byte characters " +
+        "spanning multiple chunks",
+    t => {
+        t.plan(5);
+        const source = new Readable({ objectMode: true });
+        const expectedParts = ["ø", "|", "ö", "|", "一"];
+        let i = 0;
+        source
+            .pipe(join("|"))
+            .on("data", part => {
+                expect(part).to.equal(expectedParts[i]);
+                t.pass();
+                i++;
+            })
+            .on("error", t.end)
+            .on("end", t.end);
+
+        source.push(Buffer.from("ø").slice(0, 1)); // 2-byte character spanning two chunks
+        source.push(Buffer.from("ø").slice(1, 2));
+        source.push(Buffer.from("ö").slice(0, 1)); // 2-byte character spanning two chunks
+        source.push(Buffer.from("ö").slice(1, 2));
+        source.push(Buffer.from("一").slice(0, 1)); // 3-byte character spanning three chunks
+        source.push(Buffer.from("一").slice(1, 2));
+        source.push(Buffer.from("一").slice(2, 3));
+        source.push(null);
+    },
+);

tests/last.spec.ts

@@ -0,0 +1,16 @@
+import { Readable } from "stream";
+import test from "ava";
+import { expect } from "chai";
+import mhysa from "../src";
+const { last } = mhysa();
+
+test("last() resolves to the last chunk streamed by the given readable stream", async t => {
+    const source = new Readable({ objectMode: true });
+    const lastPromise = last(source);
+    source.push("ab");
+    source.push("cd");
+    source.push("ef");
+    source.push(null);
+    const lastChunk = await lastPromise;
+    expect(lastChunk).to.equal("ef");
+});

tests/map.spec.ts

@@ -0,0 +1,57 @@
+import { Readable } from "stream";
+import test from "ava";
+import { expect } from "chai";
+import mhysa from "../src";
+const { map } = mhysa();
+
+test.cb("map() maps elements synchronously", t => {
+    t.plan(3);
+    const source = new Readable({ objectMode: true });
+    const mapStream = map((element: string) => element.toUpperCase(), {
+        objectMode: true,
+    });
+    const expectedElements = ["A", "B", "C"];
+    let i = 0;
+    source
+        .pipe(mapStream)
+        .on("data", (element: string) => {
+            expect(element).to.equal(expectedElements[i]);
+            t.pass();
+            i++;
+        })
+        .on("error", t.end)
+        .on("end", t.end);
+
+    source.push("a");
+    source.push("b");
+    source.push("c");
+    source.push(null);
+});
+
+test.cb("map() maps elements asynchronously", t => {
+    t.plan(3);
+    const source = new Readable({ objectMode: true });
+    const mapStream = map(
+        async (element: string) => {
+            await Promise.resolve();
+            return element.toUpperCase();
+        },
+        { objectMode: true },
+    );
+    const expectedElements = ["A", "B", "C"];
+    let i = 0;
+    source
+        .pipe(mapStream)
+        .on("data", (element: string) => {
+            expect(element).to.equal(expectedElements[i]);
+            t.pass();
+            i++;
+        })
+        .on("error", t.end)
+        .on("end", t.end);
+
+    source.push("a");
+    source.push("b");
+    source.push("c");
+    source.push(null);
+});

tests/merge.spec.ts

@@ -0,0 +1,61 @@
+import { Readable } from "stream";
+import test from "ava";
+import { expect } from "chai";
+import mhysa from "../src";
+const { merge } = mhysa();
+
+test.cb(
+    "merge() merges multiple readable streams in chunk arrival order",
+    t => {
+        t.plan(6);
+        const source1 = new Readable({ objectMode: true, read: () => ({}) });
+        const source2 = new Readable({ objectMode: true, read: () => ({}) });
+        const expectedElements = ["a", "d", "b", "e", "c", "f"];
+        let i = 0;
+        merge(source1, source2)
+            .on("data", (element: string) => {
+                expect(element).to.equal(expectedElements[i]);
+                t.pass();
+                i++;
+            })
+            .on("error", t.end)
+            .on("end", t.end);
+
+        source1.push("a");
+        setTimeout(() => source2.push("d"), 10);
+        setTimeout(() => source1.push("b"), 20);
+        setTimeout(() => source2.push("e"), 30);
+        setTimeout(() => source1.push("c"), 40);
+        setTimeout(() => source2.push("f"), 50);
+        setTimeout(() => source2.push(null), 60);
+        setTimeout(() => source1.push(null), 70);
+    },
+);
+
+test.cb("merge() merges a readable stream", t => {
+    t.plan(3);
+    const source = new Readable({ objectMode: true, read: () => ({}) });
+    const expectedElements = ["a", "b", "c"];
+    let i = 0;
+    merge(source)
+        .on("data", (element: string) => {
+            expect(element).to.equal(expectedElements[i]);
+            t.pass();
+            i++;
+        })
+        .on("error", t.end)
+        .on("end", t.end);
+
+    source.push("a");
+    source.push("b");
+    source.push("c");
+    source.push(null);
+});
+
+test.cb("merge() merges an empty list of readable streams", t => {
+    t.plan(0);
+    merge()
+        .on("data", () => t.pass())
+        .on("error", t.end)
+        .on("end", t.end);
+});

tests/parallelMap.spec.ts

@@ -0,0 +1,78 @@
+import { Readable } from "stream";
+import { performance } from "perf_hooks";
+import test from "ava";
+import { expect } from "chai";
+import mhysa from "../src";
+import { sleep } from "../src/helpers";
+const { parallelMap } = mhysa({ objectMode: true });
+
+test.cb("parallelMap() parallel mapping", t => {
+    t.plan(6);
+    const offset = 50;
+    const source = new Readable({ objectMode: true });
+    const expectedElements = [
+        "a_processed",
+        "b_processed",
+        "c_processed",
+        "d_processed",
+        "e_processed",
+        "f_processed",
+    ];
+    interface IPerfData {
+        start: number;
+        output?: string;
+        finish?: number;
+    }
+    const orderedResults: IPerfData[] = [];
+    source
+        .pipe(
+            parallelMap(async (data: any) => {
+                const perfData: IPerfData = { start: performance.now() };
+                const c = data + "_processed";
+                perfData.output = c;
+                await sleep(offset);
+                perfData.finish = performance.now();
+                orderedResults.push(perfData);
+                return c;
+            }, 2),
+        )
+        .on("data", (element: string) => {
+            t.true(expectedElements.includes(element));
+        })
+        .on("error", t.end)
+        .on("end", async () => {
+            expect(orderedResults[0].finish).to.be.lessThan(
+                orderedResults[2].start,
+            );
+            expect(orderedResults[1].finish).to.be.lessThan(
+                orderedResults[3].start,
+            );
+            expect(orderedResults[2].finish).to.be.lessThan(
+                orderedResults[4].start,
+            );
+            expect(orderedResults[3].finish).to.be.lessThan(
+                orderedResults[5].start,
+            );
+            expect(orderedResults[0].start).to.be.lessThan(
+                orderedResults[2].start + offset,
+            );
+            expect(orderedResults[1].start).to.be.lessThan(
+                orderedResults[3].start + offset,
+            );
+            expect(orderedResults[2].start).to.be.lessThan(
+                orderedResults[4].start + offset,
+            );
+            expect(orderedResults[3].start).to.be.lessThan(
+                orderedResults[5].start + offset,
+            );
+            t.end();
+        });
+
+    source.push("a");
+    source.push("b");
+    source.push("c");
+    source.push("d");
+    source.push("e");
+    source.push("f");
+    source.push(null);
+});

tests/parse.spec.ts

@@ -0,0 +1,45 @@
+import { Readable, finished } from "stream";
+import test from "ava";
+import { expect } from "chai";
+import mhysa from "../src";
+const { parse } = mhysa();
+
+test.cb("parse() parses the streamed elements as JSON", t => {
+    t.plan(3);
+    const source = new Readable({ objectMode: true });
+    const expectedElements = ["abc", {}, []];
+    let i = 0;
+    source
+        .pipe(parse())
+        .on("data", part => {
+            expect(part).to.deep.equal(expectedElements[i]);
+            t.pass();
+            i++;
+        })
+        .on("error", t.end)
+        .on("end", t.end);
+
+    source.push('"abc"');
+    source.push("{}");
+    source.push("[]");
+    source.push(null);
+});
+
+test.cb("parse() emits errors on invalid JSON", t => {
+    t.plan(1);
+    const source = new Readable({ objectMode: true });
+
+    source
+        .pipe(parse())
+        .resume()
+        .on("error", (d: any) => {
+            t.pass();
+            t.end();
+        })
+        .on("end", t.fail);
+
+    source.push("{}");
+    source.push({});
+    source.push([]);
+    source.push(null);
+});

tests/rate.spec.ts

@@ -0,0 +1,90 @@
+import { Readable } from "stream";
+import { performance } from "perf_hooks";
+import test from "ava";
+import { expect } from "chai";
+import mhysa from "../src";
+const { rate } = mhysa({ objectMode: true });
+
+test.cb("rate() sends data at a rate of 150", t => {
+    t.plan(5);
+    const targetRate = 150;
+    const source = new Readable({ objectMode: true });
+    const expectedElements = ["a", "b", "c", "d", "e"];
+    const start = performance.now();
+    let i = 0;
+
+    source
+        .pipe(rate(targetRate))
+        .on("data", (element: string[]) => {
+            const currentRate = (i / (performance.now() - start)) * 1000;
+            expect(element).to.deep.equal(expectedElements[i]);
+            expect(currentRate).lessThan(targetRate);
+            t.pass();
+            i++;
+        })
+        .on("error", t.end)
+        .on("end", t.end);
+
+    source.push("a");
+    source.push("b");
+    source.push("c");
+    source.push("d");
+    source.push("e");
+    source.push(null);
+});
+
+test.cb("rate() sends data at a rate of 50", t => {
+    t.plan(5);
+    const targetRate = 50;
+    const source = new Readable({ objectMode: true });
+    const expectedElements = ["a", "b", "c", "d", "e"];
+    const start = performance.now();
+    let i = 0;
+
+    source
+        .pipe(rate(targetRate))
+        .on("data", (element: string[]) => {
+            const currentRate = (i / (performance.now() - start)) * 1000;
+            expect(element).to.deep.equal(expectedElements[i]);
+            expect(currentRate).lessThan(targetRate);
+            t.pass();
+            i++;
+        })
+        .on("error", t.end)
+        .on("end", t.end);
+
+    source.push("a");
+    source.push("b");
+    source.push("c");
+    source.push("d");
+    source.push("e");
+    source.push(null);
+});
+
+test.cb("rate() sends data at a rate of 1", t => {
+    t.plan(5);
+    const targetRate = 1;
+    const source = new Readable({ objectMode: true });
+    const expectedElements = ["a", "b", "c", "d", "e"];
+    const start = performance.now();
+    let i = 0;
+
+    source
+        .pipe(rate(targetRate))
+        .on("data", (element: string[]) => {
+            const currentRate = (i / (performance.now() - start)) * 1000;
+            expect(element).to.deep.equal(expectedElements[i]);
+            expect(currentRate).lessThan(targetRate);
+            t.pass();
+            i++;
+        })
+        .on("error", t.end)
+        .on("end", t.end);
+
+    source.push("a");
+    source.push("b");
+    source.push("c");
+    source.push("d");
+    source.push("e");
+    source.push(null);
+});

tests/reduce.spec.ts

@@ -0,0 +1,99 @@
+import { Readable } from "stream";
+import test from "ava";
+import { expect } from "chai";
+import mhysa from "../src";
+const { reduce } = mhysa({ objectMode: true });
+
+test.cb("reduce() reduces elements synchronously", t => {
+    t.plan(1);
+    const source = new Readable({ objectMode: true });
+    const expectedValue = 6;
+    source
+        .pipe(reduce((acc: number, element: string) => acc + element.length, 0))
+        .on("data", (element: string) => {
+            expect(element).to.equal(expectedValue);
+            t.pass();
+        })
+        .on("error", t.end)
+        .on("end", t.end);
+
+    source.push("ab");
+    source.push("cd");
+    source.push("ef");
+    source.push(null);
+});
+
+test.cb("reduce() reduces elements asynchronously", t => {
+    t.plan(1);
+    const source = new Readable({ objectMode: true });
+    const expectedValue = 6;
+    source
+        .pipe(
+            reduce(async (acc: number, element: string) => {
+                await Promise.resolve();
+                return acc + element.length;
+            }, 0),
+        )
+        .on("data", (element: string) => {
+            expect(element).to.equal(expectedValue);
+            t.pass();
+        })
+        .on("error", t.end)
+        .on("end", t.end);
+
+    source.push("ab");
+    source.push("cd");
+    source.push("ef");
+    source.push(null);
+});
+
+test.cb.skip("reduce() emits errors during synchronous reduce", t => {
+    t.plan(2);
+    const source = new Readable({ objectMode: true });
+    source
+        .pipe(
+            reduce((acc: number, element: string) => {
+                if (element !== "ab") {
+                    throw new Error("Failed reduce");
+                }
+                return acc + element.length;
+            }, 0),
+        )
+        .resume()
+        .on("error", err => {
+            expect(err.message).to.equal("Failed reduce");
+            t.pass();
+        })
+        .on("end", t.end);
+
+    source.push("ab");
+    source.push("cd");
+    source.push("ef");
+    source.push(null);
+});
+
+test.cb.skip("reduce() emits errors during asynchronous reduce", t => {
+    t.plan(2);
+    const source = new Readable({ objectMode: true });
+    source
+        .pipe(
+            reduce(async (acc: number, element: string) => {
+                await Promise.resolve();
+                if (element !== "ab") {
+                    throw new Error("Failed mapping");
+                }
+                return acc + element.length;
+            }, 0),
+        )
+        .resume()
+        .on("error", err => {
+            expect(err.message).to.equal("Failed mapping");
+            t.pass();
+        })
+        .on("end", t.end);
+
+    source.push("ab");
+    source.push("cd");
+    source.push("ef");
+    source.push(null);
+});

tests/replace.spec.ts

@@ -0,0 +1,81 @@
+import { Readable } from "stream";
+import test from "ava";
+import { expect } from "chai";
+import mhysa from "../src";
+const { replace } = mhysa();
+
+test.cb(
+    "replace() replaces occurrences of the given string in the streamed elements with the specified " +
+        "replacement string",
+    t => {
+        t.plan(3);
+        const source = new Readable({ objectMode: true });
+        const expectedElements = ["abc", "xyf", "ghi"];
+        let i = 0;
+        source
+            .pipe(replace("de", "xy"))
+            .on("data", part => {
+                expect(part).to.equal(expectedElements[i]);
+                t.pass();
+                i++;
+            })
+            .on("error", t.end)
+            .on("end", t.end);
+
+        source.push("abc");
+        source.push("def");
+        source.push("ghi");
+        source.push(null);
+    },
+);
+
+test.cb(
+    "replace() replaces occurrences of the given regular expression in the streamed elements with " +
+        "the specified replacement string",
+    t => {
+        t.plan(3);
+        const source = new Readable({ objectMode: true });
+        const expectedElements = ["abc", "xyz", "ghi"];
+        let i = 0;
+        source
+            .pipe(replace(/^def$/, "xyz"))
+            .on("data", part => {
+                expect(part).to.equal(expectedElements[i]);
+                t.pass();
+                i++;
+            })
+            .on("error", t.end)
+            .on("end", t.end);
+
+        source.push("abc");
+        source.push("def");
+        source.push("ghi");
+        source.push(null);
+    },
+);
+
+test.cb(
+    "replace() replaces occurrences of the given multi-byte character even if it spans multiple chunks",
+    t => {
+        t.plan(3);
+        const source = new Readable({ objectMode: true });
+        const expectedElements = ["ø", "O", "a"];
+        let i = 0;
+        source
+            .pipe(replace("ö", "O"))
+            .on("data", part => {
+                expect(part).to.equal(expectedElements[i]);
+                t.pass();
+                i++;
+            })
+            .on("error", t.end)
+            .on("end", t.end);
+
+        source.push(Buffer.from("ø").slice(0, 1)); // 2-byte character spanning two chunks
+        source.push(Buffer.from("ø").slice(1, 2));
+        source.push(Buffer.from("ö").slice(0, 1)); // 2-byte character spanning two chunks
+        source.push(Buffer.from("ö").slice(1, 2));
+        source.push("a");
+        source.push(null);
+    },
+);

tests/split.spec.ts

@@ -0,0 +1,99 @@
+import { Readable } from "stream";
+import test from "ava";
+import { expect } from "chai";
+import mhysa from "../src";
+const { split } = mhysa();
+
+test.cb("split() splits chunks using the default separator (\\n)", t => {
+    t.plan(5);
+    const source = new Readable({ objectMode: true });
+    const expectedParts = ["ab", "c", "d", "ef", ""];
+    let i = 0;
+    source
+        .pipe(split())
+        .on("data", part => {
+            expect(part).to.equal(expectedParts[i]);
+            t.pass();
+            i++;
+        })
+        .on("error", t.end)
+        .on("end", t.end);
+
+    source.push("ab\n");
+    source.push("c");
+    source.push("\n");
+    source.push("d");
+    source.push("\nef\n");
+    source.push(null);
+});
+
+test.cb("split() splits chunks using the specified separator", t => {
+    t.plan(6);
+    const source = new Readable({ objectMode: true });
+    const expectedParts = ["ab", "c", "d", "e", "f", ""];
+    let i = 0;
+    source
+        .pipe(split("|"))
+        .on("data", (part: string) => {
+            expect(part).to.equal(expectedParts[i]);
+            t.pass();
+            i++;
+        })
+        .on("error", t.end)
+        .on("end", t.end);
+
+    source.push("ab|");
+    source.push("c|d");
+    source.push("|");
+    source.push("e");
+    source.push("|f|");
+    source.push(null);
+});
+
+test.cb(
+    "split() splits utf8 encoded buffers using the specified separator",
+    t => {
+        t.plan(3);
+        const expectedElements = ["a", "b", "c"];
+        let i = 0;
+        const through = split(",");
+        const buf = Buffer.from("a,b,c");
+        through
+            .on("data", element => {
+                expect(element).to.equal(expectedElements[i]);
+                i++;
+                t.pass();
+            })
+            .on("error", t.end)
+            .on("end", t.end);
+
+        for (let j = 0; j < buf.length; ++j) {
+            through.write(buf.slice(j, j + 1));
+        }
+        through.end();
+    },
+);
+
+test.cb(
+    "split() splits utf8 encoded buffers with multi-byte characters using the specified separator",
+    t => {
+        t.plan(3);
+        const expectedElements = ["一", "一", "一"];
+        let i = 0;
+        const through = split(",");
+        const buf = Buffer.from("一,一,一"); // Those spaces are multi-byte utf8 characters (code: 4E00)
+        through
+            .on("data", element => {
+                expect(element).to.equal(expectedElements[i]);
+                i++;
+                t.pass();
+            })
+            .on("error", t.end)
+            .on("end", t.end);
+
+        for (let j = 0; j < buf.length; ++j) {
+            through.write(buf.slice(j, j + 1));
+        }
+        through.end();
+    },
+);

tests/stringify.spec.ts

@@ -0,0 +1,62 @@
+import { Readable } from "stream";
+import test from "ava";
+import { expect } from "chai";
+import mhysa from "../src";
+const { stringify } = mhysa();
+
+test.cb("stringify() stringifies the streamed elements as JSON", t => {
+    t.plan(4);
+    const source = new Readable({ objectMode: true });
+    const expectedElements = [
+        '"abc"',
+        "0",
+        '{"a":"a","b":"b","c":"c"}',
+        '["a","b","c"]',
+    ];
+    let i = 0;
+    source
+        .pipe(stringify())
+        .on("data", part => {
+            expect(part).to.deep.equal(expectedElements[i]);
+            t.pass();
+            i++;
+        })
+        .on("error", t.end)
+        .on("end", t.end);
+
+    source.push("abc");
+    source.push(0);
+    source.push({ a: "a", b: "b", c: "c" });
+    source.push(["a", "b", "c"]);
+    source.push(null);
+});
+
+test.cb(
+    "stringify() stringifies the streamed elements as pretty-printed JSON",
+    t => {
+        t.plan(4);
+        const source = new Readable({ objectMode: true });
+        const expectedElements = [
+            '"abc"',
+            "0",
+            '{\n  "a": "a",\n  "b": "b",\n  "c": "c"\n}',
+            '[\n  "a",\n  "b",\n  "c"\n]',
+        ];
+        let i = 0;
+        source
+            .pipe(stringify({ pretty: true }))
+            .on("data", part => {
+                expect(part).to.deep.equal(expectedElements[i]);
+                t.pass();
+                i++;
+            })
+            .on("error", t.end)
+            .on("end", t.end);
+
+        source.push("abc");
+        source.push(0);
+        source.push({ a: "a", b: "b", c: "c" });
+        source.push(["a", "b", "c"]);
+        source.push(null);
+    },
+);

tests/unbatch.spec.ts

@@ -0,0 +1,27 @@
+import { Readable } from "stream";
+import test from "ava";
+import { expect } from "chai";
+import mhysa from "../src";
+const { unbatch, batch } = mhysa({ objectMode: true });
+
+test.cb("unbatch() unbatches", t => {
+    t.plan(3);
+    const source = new Readable({ objectMode: true });
+    const expectedElements = ["a", "b", "c"];
+    let i = 0;
+    source
+        .pipe(batch(3))
+        .pipe(unbatch())
+        .on("data", (element: string) => {
+            expect(element).to.equal(expectedElements[i]);
+            t.pass();
+            i++;
+        })
+        .on("error", t.end)
+        .on("end", t.end);
+
+    source.push("a");
+    source.push("b");
+    source.push("c");
+    source.push(null);
+});

tests/utils/collected.spec.ts

@@ -0,0 +1,9 @@
+import test from "ava";
+import { collected } from "../../src/utils";
+import mhysa from "../../src";
+const { fromArray, collect } = mhysa({ objectMode: true });
+
+test("collected returns a promise for the first data point", async t => {
+    const data = collected(fromArray([1, 2, 3, 4]).pipe(collect()));
+    t.deepEqual(await data, [1, 2, 3, 4]);
+});

tsconfig.json

@@ -3,16 +3,19 @@
     "noImplicitAny": true,
     "strictNullChecks": true,
     "noImplicitReturns": true,
-        "noUnusedLocals": true,
+    "noUnusedLocals": false,
     "noImplicitThis": true,
     "forceConsistentCasingInFileNames": true,
     "suppressImplicitAnyIndexErrors": true,
     "outDir": "./dist",
-        "module": "commonjs",
-        "target": "es5",
-        "lib": ["es2016"],
-        "sourceMap": true,
-        "declaration": true
+    "module": "commonjs"
   },
-    "include": ["src/**/*.ts"]
+  "target": "es5",
+  "lib": [
+    "es2016"
+  ],
+  "sourceMap": true,
+  "declaration": true,
+  "include": ["src/**/*"],
+  "exclude": ["tests", "node_modules"]
 }

tslint.json

@@ -9,6 +9,7 @@
         "no-implicit-dependencies": [true, "dev"],
         "prettier": [true, ".prettierrc"],
         "ordered-imports": false,
-        "interface-name": false
+        "interface-name": false,
+	"object-literal-sort-keys": false
     }
 }