Add demux

src/functions/baseFunctions.ts

@@ -20,3 +20,4 @@ export { split } from "./split";
 export { stringify } from "./stringify";
 export { unbatch } from "./unbatch";
 export { compose } from "./compose";
+export { demux } from "./demux";

src/functions/demux.ts

@@ -0,0 +1,49 @@
+import { WritableOptions, Writable } from "stream";
+
+/**
+ * Return a Duplex stream that is pushed data from multiple sources
+ * @param streams Source streams to multiplex
+ * @param options Duplex stream options
+ */
+export function demux(
+    construct: () => NodeJS.WritableStream | NodeJS.ReadWriteStream,
+    demuxBy: { key?: string; keyBy?: (chunk: any) => string },
+    options?: WritableOptions,
+): Writable {
+    return new Demux(construct, demuxBy, options);
+}
+
+class Demux extends Writable {
+    private keyMap: object;
+    private demuxer: (chunk: any) => string;
+    private construct: (
+        destKey?: string,
+    ) => NodeJS.WritableStream | NodeJS.ReadWriteStream;
+    constructor(
+        construct: (
+            destKey?: string,
+        ) => NodeJS.WritableStream | NodeJS.ReadWriteStream,
+        demuxBy: { key?: string; keyBy?: (chunk: any) => string },
+        options?: WritableOptions,
+    ) {
+        super(options);
+        if (demuxBy.keyBy === undefined && demuxBy.key === undefined) {
+            throw new Error("Need one");
+        }
+        this.demuxer = demuxBy.keyBy || ((chunk: any) => chunk[demuxBy.key!]);
+        this.construct = construct;
+        this.keyMap = {};
+    }
+
+    public write(chunk: any, encoding?: any, cb?: any): boolean {
+        const destKey = this.demuxer(chunk);
+        if (this.keyMap[destKey] === undefined) {
+            this.keyMap[destKey] = this.construct(destKey);
+        }
+        const writeRes = this.keyMap[destKey].write(chunk);
+        if (cb !== undefined) {
+            cb();
+        }
+        return writeRes;
+    }
+}

src/functions/index.ts

@@ -297,3 +297,13 @@ export function compose(
         options,
     );
 }
+
+export function demux(
+    construct: (
+        destKey?: string,
+    ) => NodeJS.WritableStream | NodeJS.ReadWriteStream,
+    demuxer: { key?: string; keyBy?: (chunk: any) => string },
+    options?: DuplexOptions,
+) {
+    return baseFunctions.demux(construct, demuxer, options);
+}

src/index.ts

@@ -22,4 +22,5 @@ export {
     accumulator,
     accumulatorBy,
     compose,
+    demux,
 } from "./functions";