diff --git a/src/functions/definitions.ts b/src/functions/definitions.ts
index 2729b65..6cc97a8 100644
--- a/src/functions/definitions.ts
+++ b/src/functions/definitions.ts
@@ -26,3 +26,5 @@ export enum FlushStrategy {
     rolling = "rolling",
     sliding = "sliding",
 }
+
+export type AccumulatorByIteratee<T> = (event: T, bufferChunk: T) => boolean;
diff --git a/src/functions/functions.spec.ts b/src/functions/functions.spec.ts
index 6d1b31e..f174928 100644
--- a/src/functions/functions.spec.ts
+++ b/src/functions/functions.spec.ts
@@ -25,7 +25,9 @@ import {
     rate,
     parallelMap,
     accumulator,
+    accumulatorBy,
 } from ".";
+import { FlushStrategy } from "./definitions";
 import { sleep } from "../helpers";
 
 test.cb("fromArray() streams array elements in flowing mode", t => {
@@ -1417,7 +1419,7 @@ test.cb("accumulator() rolling", t => {
     const flushes = [firstFlush, secondFlush, thirdFlush];
 
     source
-        .pipe(accumulator(2, undefined, "rolling"))
+        .pipe(accumulator(2, undefined, FlushStrategy.rolling))
         .on("data", (flush: TestObject[]) => {
             t.deepEqual(flush, flushes[chunkIndex]);
             chunkIndex++;
@@ -1452,7 +1454,50 @@ test.cb("accumulator() rolling with key", t => {
     const flushes = [firstFlush, secondFlush];
 
     source
-        .pipe(accumulator(3, undefined, "rolling", "ts"))
+        .pipe(accumulator(3, undefined, FlushStrategy.rolling, "ts"))
+        .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("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(
+                undefined,
+                FlushStrategy.rolling,
+                (event: TestObject, bufferChunk: TestObject) => {
+                    return bufferChunk.ts + 3 <= event.ts;
+                },
+            ),
+        )
         .on("data", (flush: TestObject[]) => {
             t.deepEqual(flush, flushes[chunkIndex]);
             chunkIndex++;
@@ -1498,7 +1543,7 @@ test.cb("accumulator() sliding", t => {
 
     const flushes = [firstFlush, secondFlush, thirdFlush, fourthFlush];
     source
-        .pipe(accumulator(3, undefined, "sliding"))
+        .pipe(accumulator(3, undefined, FlushStrategy.sliding))
         .on("data", (flush: TestObject[]) => {
             t.deepEqual(flush, flushes[chunkIndex]);
             chunkIndex++;
@@ -1555,7 +1600,72 @@ test.cb("accumulator() sliding with key", t => {
         sixthFlush,
     ];
     source
-        .pipe(accumulator(3, undefined, "sliding", "ts"))
+        .pipe(accumulator(3, undefined, FlushStrategy.sliding, "ts"))
+        .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("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(
+                undefined,
+                FlushStrategy.sliding,
+                (event: TestObject, bufferChunk: TestObject) => {
+                    return bufferChunk.ts + 3 <= event.ts ? true : false;
+                },
+            ),
+        )
         .on("data", (flush: TestObject[]) => {
             t.deepEqual(flush, flushes[chunkIndex]);
             chunkIndex++;
diff --git a/src/functions/functions.ts b/src/functions/functions.ts
index 2df000e..49eea1f 100644
--- a/src/functions/functions.ts
+++ b/src/functions/functions.ts
@@ -10,6 +10,7 @@ import {
     JsonValue,
     JsonParseOptions,
     FlushStrategy,
+    AccumulatorByIteratee,
 } from "./definitions";
 import { sleep } from "../helpers";
 
@@ -622,7 +623,7 @@ function _accumulator<T>(
     });
 }
 
-function _slidingBy<T>(
+function _sliding<T>(
     windowLength: number,
     rate: number | undefined,
     key?: string,
@@ -631,7 +632,7 @@ function _slidingBy<T>(
         if (key) {
             let index = 0;
             while (
-                buffer.length > 0 &&
+                index < buffer.length &&
                 buffer[index][key] + windowLength <= event[key]
             ) {
                 index++;
@@ -645,7 +646,37 @@ function _slidingBy<T>(
     };
 }
 
-function _rollingBy<T>(
+function _slidingByFunction<T>(
+    rate: number | undefined,
+    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>(
+    rate: number | undefined,
+    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,
     rate: number | undefined,
     key?: string,
@@ -691,13 +722,31 @@ export function accumulator(
     }
 }
 
+export function accumulatorBy<T, S extends FlushStrategy>(
+    batchRate: number | undefined,
+    flushStrategy: S,
+    iteratee: AccumulatorByIteratee<T>,
+): Transform {
+    if (flushStrategy === FlushStrategy.sliding) {
+        return slidingBy(batchRate, iteratee);
+    } else {
+        return rollingBy(batchRate, iteratee);
+    }
+}
+
 export function sliding(
     windowLength: number,
     rate: number | undefined,
     key?: string,
 ): Transform {
-    const slidingByFn = _slidingBy(windowLength, rate, key);
-    return _accumulator(slidingByFn, false);
+    return _accumulator(_sliding(windowLength, rate, key), false);
+}
+
+export function slidingBy<T>(
+    rate: number | undefined,
+    iteratee: AccumulatorByIteratee<T>,
+): Transform {
+    return _accumulator(_slidingByFunction(rate, iteratee), false);
 }
 
 export function rolling(
@@ -705,6 +754,12 @@ export function rolling(
     rate: number | undefined,
     key?: string,
 ): Transform {
-    const rollingByFn = _rollingBy(windowLength, rate, key);
-    return _accumulator(rollingByFn);
+    return _accumulator(_rolling(windowLength, rate, key));
+}
+
+export function rollingBy<T>(
+    rate: number | undefined,
+    iteratee: AccumulatorByIteratee<T>,
+): Transform {
+    return _accumulator(_rollingByFunction(rate, iteratee));
 }
diff --git a/src/functions/index.ts b/src/functions/index.ts
index 6265a92..2c511c0 100644
--- a/src/functions/index.ts
+++ b/src/functions/index.ts
@@ -8,6 +8,7 @@ import {
     WithEncoding,
     JsonParseOptions,
     FlushStrategy,
+    AccumulatorByIteratee,
 } from "./definitions";
 
 /**
@@ -260,3 +261,11 @@ export function accumulator(
         keyBy,
     );
 }
+
+export function accumulatorBy<T, S extends FlushStrategy>(
+    batchRate: number | undefined,
+    flushStrategy: S,
+    iteratee: AccumulatorByIteratee<T>,
+) {
+    return baseFunctions.accumulatorBy(batchRate, flushStrategy, iteratee);
+}