fix(bufferCount): will behave as expected when startBufferEvery is less than bufferSize (#2076)

- fixed issue where internal `buffers` store was keeping an additional buffer for no good reason
- improved logic and performance around updating internal `buffers` list
- adds a test to ensure proper behavior

fixes #2062

Author: benlesh

spec/operators/bufferCount-spec.ts

@@ -1,4 +1,5 @@
 import * as Rx from '../../dist/cjs/Rx';
+import { expect } from 'chai';
 declare const {hot, asDiagram, expectObservable, expectSubscriptions};
 
 const Observable = Rx.Observable;
@@ -31,6 +32,26 @@ describe('Observable.prototype.bufferCount', () => {
     expectObservable(e1.bufferCount(2)).toBe(expected, values);
   });
 
+  it('should buffer properly (issue #2062)', () => {
+    const item$ = new Rx.Subject();
+    const results = [];
+    item$
+      .bufferCount(3, 1)
+      .subscribe(value => {
+        results.push(value);
+
+        if (value.join() === '1,2,3') {
+          item$.next(4);
+        }
+      });
+
+    item$.next(1);
+    item$.next(2);
+    item$.next(3);
+
+    expect(results).to.deep.equal([[1, 2, 3], [2, 3, 4]]);
+  });
+
   it('should emit partial buffers if source completes before reaching specified buffer count', () => {
     const e1 =   hot('--a--b--c--d--|');
     const expected = '--------------(x|)';

src/operator/bufferCount.ts

@@ -62,38 +62,30 @@ class BufferCountOperator<T> implements Operator<T, T[]> {
  * @extends {Ignored}
  */
 class BufferCountSubscriber<T> extends Subscriber<T> {
-  private buffers: Array<T[]> = [[]];
+  private buffers: Array<T[]> = [];
   private count: number = 0;
 
   constructor(destination: Subscriber<T[]>, private bufferSize: number, private startBufferEvery: number) {
     super(destination);
   }
 
   protected _next(value: T) {
-    const count = (this.count += 1);
-    const destination = this.destination;
-    const bufferSize = this.bufferSize;
-    const startBufferEvery = (this.startBufferEvery == null) ? bufferSize : this.startBufferEvery;
-    const buffers = this.buffers;
-    const len = buffers.length;
-    let remove = -1;
+    const count = this.count++;
+    const { destination, bufferSize, startBufferEvery, buffers } = this;
+    const startOn = (startBufferEvery == null) ? bufferSize : startBufferEvery;
 
-    if (count % startBufferEvery === 0) {
+    if (count % startOn === 0) {
       buffers.push([]);
     }
 
-    for (let i = 0; i < len; i++) {
+    for (let i = buffers.length; i--; ) {
       const buffer = buffers[i];
       buffer.push(value);
       if (buffer.length === bufferSize) {
-        remove = i;
+        buffers.splice(i, 1);
         destination.next(buffer);
       }
     }
-
-    if (remove !== -1) {
-      buffers.splice(remove, 1);
-    }
   }
 
   protected _complete() {