feat(skipLast): add skipLast operator (#2316)

* feat(skipLast): adds skipLast operator

Adds skipLast operator from RxJS 4. Its internals and tests are based on takeLast for better

performance.

Closes #1404

* docs(skipLast): updated decision tree, MIGRATION.md and operators.md

* docs(skipLast): fix tree.yml as suggested by @staltz

* refactor(skipLast): refactored skipLast operator

Author: martinsik

MIGRATION.md

@@ -110,7 +110,6 @@ enabling "composite" subscription behavior.
 |`shareValue`|No longer implemented|
 |`singleInstance`|`share`|
 |`skipLastWithTime`|No longer implemented|
-|`skipLast`|No longer implemented|
 |`skipUntilWithTime`|No longer implemented|
 |`slice(start, end)`|`skip(start).take(end - start)`|
 |`some`|`first(fn, () => true, false)`|

doc/decision-tree-widget/tree.yml

@@ -59,6 +59,9 @@ children:
         - label: based on custom logic
           children:
           - label: skipWhile
+      - label: from the end of the Observable
+        children:
+        - label: skipLast
       - label: until another Observable emits a value
         children:
         - label: skipUntil

doc/operators.md

@@ -178,6 +178,7 @@ There are operators for different purposes, and they may be categorized as: crea
 - [`sampleTime`](../class/es6/Observable.js~Observable.html#instance-method-sampleTime)
 - [`single`](../class/es6/Observable.js~Observable.html#instance-method-single)
 - [`skip`](../class/es6/Observable.js~Observable.html#instance-method-skip)
+- [`skipLast`](../class/es6/Observable.js~Observable.html#instance-method-skipLast)
 - [`skipUntil`](../class/es6/Observable.js~Observable.html#instance-method-skipUntil)
 - [`skipWhile`](../class/es6/Observable.js~Observable.html#instance-method-skipWhile)
 - [`take`](../class/es6/Observable.js~Observable.html#instance-method-take)

perf/micro/current-thread-scheduler/operators/skiplast.js

@@ -0,0 +1,18 @@
+var RxOld = require('rx');
+var RxNew = require('../../../../index');
+
+module.exports = function (suite) {
+  var oldSkipLastWithImmediateScheduler = RxOld.Observable.range(0, 500, RxOld.Scheduler.currentThread).skipLast(50);
+  var newSkipLastWithImmediateScheduler = RxNew.Observable.range(0, 500, RxNew.Scheduler.queue).skipLast(50);
+
+  function _next(x) { }
+  function _error(e) { }
+  function _complete() { }
+  return suite
+    .add('old skipLast with current thread scheduler', function () {
+      oldSkipLastWithImmediateScheduler.subscribe(_next, _error, _complete);
+    })
+    .add('new skipLast with current thread scheduler', function () {
+      newSkipLastWithImmediateScheduler.subscribe(_next, _error, _complete);
+    });
+};

perf/micro/immediate-scheduler/operators/skiplast.js

@@ -0,0 +1,18 @@
+var RxOld = require('rx');
+var RxNew = require('../../../../index');
+
+module.exports = function (suite) {
+  var oldSkipLastWithImmediateScheduler = RxOld.Observable.range(0, 500, RxOld.Scheduler.immediate).skipLast(50);
+  var newSkipLastWithImmediateScheduler = RxNew.Observable.range(0, 500).skipLast(50);
+
+  function _next(x) { }
+  function _error(e) { }
+  function _complete() { }
+  return suite
+      .add('old skipLast with immediate scheduler', function () {
+        oldSkipLastWithImmediateScheduler.subscribe(_next, _error, _complete);
+      })
+      .add('new skipLast with immediate scheduler', function () {
+        newSkipLastWithImmediateScheduler.subscribe(_next, _error, _complete);
+      });
+};

spec/operators/skipLast-spec.ts

@@ -0,0 +1,155 @@
+import {expect} from 'chai';
+import * as Rx from '../../dist/cjs/Rx';
+declare const {hot, cold, asDiagram, expectObservable, expectSubscriptions};
+
+const Observable = Rx.Observable;
+
+/** @test {takeLast} */
+describe('Observable.prototype.skipLast', () => {
+  asDiagram('skipLast(2)')('should skip two values of an observable with many values', () => {
+    const e1 =  cold('--a-----b----c---d--|');
+    const e1subs =   '^                   !';
+    const expected = '-------------a---b--|';
+
+    expectObservable(e1.skipLast(2)).toBe(expected);
+    expectSubscriptions(e1.subscriptions).toBe(e1subs);
+  });
+
+  it('should skip last three values', () => {
+    const e1 =  cold('--a-----b----c---d--|');
+    const e1subs =   '^                   !';
+    const expected = '-----------------a--|';
+
+    expectObservable(e1.skipLast(3)).toBe(expected);
+    expectSubscriptions(e1.subscriptions).toBe(e1subs);
+  });
+
+  it('should skip all values when trying to take larger then source', () => {
+    const e1 =  cold('--a-----b----c---d--|');
+    const e1subs =   '^                   !';
+    const expected = '--------------------|';
+
+    expectObservable(e1.skipLast(5)).toBe(expected);
+    expectSubscriptions(e1.subscriptions).toBe(e1subs);
+  });
+
+  it('should skip all element when try to take exact', () => {
+    const e1 =  cold('--a-----b----c---d--|');
+    const e1subs =   '^                   !';
+    const expected = '--------------------|';
+
+    expectObservable(e1.skipLast(4)).toBe(expected);
+    expectSubscriptions(e1.subscriptions).toBe(e1subs);
+  });
+
+  it('should not skip any values', () => {
+    const e1 =  cold('--a-----b----c---d--|');
+    const e1subs =   '^                   !';
+    const expected = '--a-----b----c---d--|';
+
+    expectObservable(e1.skipLast(0)).toBe(expected);
+    expectSubscriptions(e1.subscriptions).toBe(e1subs);
+  });
+
+  it('should work with empty', () => {
+    const e1 =  cold('|');
+    const e1subs =   '(^!)';
+    const expected = '|';
+
+    expectObservable(e1.skipLast(42)).toBe(expected);
+    expectSubscriptions(e1.subscriptions).toBe(e1subs);
+  });
+
+  it('should go on forever on never', () => {
+    const e1 =  cold('-');
+    const e1subs =   '^';
+    const expected = '-';
+
+    expectObservable(e1.skipLast(42)).toBe(expected);
+    expectSubscriptions(e1.subscriptions).toBe(e1subs);
+  });
+
+  it('should skip one value from an observable with one value', () => {
+    const e1 =   hot('---(a|)');
+    const e1subs =   '^  !   ';
+    const expected = '---|   ';
+
+    expectObservable(e1.skipLast(1)).toBe(expected);
+    expectSubscriptions(e1.subscriptions).toBe(e1subs);
+  });
+
+  it('should skip one value from an observable with many values', () => {
+    const e1 = hot('--a--^--b----c---d--|');
+    const e1subs =      '^              !';
+    const expected =    '--------b---c--|';
+
+    expectObservable(e1.skipLast(1)).toBe(expected);
+    expectSubscriptions(e1.subscriptions).toBe(e1subs);
+  });
+
+  it('should work with empty and early emission', () => {
+    const e1 = hot('--a--^----|');
+    const e1subs =      '^    !';
+    const expected =    '-----|';
+
+    expectObservable(e1.skipLast(42)).toBe(expected);
+    expectSubscriptions(e1.subscriptions).toBe(e1subs);
+  });
+
+  it('should propagate error from the source observable', () => {
+    const e1 = hot('---^---#', null, 'too bad');
+    const e1subs =    '^   !';
+    const expected =  '----#';
+
+    expectObservable(e1.skipLast(42)).toBe(expected, null, 'too bad');
+    expectSubscriptions(e1.subscriptions).toBe(e1subs);
+  });
+
+  it('should propagate error from an observable with values', () => {
+    const e1 = hot('---^--a--b--#');
+    const e1subs =    '^        !';
+    const expected =  '---------#';
+
+    expectObservable(e1.skipLast(42)).toBe(expected);
+    expectSubscriptions(e1.subscriptions).toBe(e1subs);
+  });
+
+  it('should allow unsubscribing explicitly and early', () => {
+    const e1 = hot('---^--a--b-----c--d--e--|');
+    const unsub =     '         !            ';
+    const e1subs =    '^        !            ';
+    const expected =  '----------            ';
+
+    expectObservable(e1.skipLast(42), unsub).toBe(expected);
+    expectSubscriptions(e1.subscriptions).toBe(e1subs);
+  });
+
+  it('should work with throw', () => {
+    const e1 =  cold('#');
+    const e1subs =   '(^!)';
+    const expected = '#';
+
+    expectObservable(e1.skipLast(42)).toBe(expected);
+    expectSubscriptions(e1.subscriptions).toBe(e1subs);
+  });
+
+  it('should throw if total is less than zero', () => {
+    expect(() => { Observable.range(0, 10).skipLast(-1); })
+      .to.throw(Rx.ArgumentOutOfRangeError);
+  });
+
+  it('should not break unsubscription chain when unsubscribed explicitly', () => {
+    const e1 = hot('---^--a--b-----c--d--e--|');
+    const unsub =     '         !            ';
+    const e1subs =    '^        !            ';
+    const expected =  '----------            ';
+
+    const result = e1
+      .mergeMap((x: string) => Observable.of(x))
+      .skipLast(42)
+      .mergeMap((x: string) => Observable.of(x));
+
+    expectObservable(result, unsub).toBe(expected);
+    expectSubscriptions(e1.subscriptions).toBe(e1subs);
+  });
+});

src/Rx.ts

@@ -112,6 +112,7 @@ import './add/operator/sequenceEqual';
 import './add/operator/share';
 import './add/operator/single';
 import './add/operator/skip';
+import './add/operator/skipLast';
 import './add/operator/skipUntil';
 import './add/operator/skipWhile';
 import './add/operator/startWith';

src/add/operator/skipLast.ts

@@ -0,0 +1,10 @@
+import { Observable } from '../../Observable';
+import { skipLast } from '../../operator/skipLast';
+
+Observable.prototype.skipLast = skipLast;
+
+declare module '../../Observable' {
+  interface Observable<T> {
+    skipLast: typeof skipLast;
+  }
+}

src/operator/skipLast.ts

@@ -0,0 +1,90 @@
+import { Operator } from '../Operator';
+import { Subscriber } from '../Subscriber';
+import { ArgumentOutOfRangeError } from '../util/ArgumentOutOfRangeError';
+import { Observable } from '../Observable';
+import { TeardownLogic } from '../Subscription';
+
+/**
+ * Skip the last `count` values emitted by the source Observable.
+ *
+ * <img src="./img/skipLast.png" width="100%">
+ *
+ * `skipLast` returns an Observable that accumulates a queue with a length
+ * enough to store the first `count` values. As more values are received,
+ * values are taken from the front of the queue and produced on the result
+ * sequence. This causes values to be delayed.
+ *
+ * @example <caption>Skip the last 2 values of an Observable with many values</caption>
+ * var many = Rx.Observable.range(1, 5);
+ * var skipLastTwo = many.skipLast(2);
+ * skipLastTwo.subscribe(x => console.log(x));
+ *
+ * // Results in:
+ * // 1 2 3
+ *
+ * @see {@link skip}
+ * @see {@link skipUntil}
+ * @see {@link skipWhile}
+ * @see {@link take}
+ *
+ * @throws {ArgumentOutOfRangeError} When using `skipLast(i)`, it throws
+ * ArgumentOutOrRangeError if `i < 0`.
+ *
+ * @param {number} count Number of elements to skip from the end of the source Observable.
+ * @returns {Observable<T>} An Observable that skips the last count values
+ * emitted by the source Observable.
+ * @method skipLast
+ * @owner Observable
+ */
+export function skipLast<T>(this: Observable<T>, count: number): Observable<T> {
+  return this.lift(new SkipLastOperator(count));
+}
+
+class SkipLastOperator<T> implements Operator<T, T> {
+  constructor(private _skipCount: number) {
+    if (this._skipCount < 0) {
+      throw new ArgumentOutOfRangeError;
+    }
+  }
+
+  call(subscriber: Subscriber<T>, source: any): TeardownLogic {
+    if (this._skipCount === 0) {
+      // If we don't want to skip any values then just subscribe
+      // to Subscriber without any further logic.
+      return source.subscribe(new Subscriber(subscriber));
+    } else {
+      return source.subscribe(new SkipLastSubscriber(subscriber, this._skipCount));
+    }
+  }
+}
+
+/**
+ * We need this JSDoc comment for affecting ESDoc.
+ * @ignore
+ * @extends {Ignored}
+ */
+class SkipLastSubscriber<T> extends Subscriber<T> {
+  private _ring: T[];
+  private _count: number = 0;
+
+  constructor(destination: Subscriber<T>, private _skipCount: number) {
+    super(destination);
+    this._ring = new Array<T>(_skipCount);
+  }
+
+  protected _next(value: T): void {
+    const skipCount = this._skipCount;
+    const count = this._count++;
+
+    if (count < skipCount) {
+      this._ring[count] = value;
+    } else {
+      const currentIndex = count % skipCount;
+      const ring = this._ring;
+      const oldValue = ring[currentIndex];
+
+      ring[currentIndex] = value;
+      this.destination.next(oldValue);
+    }
+  }
+}