Use SimpleQueue in WritableStream implementation

third_party/WebKit/Source/core/streams/SimpleQueue.js

+// Copyright 2017 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+// Queue implementation used by ReadableStream and WritableStream.
+
+(function(global, binding, v8) {
+  'use strict';
+
+  // Simple queue structure. Avoids scalability issues with using
+  // InternalPackedArray directly by using multiple arrays in a linked list and
+  // keeping the array size bounded.
+  const QUEUE_MAX_ARRAY_SIZE = 16384;
+  class SimpleQueue {
+    constructor() {
+      this.front = {
+        elements: new v8.InternalPackedArray(),
+        next: undefined,
+      };
+      this.back = this.front;
+      // The cursor is used to avoid calling InternalPackedArray.shift().
+      this.cursor = 0;
+      this.size = 0;
+    }
+
+    get length() {
+      return this.size;
+    }
+
+    push(element) {
+      ++this.size;
+      if (this.back.elements.length === QUEUE_MAX_ARRAY_SIZE) {
+        const oldBack = this.back;
+        this.back = {
+          elements: new v8.InternalPackedArray(),
+          next: undefined,
+        };
+        oldBack.next = this.back;
+      }
+      this.back.elements.push(element);
+    }
+
+    shift() {
+      // assert(this.size > 0);
+      --this.size;
+      if (this.front.elements.length === this.cursor) {
+        // assert(this.cursor === QUEUE_MAX_ARRAY_SIZE);
+        // assert(this.front.next !== undefined);
+        this.front = this.front.next;
+        this.cursor = 0;
+      }
+      const element = this.front.elements[this.cursor];
+      // Permit shifted element to be garbage collected.
+      this.front.elements[this.cursor] = undefined;
+      ++this.cursor;
+
+      return element;
+    }
+
+    forEach(callback) {
+      let i = this.cursor;
+      let node = this.front;
+      let elements = node.elements;
+      while (i !== elements.length || node.next !== undefined) {
+        if (i === elements.length) {
+          // assert(node.next !== undefined);
+          // assert(i === QUEUE_MAX_ARRAY_SIZE);
+          node = node.next;
+          elements = node.elements;
+          i = 0;
+        }
+        callback(elements[i]);
+        ++i;
+      }
+    }
+
+    // Return the element that would be returned if shift() was called now,
+    // without modifying the queue.
+    peek() {
+      // assert(this.size > 0);
+      if (this.front.elements.length === this.cursor) {
+        // assert(this.cursor === QUEUE_MAX_ARRAY_SIZE)
+        // assert(this.front.next !== undefined);
+        return this.front.next.elements[0];
+      }
+      return this.front.elements[this.cursor];
+    }
+  }
+
+  binding.SimpleQueue = SimpleQueue;
+});