Alternative implementation

Author: tshemsedinov

JavaScript/5-spare.js

@@ -0,0 +1,114 @@
+'use strict';
+
+class QueueNode {
+  length = 0;
+
+  constructor({ size }) {
+    this.size = size;
+    this.buffer = new Array(size);
+    this.reset();
+  }
+
+  reset() {
+    this.readIndex = 0;
+    this.writeIndex = 0;
+    this.next = null;
+  }
+
+  enqueue(item) {
+    if (this.writeIndex >= this.size) return false;
+    this.buffer[this.writeIndex++] = item;
+    this.length++;
+    return true;
+  }
+
+  dequeue() {
+    if (this.length === 0) return null;
+    const index = this.readIndex++;
+    const item = this.buffer[index];
+    this.buffer[index] = null;
+    this.length--;
+    return item;
+  }
+}
+
+class UnrolledQueue {
+  #length = 0;
+  #nodeSize = 2048;
+  #head = null;
+  #tail = null;
+  #spare = null;
+
+  constructor(options = {}) {
+    const { nodeSize } = options;
+    if (nodeSize) this.#nodeSize = nodeSize;
+    const node = this.#createNode();
+    this.#head = node;
+    this.#tail = node;
+  }
+
+  #createNode() {
+    return new QueueNode({ size: this.#nodeSize });
+  }
+
+  get length() {
+    return this.#length;
+  }
+
+  enqueue(item) {
+    if (!this.#head.enqueue(item)) {
+      let node = this.#spare;
+      if (node) this.#spare = null;
+      else node = this.#createNode();
+      this.#head.next = node;
+      this.#head = node;
+      this.#head.enqueue(item);
+    }
+    this.#length++;
+  }
+
+  dequeue() {
+    if (this.#length === 0) return null;
+    const tail = this.#tail;
+    const item = tail.dequeue();
+    this.#length--;
+    if (tail.length > 0) return item;
+    const next = this.#tail.next;
+    if (next) {
+      this.#tail = next;
+      if (!this.#spare) {
+        tail.reset();
+        this.#spare = tail;
+      }
+    } else {
+      tail.reset();
+    }
+    return item;
+  }
+}
+
+// Usage
+
+const mq = new UnrolledQueue({ nodeSize: 8 });
+
+for (let id = 0; id < 20; id++) {
+  mq.enqueue({ id });
+}
+
+while (mq.length) {
+  const task = mq.dequeue();
+  console.log(`Processing ${task.id}`);
+}
+
+for (let id = 100; id < 125; id++) {
+  mq.enqueue({ id });
+}
+
+while (mq.length) {
+  const task = mq.dequeue();
+  console.log(`Processing ${task.id}`);
+}
+
+for (let id = 100; id < 105; id++) {
+  mq.enqueue({ id });
+}