fixed_queue.js

'use strict';

const {
  Array,
} = primordials;

// Currently optimal queue size, tested on V8 6.0 - 6.6. Must be power of two.
const kSize = 2048;
const kMask = kSize - 1;

// The FixedQueue is implemented as a singly-linked list of fixed-size
// circular buffers. It looks something like this:
//
//  head                                                       tail
//    |                                                          |
//    v                                                          v
// +-----------+ <-----\       +-----------+ <------\         +-----------+
// |  [null]   |        \----- |   next    |         \------- |   next    |
// +-----------+               +-----------+                  +-----------+
// |   item    | <-- bottom    |   item    | <-- bottom       |  [empty]  |
// |   item    |               |   item    |                  |  [empty]  |
// |   item    |               |   item    |                  |  [empty]  |
// |   item    |               |   item    |                  |  [empty]  |
// |   item    |               |   item    |       bottom --> |   item    |
// |   item    |               |   item    |                  |   item    |
// |    ...    |               |    ...    |                  |    ...    |
// |   item    |               |   item    |                  |   item    |
// |   item    |               |   item    |                  |   item    |
// |  [empty]  | <-- top       |   item    |                  |   item    |
// |  [empty]  |               |   item    |                  |   item    |
// |  [empty]  |               |  [empty]  | <-- top  top --> |  [empty]  |
// +-----------+               +-----------+                  +-----------+
//
// Or, if there is only one circular buffer, it looks something
// like either of these:
//
//  head   tail                                 head   tail
//    |     |                                     |     |
//    v     v                                     v     v
// +-----------+                               +-----------+
// |  [null]   |                               |  [null]   |
// +-----------+                               +-----------+
// |  [empty]  |                               |   item    |
// |  [empty]  |                               |   item    |
// |   item    | <-- bottom            top --> |  [empty]  |
// |   item    |                               |  [empty]  |
// |  [empty]  | <-- top            bottom --> |   item    |
// |  [empty]  |                               |   item    |
// +-----------+                               +-----------+
//
// Adding a value means moving `top` forward by one, removing means
// moving `bottom` forward by one. After reaching the end, the queue
// wraps around.
//
// When `top === bottom` the current queue is empty and when
// `top + 1 === bottom` it's full. This wastes a single space of storage
// but allows much quicker checks.

class FixedCircularBuffer {
  constructor() {
    this.bottom = 0;
    this.top = 0;
    this.list = new Array(kSize);
    this.next = null;
  }

  isEmpty() {
    return this.top === this.bottom;
  }

  isFull() {
    return ((this.top + 1) & kMask) === this.bottom;
  }

  push(data) {
    this.list[this.top] = data;
    this.top = (this.top + 1) & kMask;
  }

  shift() {
    if (this.isEmpty())
      return null;
    const data = this.list[this.bottom];
    this.list[this.bottom] = undefined;
    this.bottom = (this.bottom + 1) & kMask;
    return data;
  }
}

module.exports = class FixedQueue {
  constructor() {
    this.head = this.tail = this.buffer = new FixedCircularBuffer();
  }

  isEmpty() {
    return this.head.isEmpty();
  }

  push(data) {
    const head = this.head;
    if (head.isFull()) {
      // Head is full: Reuses the empty tail or creates a new queue,
      // sets the old queue's `.next` to it, and sets it as the new main queue.
      const buffer = this.buffer;
      if (buffer.isEmpty()) {
        this.buffer = buffer.next;
        buffer.next = null;
        this.head = head.next = buffer;
      } else {
        this.head = head.next = new FixedCircularBuffer();
      }
    }
    this.head.push(data);
  }

  shift() {
    const tail = this.tail;
    const data = tail.shift();
    if (tail.isEmpty() && tail.next !== null) {
      // If there is another queue, it forms the new tail.
      this.tail = tail.next;
    }
    return data;
  }
};