All files / lib/internal fixed_queue.js

94.02% Statements 110/117
87.5% Branches 14/16
100% Functions 9/9
94.02% Lines 110/117

Press n or j to go to the next uncovered block, b, p or k for the previous block.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 11895x 95x 95x 95x 95x 95x 95x 95x 95x 95x 95x 95x 95x 95x 95x 95x 95x 95x 95x 95x 95x 95x 95x 95x 95x 95x 95x 95x 95x 95x 95x 95x 95x 95x 95x 95x 95x 95x 95x 95x 95x 95x 95x 95x 95x 95x 95x 95x 95x 95x 95x 95x 95x 95x 95x 95x 95x 95x 95x 95x 95x 95x 95x 95x 95x 95x 95x 13492x 13492x 95x 95x 4561x 4561x 95x 95x 4561x 4561x 4561x 95x 95x 6746x 6746x 6746x 4561x 4561x 4561x 6746x 95x 95x 95x 95x 95x 95x 95x 95x 6746x 6746x 95x 95x 4561x         4561x 4561x 95x 95x 6746x 6746x 6746x       6746x 6746x 95x  
'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.
 
const FixedCircularBuffer = 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() {
    const nextItem = this.list[this.bottom];
    if (nextItem === undefined)
      return null;
    this.list[this.bottom] = undefined;
    this.bottom = (this.bottom + 1) & kMask;
    return nextItem;
  }
};
 
module.exports = class FixedQueue {
  constructor() {
    this.head = this.tail = new FixedCircularBuffer();
  }
 
  isEmpty() {
    return this.head.isEmpty();
  }
 
  push(data) {
    if (this.head.isFull()) {
      // Head is full: Creates a new queue, sets the old queue's `.next` to it,
      // and sets it as the new main queue.
      this.head = this.head.next = new FixedCircularBuffer();
    }
    this.head.push(data);
  }
 
  shift() {
    const tail = this.tail;
    const next = tail.shift();
    if (tail.isEmpty() && tail.next !== null) {
      // If there is another queue, it forms the new tail.
      this.tail = tail.next;
    }
    return next;
  }
};