162 lines
4.7 KiB
JavaScript
162 lines
4.7 KiB
JavaScript
'use strict';
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// Extracted from node/lib/internal/fixed_queue.js
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// Currently optimal queue size, tested on V8 6.0 - 6.6. Must be power of two.
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const kSize = 2048;
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const kMask = kSize - 1;
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// The FixedQueue is implemented as a singly-linked list of fixed-size
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// circular buffers. It looks something like this:
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//
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// head tail
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// | |
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// v v
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// +-----------+ <-----\ +-----------+ <------\ +-----------+
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// | [null] | \----- | next | \------- | next |
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// +-----------+ +-----------+ +-----------+
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// | item | <-- bottom | item | <-- bottom | undefined |
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// | item | | item | | undefined |
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// | item | | item | | undefined |
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// | item | | item | | undefined |
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// | item | | item | bottom --> | item |
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// | item | | item | | item |
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// | ... | | ... | | ... |
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// | item | | item | | item |
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// | item | | item | | item |
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// | undefined | <-- top | item | | item |
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// | undefined | | item | | item |
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// | undefined | | undefined | <-- top top --> | undefined |
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// +-----------+ +-----------+ +-----------+
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//
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// Or, if there is only one circular buffer, it looks something
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// like either of these:
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//
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// head tail head tail
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// | | | |
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// v v v v
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// +-----------+ +-----------+
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// | [null] | | [null] |
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// +-----------+ +-----------+
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// | undefined | | item |
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// | undefined | | item |
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// | item | <-- bottom top --> | undefined |
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// | item | | undefined |
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// | undefined | <-- top bottom --> | item |
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// | undefined | | item |
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// +-----------+ +-----------+
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//
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// Adding a value means moving `top` forward by one, removing means
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// moving `bottom` forward by one. After reaching the end, the queue
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// wraps around.
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//
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// When `top === bottom` the current queue is empty and when
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// `top + 1 === bottom` it's full. This wastes a single space of storage
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// but allows much quicker checks.
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/**
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* @type {FixedCircularBuffer}
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* @template T
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*/
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class FixedCircularBuffer {
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constructor() {
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/**
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* @type {number}
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*/
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this.bottom = 0;
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/**
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* @type {number}
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*/
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this.top = 0;
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/**
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* @type {Array<T|undefined>}
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*/
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this.list = new Array(kSize).fill(undefined);
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/**
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* @type {T|null}
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*/
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this.next = null;
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}
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/**
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* @returns {boolean}
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*/
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isEmpty() {
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return this.top === this.bottom;
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}
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/**
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* @returns {boolean}
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*/
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isFull() {
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return ((this.top + 1) & kMask) === this.bottom;
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}
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/**
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* @param {T} data
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* @returns {void}
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*/
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push(data) {
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this.list[this.top] = data;
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this.top = (this.top + 1) & kMask;
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}
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/**
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* @returns {T|null}
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*/
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shift() {
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const nextItem = this.list[this.bottom];
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if (nextItem === undefined) {
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return null;
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}
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this.list[this.bottom] = undefined;
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this.bottom = (this.bottom + 1) & kMask;
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return nextItem;
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}
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}
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/**
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* @template T
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*/
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module.exports = class FixedQueue {
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constructor() {
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/**
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* @type {FixedCircularBuffer<T>}
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*/
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this.head = this.tail = new FixedCircularBuffer();
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}
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/**
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* @returns {boolean}
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*/
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isEmpty() {
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return this.head.isEmpty();
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}
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/**
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* @param {T} data
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*/
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push(data) {
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if (this.head.isFull()) {
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// Head is full: Creates a new queue, sets the old queue's `.next` to it,
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// and sets it as the new main queue.
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this.head = this.head.next = new FixedCircularBuffer();
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}
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this.head.push(data);
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}
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/**
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* @returns {T|null}
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*/
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shift() {
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const tail = this.tail;
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const next = tail.shift();
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if (tail.isEmpty() && tail.next !== null) {
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// If there is another queue, it forms the new tail.
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this.tail = tail.next;
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tail.next = null;
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}
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return next;
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}
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};
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