regglee / js-priority-queue

Priority queues in JavaScript

Priority Queue

A priority queue is a data structure with these operations:

Operation	Syntax (js-priority-queue)	Description
Create	var queue = new PriorityQueue();	Creates a priority queue
Queue	queue.queue(value);	Inserts a new value in the queue
Length	var length = queue.length;	Returns the number of elements in the queue
Peek	var firstItem = queue.peek();	Returns the smallest item in the queue and leaves the queue unchanged
Dequeue	var firstItem = queue.dequeue();	Returns the smallest item in the queue and removes it from the queue
Clear	queue.clear();	Removes all values from the queue

You cannot access the data in any other way: you must dequeue or peek.

Why use this library? Two reasons:

1. It's easier to use than an Array, and it's clearer.
2. It can make your code execute more quickly.

Installing

You can npm install js-priority-queue or bower install js-priority-queue. Alternatively, just download priority-queue.js from this directory.

Include it through RequireJS or Browserify. Or, to pollute your global scope, insert this in your HTML:

<script src="priority-queue.js"></script>

Then write code like this:

var queue = new PriorityQueue({ comparator: function(a, b) { return b - a; }});
queue.queue(5);
queue.queue(3);
queue.queue(2);
var lowest = queue.dequeue(); // returns 5

Options

How exactly will these elements be ordered? Let's use the comparator option. This is the argument we would pass to Array.prototype.sort:

var compareNumbers = function(a, b) { return a - b; };
var queue = new PriorityQueue({ comparator: compareNumbers });

You can also pass initial values, in any order. With lots of values, it's faster to load them all at once than one at a time.

var queue = new PriorityQueue({ initialValues: [ 1, 2, 3 ] })

Strategies

We can implement this with a regular Array. We'll keep it sorted inversely, so queue.dequeue() maps to array.pop().

But with an Array, we'll need to splice(), which can affect every single element in the array. An alternative is to create a Binary Heap, which writes far fewer array elements when queueing (though each element is written more slowly).

Finally, we can use a B-Heap. It's like a binary heap, except it orders elements such that during a single operation, writes occur closer to each other in memory. Unfortunately, it's slower to calculate where in memory each write should occur (it costs a function call instead of a bit-shift). So while it's fast in theory, it's slower in practice.

Create the queues like this:

var queue = new PriorityQueue({ strategy: PriorityQueue.ArrayStrategy }); // Array
var queue = new PriorityQueue({ strategy: PriorityQueue.BinaryHeapStrategy }); // Default
var queue = new PriorityQueue({ strategy: PriorityQueue.BHeapStrategy }); // Slower

You'll see running times like this:

Operation	Array	Binary heap	B-Heap
Create	O(n lg n)	O(n)	O(n)
Queue	O(n) (often slow)	O(lg n) (fast)	O(lg n)
Peek	O(1)	O(1)	O(1)
Dequeue	O(1) (fast)	O(lg n)	O(lg n)

According to JsPerf, the fastest strategy for most cases is BinaryHeapStrategy. Only use ArrayStrategy only if you're queuing items in a very particular order. Don't use BHeapStrategy, except as a lesson in how sometimes miracles in one programming language aren't great in other languages.

Contributing

1. Fork this repository
2. Run npm install
3. Write the behavior you expect in spec-coffee/
4. Edit files in coffee/ until gulp test says you're done
5. Run gulp to update priority-queue.js and priority-queue.min.js
6. Submit a pull request

License

I, Adam Hooper, the sole author of this project, waive all my rights to it and release it under the Public Domain. Do with it what you will.