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(). Each queue() is a splice(), which rewrites the entire array. This is fast for tiny queues.

An alternative is a Binary Heap: it modifies just a few array elements when queueing (though each modification has a cost).

Finally, we can use a B-Heap. It's like a binary heap, except its modifications often occur close together in memory. Unfortunately, calculating where in memory the modifications should occur is slower. (It costs a function call instead of a bit-shift.) So while B-heap is fast in theory, it's slow 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. Use ArrayStrategy in edge cases, after performance-testing your specific data. Don't use BHeapStrategy: it's a lesson that a miracle in C can flop in JavaScript.

The default strategy is BinaryHeapStrategy.

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.