js-priority-queue

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:

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.