js-priority-queue
Priority queue data structures
npm install js-priority-queue
Want to see pretty graphs? Log in now!
2 | downloads in the last week |
3 | downloads in the last month |
Last Published By | |
---|---|
Version | 0.0.4 last updated 3 months ago |
License | Public Domain |
Keywords | priority, queue, bheap, b-heap, binary, heap, dequeue, comparator |
Repository | https://github.com/adamhooper/js-priority-queue (git) |
Homepage | https://github.com/adamhooper/js-priority-queue |
Bugs | https://github.com/adamhooper/js-priority-queue/issues |
Dependencies | None |
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 |
You cannot access the data in any other way: you must dequeue or peek.
Why use this library? Two reasons:
- It's easier to use than an Array, and it's clearer.
- It can make your code execute more quickly.
Installing
Download priority-queue.js
. Alternatively, install through Bower:
bower install js-priority-queue
Include it through RequireJS.
Then write code like this:
require([ 'vendor/priority-queue' ], function(PriorityQueue) {
var queue = new PriorityQueue({ comparator: function(a, b) { return b - a; });
queue.queue(5);
queue.queue(3);
queue.queue(2);
var lowest = queue.pop(); // returns 2
});
If you don't like RequireJS, you can download the standalone version,
priority-queue.no-require.js
, and write:
var queue = new PriorityQueue({ comparator: function(a, b) { return b - a; });
queue.queue(5);
queue.queue(3);
queue.queue(2);
var lowest = queue.pop(); // returns 2
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
- Fork this repository
- Run
npm install
- Write the behavior you expect in
spec-coffee/
- Edit files in
coffee/
untilgrunt test
says you're done - Run
grunt
to updatepriority-queue.js
andpriority-queue.min.js
- 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.