js-priority-queue
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    0.1.5 • Public • Published

    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.

    Install

    npm i js-priority-queue

    DownloadsWeekly Downloads

    9,488

    Version

    0.1.5

    License

    Public Domain

    Last publish

    Collaborators

    • adamhooper