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https://github.com/ezimuel/fastpriorityqueue

Efficient implementation of an integer proprity queue in PHP
https://github.com/ezimuel/fastpriorityqueue

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Efficient implementation of an integer proprity queue in PHP

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## Fast Priority Queue implementation



[![Build Status](https://secure.travis-ci.org/ezimuel/FastPriorityQueue.svg?branch=master)](https://secure.travis-ci.org/ezimuel/FastPriorityQueue)



This is an efficient implementation of an **integer priority queue** in PHP.

PHP offers the [SplPriorityQueue](http://php.net/manual/en/class.splpriorityqueue.php)

class that implements a priority queue, but this component has a "strange"

behaviour, see PHP request [#60926](https://bugs.php.net/bug.php?id=60926)

and PHP bug [#53710](https://bugs.php.net/bug.php?id=53710).

Basically, elements with the same priority are extracted in arbitrary order and

this can be a problem in many situations. Even, though, the [definition](https://en.wikipedia.org/wiki/Priority_queue)

of Priority Queue says:



> In a priority queue, an element with high priority is served before an

> element with low priority. If two elements have the same priority, they

> are served according to their order in the queue.



Read the post [Taming SplPriorityQueue](https://mwop.net/blog/253-Taming-SplPriorityQueue.html)

by [Matthew Weier O'Phinney](https://github.com/weierophinney) for more

information about this `SplPriorityQueue` issue.



## Implementation details



I did not use the usual approach to implement the priority queue with a [heap](https://en.wikipedia.org/wiki/Heap_%28data_structure%29).

I've used [ordered arrays](https://github.com/ezimuel/FastPriorityQueue/blob/master/src/PriorityQueue.php#L19)

grouped by [priorities](https://github.com/ezimuel/FastPriorityQueue/blob/master/src/PriorityQueue.php#L26).

For the array iteration I used the [next()](http://php.net/manual/en/function.next.php),

and [current()](http://php.net/manual/en/function.current.php) functions of PHP.



To get the priorities in order, I use the [max()](http://php.net/manual/en/function.max.php)

function of PHP. To retrieve the next priority, I remove the previous one from

the array, using [unset()](http://php.net/manual/en/function.unset.php), and I

re-apply the `max()` function to the remaining values.



This solution is very simple and offers very good performance (see the benchmark

below).



## Benchmark



I provided a simple script to test the performance of the implementation. You

can execute this test running the following command:



```

php benchmark/test.php

```



This script executes 50'000 insert and extract operations using different

priority queue implementations:



- [SplPriorityQueue](http://php.net/manual/en/class.splpriorityqueue.php)

- [Zend\Stdlib\SplPriorityQueue](https://github.com/zendframework/zend-stdlib/blob/master/src/SplPriorityQueue.php)

- [Zend\Stdlib\PriorityQueue](https://github.com/zendframework/zend-stdlib/blob/master/src/PriorityQueue.php)



I have also included the `SplPriorityQueue` of PHP to have a starter point for the

comparison, even though the results of this component are not correct, as

mentioned above.



I executed the benchmark using an Intel Core i5-2500 at 3.30GHz with 8 Gb of RAM

running Ubuntu Linux 14.04 and PHP 5.5.9. Here the results:



```

--- Benchmark 50,000 insert and extract with a fixed priority

SplPriorityQueue                : 0.05173206 (sec)

FastPriorityQueue\PriorityQueue : 0.07072687 (sec)

Zend\Stdlib\SplPriorityQueue    : 0.23528290 (sec)

Zend\Stdlib\PriorityQueue       : 0.39357114 (sec)



--- Benchmark 50,000 insert and extract with random priority (1,100)

SplPriorityQueue                : 0.06713820 (sec)

FastPriorityQueue\PriorityQueue : 0.10090804 (sec)

Zend\Stdlib\SplPriorityQueue    : 0.44940901 (sec)

Zend\Stdlib\PriorityQueue       : 0.65850401 (sec)

```



As you can see the execution time of `FastPriorityQueue` is very good, about **3x

to 6x faster** than the other implementations (except the `SplPriorityQueue` that

is out of the comparison).



## Unit Tests



You can run the unit tests by executing the following commmand after the installation

using [composer](https://getcomposer.org/).



```

vendor/bin/phpunit

```



## Copyright



This work is licensed under a [Creative Commons Attribution 4.0 International License](http://creativecommons.org/licenses/by/4.0/).



(C) Copyright 2015 by [Enrico Zimuel](http://www.zimuel.it).