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✎ Editing: Queue.php ← Back

<?php

/**

* A simple array-backed queue, based off of the classic Okasaki

* persistent amortized queue.  The basic idea is to maintain two

* stacks: an input stack and an output stack.  When the output

* stack runs out, reverse the input stack and use it as the output

* stack.

* We don't use the SPL implementation because it's only supported

* on PHP 5.3 and later.

* Exercise: Prove that push/pop on this queue take amortized O(1) time.

* Exercise: Extend this queue to be a deque, while preserving amortized

* O(1) time.  Some care must be taken on rebalancing to avoid quadratic

* behaviour caused by repeatedly shuffling data from the input stack

* to the output stack and back.

class HTMLPurifier_Queue {

private $input;

private $output;

public function __construct($input = array()) {

$this->input = $input;

$this->output = array();

}

/**

* Shifts an element off the front of the queue.

public function shift() {

if (empty($this->output)) {

$this->output = array_reverse($this->input);

$this->input = array();

}

if (empty($this->output)) {

return NULL;

}

return array_pop($this->output);

}

/**

* Pushes an element onto the front of the queue.

public function push($x) {

array_push($this->input, $x);

}

/**

* Checks if it's empty.

public function isEmpty() {

return empty($this->input) && empty($this->output);

}

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