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57 lines
1.5 KiB
57 lines
1.5 KiB
4 years ago
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<?php
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/**
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* A simple array-backed queue, based off of the classic Okasaki
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* persistent amortized queue. The basic idea is to maintain two
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* stacks: an input stack and an output stack. When the output
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* stack runs out, reverse the input stack and use it as the output
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* stack.
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*
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* We don't use the SPL implementation because it's only supported
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* on PHP 5.3 and later.
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*
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* Exercise: Prove that push/pop on this queue take amortized O(1) time.
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*
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* Exercise: Extend this queue to be a deque, while preserving amortized
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* O(1) time. Some care must be taken on rebalancing to avoid quadratic
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* behaviour caused by repeatedly shuffling data from the input stack
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* to the output stack and back.
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*/
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class HTMLPurifier_Queue {
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private $input;
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private $output;
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public function __construct($input = array()) {
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$this->input = $input;
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$this->output = array();
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}
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/**
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* Shifts an element off the front of the queue.
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*/
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public function shift() {
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if (empty($this->output)) {
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$this->output = array_reverse($this->input);
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$this->input = array();
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}
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if (empty($this->output)) {
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return NULL;
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}
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return array_pop($this->output);
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}
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/**
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* Pushes an element onto the front of the queue.
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*/
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public function push($x) {
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array_push($this->input, $x);
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}
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/**
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* Checks if it's empty.
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*/
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public function isEmpty() {
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return empty($this->input) && empty($this->output);
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}
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}
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