1 files changed, 56 insertions, 0 deletions
diff --git a/library/HTMLPurifier/Queue.php b/library/HTMLPurifier/Queue.php
new file mode 100644
index 000000000..f58db9042
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+++ b/library/HTMLPurifier/Queue.php
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+<?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);
+    }
+}