Implementation of a Zipper, for efficient splice.

Signed-off-by: Edward Z. Yang <ezyang@mit.edu>

library/HTMLPurifier.includes.php

@@ -73,6 +73,7 @@ require 'HTMLPurifier/URISchemeRegistry.php';
 require 'HTMLPurifier/UnitConverter.php';
 require 'HTMLPurifier/VarParser.php';
 require 'HTMLPurifier/VarParserException.php';
+require 'HTMLPurifier/Zipper.php';
 require 'HTMLPurifier/AttrDef/CSS.php';
 require 'HTMLPurifier/AttrDef/Clone.php';
 require 'HTMLPurifier/AttrDef/Enum.php';

library/HTMLPurifier.safe-includes.php

@@ -67,6 +67,7 @@ require_once $__dir . '/HTMLPurifier/URISchemeRegistry.php';
 require_once $__dir . '/HTMLPurifier/UnitConverter.php';
 require_once $__dir . '/HTMLPurifier/VarParser.php';
 require_once $__dir . '/HTMLPurifier/VarParserException.php';
+require_once $__dir . '/HTMLPurifier/Zipper.php';
 require_once $__dir . '/HTMLPurifier/AttrDef/CSS.php';
 require_once $__dir . '/HTMLPurifier/AttrDef/Clone.php';
 require_once $__dir . '/HTMLPurifier/AttrDef/Enum.php';

library/HTMLPurifier/Zipper.php

@@ -0,0 +1,145 @@
+<?php
+
+/**
+ * A zipper is a purely-functional data structure which contains
+ * a focus that can be efficiently manipulated.  It is known as
+ * a "one-hole context".  This mutable variant implements a zipper
+ * for a list as a pair of two arrays, laid out as follows:
+ *
+ *      Base list: 1 2 3 4 [ ] 6 7 8 9
+ *      Front list: 1 2 3 4
+ *      Back list: 9 8 7 6
+ *
+ * User is expected to keep track of the "current element" and properly
+ * fill it back in as necessary.  (ToDo: Maybe it's more user friendly
+ * to implicitly track the current element?)
+ *
+ * Nota bene: the current class gets confused if you try to store NULLs
+ * in the list.
+ */
+
+class HTMLPurifier_Zipper
+{
+    private $front, $back;
+
+    public function __construct($front, $back) {
+        $this->front = $front;
+        $this->back = $back;
+    }
+
+    /**
+     * Creates a zipper from an array, with a hole in the
+     * 0-index position.
+     * @param Array to zipper-ify.
+     * @return Tuple of zipper and element of first position.
+     */
+    static public function fromArray($array) {
+        $z = new self(array(), array_reverse($array));
+        $t = $z->delete(); // delete the "dummy hole"
+        return array($z, $t);
+    }
+
+    /**
+     * Convert zipper back into a normal array, optionally filling in
+     * the hole with a value. (Usually you should supply a $t, unless you
+     * are at the end of the array.)
+     */
+    public function toArray($t = NULL) {
+        $a = $this->front;
+        if ($t !== NULL) $a[] = $t;
+        for ($i = count($this->back)-1; $i >= 0; $i--) {
+            $a[] = $this->back[$i];
+        }
+        return $a;
+    }
+
+    /**
+     * Move hole to the next element.
+     * @param $t Element to fill hole with
+     * @return Original contents of new hole.
+     */
+    public function next($t) {
+        if ($t !== NULL) array_push($this->front, $t);
+        return empty($this->back) ? NULL : array_pop($this->back);
+    }
+
+    /**
+     * Iterated hole advancement.
+     * @param $t Element to fill hole with
+     * @param $i How many forward to advance hole
+     * @return Original contents of new hole, i away
+     */
+    public function advance($t, $n) {
+        for ($i = 0; $i < $n; $i++) {
+            $t = $this->next($t);
+        }
+        return $t;
+    }
+
+    /**
+     * Move hole to the previous element
+     * @param $t Element to fill hole with
+     * @return Original contents of new hole.
+     */
+    public function prev($t) {
+        if ($t !== NULL) array_push($this->back, $t);
+        return empty($this->front) ? NULL : array_pop($this->front);
+    }
+
+    /**
+     * Delete contents of current hole, shifting hole to
+     * next element.
+     * @return Original contents of new hole.
+     */
+    public function delete() {
+        return empty($this->back) ? NULL : array_pop($this->back);
+    }
+
+    /**
+     * Insert element before hole.
+     * @param Element to insert
+     */
+    public function insertBefore($t) {
+        if ($t !== NULL) array_push($this->front, $t);
+    }
+
+    /**
+     * Insert element after hole.
+     * @param Element to insert
+     */
+    public function insertAfter($t) {
+        if ($t !== NULL) array_push($this->back, $t);
+    }
+
+    /**
+     * Splice in multiple elements at hole.  Functional specification
+     * in terms of array_splice:
+     *
+     *      $r1 = array_splice($arr, $i, $delete, $replacement);
+     *
+     *      list($z, $t) = HTMLPurifier_Zipper::fromArray($arr);
+     *      $t = $z->advance($t, $i);
+     *      $t = $z->splice($t, $delete, $replacement);
+     *      $r2 = $z->toArray($t);
+     *
+     *      assert($r1 === $r2);
+     *
+     * NB: the absolute index location after this operation is
+     * *unchanged!*
+     *
+     * @param Current contents of hole.
+     */
+    public function splice($t, $delete, $replacement) {
+        // delete
+        $r = $t;
+        for ($i = $delete; $i > 0; $i--) {
+            $r = $this->delete();
+        }
+        // insert
+        for ($i = count($replacement)-1; $i >= 0; $i--) {
+            $this->insertAfter($r);
+            $r = $replacement[$i];
+        }
+        return $r;
+    }
+}

tests/HTMLPurifier/ZipperTest.php

@@ -0,0 +1,25 @@
+<?php
+
+class HTMLPurifier_ZipperTest extends HTMLPurifier_Harness
+{
+    public function testBasicNavigation() {
+        list($z, $t) = HTMLPurifier_Zipper::fromArray(array(0,1,2,3));
+        $this->assertIdentical($t, 0);
+        $t = $z->next($t);
+        $this->assertIdentical($t, 1);
+        $t = $z->prev($t);
+        $this->assertIdentical($t, 0);
+        $t = $z->advance($t, 2);
+        $this->assertIdentical($t, 2);
+        $t = $z->delete();
+        $this->assertIdentical($t, 3);
+        $z->insertBefore(4);
+        $z->insertAfter(5);
+        $this->assertIdentical($z->toArray($t), array(0,1,4,3,5));
+        $t = $z->splice($t, 2, array(6,7));
+        $this->assertIdentical($t, 6);
+        $this->assertIdentical($z->toArray($t), array(0,1,4,6,7));
+    }
+
+    // ToDo: QuickCheck style test comparing with array_splice
+}