added strategy pattern

Author: Dominik Liebler

Diff for: Strategy/Strategy.php

@@ -0,0 +1,111 @@
+<?php
+
+namespace DesignPatterns;
+
+/**
+ * strategy pattern
+ *
+ * Terminology:
+ * - Context
+ * - Strategy
+ * - Concrete Strategy
+ *
+ * Purpose:
+ * to separate strategies and to enable fast switching between them.
+ * also this pattern is a good alternative to inheritance (instead of having an abstract class that is extended)
+ *
+ * Examples:
+ * - sorting a list of objects, one strategy by date, the other by id
+ * - simplify unit testing: e.g. switching between file and in-memory storage
+ *
+ */
+
+interface Comparator
+{
+    /**
+     * @abstract
+     * @param object $a
+     * @param object $b
+     * @return bool
+     */
+    public function compare($a, $b);
+}
+
+class ObjectCollection
+{
+    private $_elements;
+    private $_comparator;
+
+    /**
+     * @param array $elements
+     */
+    public function __construct(array $elements = array())
+    {
+        $this->_elements = $elements;
+    }
+
+    /**
+     * @return array
+     */
+    public function sort()
+    {
+        $callback = array($this->_comparator, 'compare');
+        uasort($this->_elements, $callback);
+
+        return $this->_elements;
+    }
+
+    /**
+     * @param Comparator $comparator
+     * @return void
+     */
+    public function setComparator(Comparator $comparator)
+    {
+        $this->_comparator = $comparator;
+    }
+}
+
+class IdComparator implements Comparator
+{
+    public function compare($a, $b)
+    {
+        if ($a['id'] == $b['id']) {
+            return 0;
+        } else {
+            return $a['id'] < $b['id'] ? -1 : 1;
+        }
+    }
+}
+
+class DateComparator implements Comparator
+{
+    public function compare($a, $b)
+    {
+        $aDate = strtotime($a['date']);
+        $bDate = strtotime($b['date']);
+
+        if ($aDate == $bDate) {
+            return 0;
+        } else {
+            return $aDate < $bDate ? -1 : 1;
+        }
+    }
+}
+
+$elements = array(
+    array(
+        'id' => 2,
+        'date' => '2011-01-01',
+    ),
+    array(
+        'id' => 1,
+        'date' => '2011-02-01'
+    )
+);
+
+$collection = new ObjectCollection($elements);
+$collection->setComparator(new IdComparator());
+$collection->sort();
+
+$collection->setComparator(new DateComparator());
+$collection->sort();