add visitor

library/Pattern/Visitor/Product.php

@@ -0,0 +1,13 @@
+<?php
+
+class Pattern_Visitor_Product
+{
+    public $brand = null;
+    public $model = null;
+    public $price = 0;
+
+    public function applyVisitor(Pattern_Visitor_Visitor_Interface $visitor)
+    {
+        $visitor->visitProduct($this);
+    }
+}

library/Pattern/Visitor/Visitor/Discount.php

@@ -0,0 +1,12 @@
+<?php
+
+class Pattern_Visitor_Visitor_Discount implements Pattern_Visitor_Visitor_Interface
+{
+    public function visitProduct(Pattern_Visitor_Product $product)
+    {
+        if (0 === $product->price) return;
+
+        $product->price =
+            $product->price - number_format( (20 * 100) / $product->price);
+    }
+}

library/Pattern/Visitor/Visitor/Interface.php

@@ -0,0 +1,6 @@
+<?php
+
+interface Pattern_Visitor_Visitor_Interface
+{
+    function visitProduct(Pattern_Visitor_Product $product);
+}

tests/Pattern/Visitor/ProductTest.php

@@ -0,0 +1,14 @@
+<?php
+
+class Pattern_Visitor_ProductTest extends PHPUnit_Framework_TestCase
+{
+    public function testDiscount()
+    {
+        $product = new Pattern_Visitor_Product();
+        $product->price = 100;
+        $discount = new Pattern_Visitor_Visitor_Discount();
+        $product->applyVisitor($discount);
+
+        $this->assertEquals(80, $product->price);
+    }
+}