refactor: implements chain of resposibility pattern in discount calculator

Author: fbsouzas

src/Discounts/Discount.php

@@ -0,0 +1,19 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Fbsouzas\DesignPattern\Discounts;
+
+use Fbsouzas\DesignPattern\Budgets\Budget;
+
+abstract class Discount
+{
+    protected ?Discount $nextDiscount;
+
+    public function __construct(?Discount $nextDiscount)
+    {
+        $this->nextDiscount = $nextDiscount;
+    }
+
+    abstract public function calculate(Budget $budget): float;
+}

src/Discounts/DiscountCalculator.php

@@ -10,14 +10,13 @@ class DiscountCalculator
 {
     public function calculate(Budget $budget): float
     {
-        if ($budget->quantityOfItems > 5) {
-            return $budget->value * 0.1;
-        }
 
-        if ($budget->value > 500) {
-            return $budget->value * 0.05;
-        }
+        $chainOfDiscount = new DiscountToMoreThan5Items(
+            new DiscountToValueHigherThan500(
+                new NoDiscount()
+            )
+        );
 
-        return 0;
+        return $chainOfDiscount->calculate($budget);
     }
 }

src/Discounts/DiscountToMoreThan5Items.php

@@ -0,0 +1,19 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Fbsouzas\DesignPattern\Discounts;
+
+use Fbsouzas\DesignPattern\Budgets\Budget;
+
+class DiscountToMoreThan5Items extends Discount
+{
+    public function calculate(Budget $budget): float
+    {
+        if ($budget->quantityOfItems > 5) {
+            return $budget->value * 0.1;
+        }
+
+        return $this->nextDiscount->calculate($budget);
+    }
+}

src/Discounts/DiscountToValueHigherThan500.php

@@ -0,0 +1,19 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Fbsouzas\DesignPattern\Discounts;
+
+use Fbsouzas\DesignPattern\Budgets\Budget;
+
+class DiscountToValueHigherThan500 extends Discount
+{
+    public function calculate(Budget $budget): float
+    {
+        if ($budget->value > 500) {
+            return $budget->value * 0.05;
+        }
+
+        return $this->nextDiscount->calculate($budget);
+    }
+}

src/Discounts/NoDiscount.php

@@ -0,0 +1,20 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Fbsouzas\DesignPattern\Discounts;
+
+use Fbsouzas\DesignPattern\Budgets\Budget;
+
+class NoDiscount extends Discount
+{
+    public function __construct()
+    {
+        parent::__construct(null);
+    }
+
+    public function calculate(Budget $budget): float
+    {
+        return 0;
+    }
+}