Add BuyXGetOneFreeDiscount

buy_x_get_one_free_discount.rb

@@ -0,0 +1,10 @@
+class BuyXGetOneFreeDiscount
+  def initialize(unit_price:, trigger_quantity: 2)
+    @unit_price = unit_price
+    @trigger_quantity = trigger_quantity.to_f
+  end
+
+  def discount_for_quantity(quantity)
+    return (quantity / @trigger_quantity).floor * @unit_price
+  end
+end