Add BuyXGetOneFreeDiscount

test/buy_x_get_one_free_discount_test.rb

@@ -0,0 +1,25 @@
+require "minitest/autorun"
+
+require 'buy_x_get_one_free_discount.rb'
+
+class BuyXGetOneFreeDiscountTest < Minitest::Test
+  def test_appropriate_discount_is_applied
+    unit_price = 4.00
+    (2..4).each do |trigger_quantity|
+      discount = BuyXGetOneFreeDiscount.new(unit_price: unit_price, trigger_quantity: trigger_quantity)
+
+      # Single discount at and above trigger quantity
+      assert_equal unit_price, discount.discount_for_quantity(trigger_quantity)
+      assert_equal unit_price, discount.discount_for_quantity(trigger_quantity + 1)
+      assert_equal unit_price, discount.discount_for_quantity(trigger_quantity + trigger_quantity - 1)
+
+
+      # No discount if less than trigger quantity
+      assert_equal 0.00, discount.discount_for_quantity(trigger_quantity - 1)
+
+      # Double discount if double trigger quantity
+      assert_equal unit_price * 2, discount.discount_for_quantity(trigger_quantity * 2)
+      assert_equal unit_price * 2, discount.discount_for_quantity((trigger_quantity * 2) + 1)
+    end
+  end
+end