Added loyalty discount

Author: ardalis

DesignPatternsInCSharp/RulesEngine/Discounts/DiscountCalculate_CalculateDiscountPercentage.cs

@@ -54,6 +54,23 @@ public void Returns10PctForCustomersWhoAreVeterans(int customerAge)
             result.Should().Be(.10m);
         }
 
+        [Theory]
+        [InlineData(1, .05)]
+        [InlineData(2, .08)]
+        [InlineData(5, .10)]
+        [InlineData(10, .12)]
+        [InlineData(15, .15)]
+        public void ReturnsCorrectLoyaltyDiscountForLongtimeCustomers(int yearsAsCustomer, decimal expectedDiscount)
+        {
+            var customer = CreateCustomer(DEFAULT_AGE, 
+                DateTime.Today.AddYears(-yearsAsCustomer).AddDays(-1));
+
+            var result = _calculator.CalculateDiscountPercentage(customer);
+
+            result.Should().Be(expectedDiscount);
+        }
+
+
         private Customer CreateCustomer(int age = DEFAULT_AGE, DateTime? firstPurchaseDate = null)
         {
             return new Customer

DesignPatternsInCSharp/RulesEngine/Discounts/DiscountCalculator.cs

@@ -10,8 +10,31 @@ public decimal CalculateDiscountPercentage(Customer customer)
             {
                 return .15m;
             }
-           
-            if(customer.IsVeteran)
+            else
+            {
+                if (customer.DateOfFirstPurchase.Value < DateTime.Now.AddYears(-15))
+                {
+                    return .15m;
+                }
+                if (customer.DateOfFirstPurchase.Value < DateTime.Now.AddYears(-10))
+                {
+                    return .12m;
+                }
+                if (customer.DateOfFirstPurchase.Value < DateTime.Now.AddYears(-5))
+                {
+                    return .10m;
+                }
+                if (customer.DateOfFirstPurchase.Value < DateTime.Now.AddYears(-2))
+                {
+                    return .08m;
+                }
+                if (customer.DateOfFirstPurchase.Value < DateTime.Now.AddYears(-1))
+                {
+                    return .05m;
+                }
+            }
+
+            if (customer.IsVeteran)
             {
                 return .10m;
             }