Added Visitor pattern

README.md

@@ -17,7 +17,7 @@ Kotlin OOP and FP Design Patterns
     * [x] [Observer](#observer)
     * [x] [State](#state)
     * [ ] [Template](#template)
-    * [ ] [Visitor](#visitor)
+    * [x] [Visitor](#visitor)
   * [Creational Patterns](#creational)
     * [ ] [Abstract Factory](#abstract-factory)
     * [ ] [Builder](#builder)
@@ -311,12 +311,64 @@ Template
 
 **In progress**
 
-Visitor
+[Visitor](/src/main/kotlin/oop/Visitor)
 ------
 
 > Represent an operation to be performed on the elements of an object structure. It lets you define a new operation without changing the classes of the elements on which it operates.
 
-**In progress**
+### Example
+
+```kotlin
+interface PaymentMethodVisitable {
+  fun accept(visitor: PaymentMethodVisitor)
+}
+
+sealed class PaymentMethod(var moneyPayed: Float, var cost: Float) : PaymentMethodVisitable {
+
+  class CashPaymentMethod(moneyPayed: Float, cost: Float) : PaymentMethod(moneyPayed, cost) {
+    override fun accept(visitor: PaymentMethodVisitor) = visitor.visit(this)
+  }
+
+  class CreditCardPaymentMethod(cost: Float) : PaymentMethod(cost, cost) {
+    override fun accept(visitor: PaymentMethodVisitor) = visitor.visit(this)
+  }
+
+}
+
+interface PaymentMethodVisitor {
+  fun visit(paymentMethod: PaymentMethod.CashPaymentMethod)
+  fun visit(paymentMethod: PaymentMethod.CreditCardPaymentMethod)
+}
+
+class CashRegister(initialAmount: Float) : PaymentMethodVisitor {
+
+  var cash = initialAmount
+    private set
+
+  override fun visit(paymentMethod: PaymentMethod.CashPaymentMethod) {
+    cash += paymentMethod.moneyPayed
+    cash -= (paymentMethod.moneyPayed - paymentMethod.cost)
+    paymentMethod.moneyPayed = (paymentMethod.moneyPayed - paymentMethod.cost)
+  }
+
+  override fun visit(paymentMethod: PaymentMethod.CreditCardPaymentMethod) {
+    cash += paymentMethod.cost
+    paymentMethod.moneyPayed = 0
+  }
+
+}
+```
+
+### Usage
+
+```kotlin
+val cashRegister = CashRegister(0.0f)
+val cash = PaymentMethod.CashPaymentMethod(15.50f, 12.50f)
+cashRegister.visit(cash)
+
+println(cashRegister.cash) // 12.50f
+println(cash.moneyPayed)   // 3f
+```
 
 Creational
 ==========

src/main/kotlin/oop/Visitor/PaymentMethodVisitable.kt

@@ -0,0 +1,19 @@
+package oop.Visitor
+
+
+interface PaymentMethodVisitable {
+  fun accept(visitor: PaymentMethodVisitor)
+}
+
+sealed class PaymentMethod(var moneyPayed: Int, var cost: Int) : PaymentMethodVisitable {
+
+  class CashPaymentMethod(moneyPayed: Float, cost: Float) :
+    PaymentMethod((moneyPayed * 100).toInt(), (cost * 100).toInt()) {
+    override fun accept(visitor: PaymentMethodVisitor) = visitor.visit(this)
+  }
+
+  class CreditCardPaymentMethod(cost: Float) : PaymentMethod((cost * 100).toInt(), (cost * 100).toInt()) {
+    override fun accept(visitor: PaymentMethodVisitor) = visitor.visit(this)
+  }
+
+}

src/main/kotlin/oop/Visitor/PaymentMethodVisitor.kt

@@ -0,0 +1,25 @@
+package oop.Visitor
+
+
+interface PaymentMethodVisitor {
+  fun visit(paymentMethod: PaymentMethod.CashPaymentMethod)
+  fun visit(paymentMethod: PaymentMethod.CreditCardPaymentMethod)
+}
+
+class CashRegister(initialAmount: Float) : PaymentMethodVisitor {
+
+  var cash = (initialAmount * 100).toInt()
+    private set
+
+  override fun visit(paymentMethod: PaymentMethod.CashPaymentMethod) {
+    cash += paymentMethod.moneyPayed
+    cash -= (paymentMethod.moneyPayed - paymentMethod.cost)
+    paymentMethod.moneyPayed = (paymentMethod.moneyPayed - paymentMethod.cost)
+  }
+
+  override fun visit(paymentMethod: PaymentMethod.CreditCardPaymentMethod) {
+    cash += paymentMethod.cost
+    paymentMethod.moneyPayed = 0
+  }
+
+}

src/test/kotlin/oop/Visitor/VisitorTest.kt

@@ -0,0 +1,53 @@
+package oop.Visitor
+
+import com.natpryce.hamkrest.Matcher
+import com.natpryce.hamkrest.assertion.assert
+import org.junit.Test
+
+class VisitorTest {
+
+  val `is` = { amount: Number -> Matcher(Number::equals, amount) }
+  fun Int.toMoney() = this.div(100f)
+
+  @Test
+  fun `cash should increment exact amount when payment is credit card`() {
+    val cashRegister = CashRegister(0f)
+    val creditCard = PaymentMethod.CreditCardPaymentMethod(15.50f)
+
+    cashRegister.visit(creditCard)
+
+    assert.that(cashRegister.cash.toMoney(), `is`(15.50f))
+  }
+
+  @Test
+  fun `moneyPayed should be 0 when payment is credit card after payment`() {
+    val cashRegister = CashRegister(0.0f)
+    val creditCard = PaymentMethod.CreditCardPaymentMethod(15.50f)
+
+    cashRegister.visit(creditCard)
+
+    assert.that(creditCard.moneyPayed.toMoney(), `is`(0.0f))
+  }
+
+  @Test
+  fun `cash should increment payedAmount and decrease exchange amount when payment is cash`() {
+    val cashRegister = CashRegister(0.0f)
+    val cash = PaymentMethod.CashPaymentMethod(20.0f, 19.99f)
+
+    cashRegister.visit(cash)
+
+    assert.that(cashRegister.cash.toMoney(), `is`(19.99f))
+  }
+
+  @Test
+  fun `moneyPayed should be equal to exchange when payment is cash`() {
+    val cashRegister = CashRegister(0.0f)
+    val cash = PaymentMethod.CashPaymentMethod(20.0f, 19.99f)
+
+    cashRegister.visit(cash)
+
+    assert.that(cash.moneyPayed.toMoney(), `is`(0.01f))
+  }
+
+
+}