KotlinClassicGangOfFourVisitor.kt

import kotlin.math.PI

fun main() {
    val shapes = listOf(
        Square(1.0),
        Square(2.0),
        Circle(3.0)
    )

    val shapeVisitors = listOf(
        AreaShapeVisitor(),
        PerimeterShapeVisitor(),
        StringifierShapeVisitor(),
        PrettyStringifierShapeVisitor()
    )

    // Here we do double dispatch:
    // Depending on the type of the shape and the type of the visitor, we run some specific function
    shapeVisitors.forEach { visitor ->
        shapes.forEach { shape ->
            println(shape.accept(visitor))
        }
    }
}

interface Shape {
    fun <T> accept(shapeVisitor: ShapeVisitor<T>): T
}

class Square(val side: Double) : Shape {
    override fun <T> accept(shapeVisitor: ShapeVisitor<T>) = shapeVisitor.visit(this)
}

class Circle(val radius: Double) : Shape {
    override fun <T> accept(shapeVisitor: ShapeVisitor<T>) = shapeVisitor.visit(this)
}

interface ShapeVisitor<T> {
    fun visit(square: Square): T
    fun visit(circle: Circle): T
}

class AreaShapeVisitor : ShapeVisitor<Double> {
    override fun visit(square: Square): Double {
        val side = square.side
        return side * side
    }

    override fun visit(circle: Circle): Double {
        val radius = circle.radius
        return PI * radius * radius
    }
}

class PerimeterShapeVisitor : ShapeVisitor<Double> {
    override fun visit(square: Square): Double {
        val side = square.side
        return 4 * side
    }

    override fun visit(circle: Circle) = 2 * PI * circle.radius
}

class StringifierShapeVisitor : ShapeVisitor<String?> {
    override fun visit(square: Square) = square::class.simpleName
    override fun visit(circle: Circle) = circle::class.simpleName
}

class PrettyStringifierShapeVisitor : ShapeVisitor<String> {
    private val stringifier = StringifierShapeVisitor()

    override fun visit(square: Square): String {
        val string = stringifier.visit(square)
        requireNotNull(string)
        return "⃞⃞⃞$string⃞⃞⃞"
    }

    override fun visit(circle: Circle): String {
        val string = stringifier.visit(circle)
        requireNotNull(string)
        return "•••$string•••"
    }
}