Shape.kt

interface Shape {
  fun perimeter(): Double
  fun area(): Double
  fun diameter(): Double
}

class Rectangle(val x: Double, val y: Double) : Shape {
  override fun perimeter() = (x + y) * 2
  override fun area() = x * y
  override fun diameter() = Math.sqrt(x * x + y * y)
}

class Circle(val radius: Double) : Shape {
  override fun perimeter() = 2 * PI * radius
  override fun area() = PI * radius * radius
  override fun diameter() = 2 * radius
}