RPN

Rpn.kt

import java.util.LinkedList
import java.util.List

fun rpn(expression : String) : Double {
  val expressionElements = expression.split(' ').toLinkedList()
  val result = foldl(rpn, Stack<Double>(), expressionElements)
  return result.pop()
}

private fun foldl<T, U>(f : (T, U) -> T, a : T, l : List<U>) : T {
  if(l.size == 0) {
    return a
  }

  return foldl(f, f(a, l.remove(0)), l)
}

private val rpn : (Stack<Double>, String) -> Stack<Double> = {
  (stack : Stack<Double>, str : String) ->
    when(str) {
      "+" -> stack.push(  stack.pop() + stack.pop())
      "-" -> stack.push( -stack.pop() + stack.pop())
      "*" -> stack.push(  stack.pop() * stack.pop())
      "/" -> stack.push(1/stack.pop() * stack.pop())
      else -> stack.push(str.toDouble())
    }
    stack
}

private class Stack<T>() {
  private val elements = LinkedList<T>()
  
  fun push(e : T) = elements.addFirst(e)
  fun pop() = elements.remove()
}

private inline fun test(b : Boolean) {
  if(!b) throw AssertionError()
}

fun main(args : Array<String>) {
  test(rpn("1 2 +") == 3.0)
  test(rpn("1 2 -") == -1.0)
  test(rpn("1 2 *") == 2.0)
  test(rpn("1 2 /") == 0.5)
  
  test(rpn("1 2 + 3 *") == 9.0)
  test(rpn("1 2 3 + *") == 5.0)
  test(rpn("1 2 - 3 4 + *") == -7.0)
  test(rpn("1 2 3 4 * * +") == 25.0)
  test(rpn("1 2 3 4 * * 5 6 + -") == 13.0)
}