inFullMobile Language Wars: Round 1 - Kotlin Solution 1 - full code by @rainqer - See: https://gist.github.com/rsp/d8bdbafa09f24f99eebc8ed60fe205c8

ifm-lw-r1-kotlin-solution-1-full.kt

package com.infullmobile.languagewars

import org.junit.Assert.assertEquals
import org.junit.Test

class ExampleUnitTest {

    @Test
    fun testHelperFunctions() {
        assertEquals(applyXZeroTimes(increment)(0), 0)
        assertEquals(applyXOnce(increment)(0), 1)
        assertEquals(applyXTwice(increment)(0), 2)
        assertEquals(applyXThousandfold(increment)(0), 1000)
    }

    @Test
    fun testApplyingOneTimeLess() {
        assertEquals(f(applyXZeroTimes)(increment)(0), 0)
        assertEquals(f(applyXOnce)(increment)(0), 0)
        assertEquals(f(applyXTwice)(increment)(0), 1)
        assertEquals(f(applyXThousandfold)(increment)(0), 999)
    }
}

// x = method to be applied
// a = method duplicating x n times
// b = method duplicating x n-1 times
// f = method taking a returning b

val applyXZeroTimes: ((Int) -> Int) -> (Int) -> Int = { x -> { input -> input } }

val applyXOnce: ((Int) -> Int) -> (Int) -> Int = { x -> x }

val applyXTwice: ((Int) -> Int) -> (Int) -> Int = { x -> applyXTwice(x) }

fun applyXTwice(x: (Int) -> Int): (Int) -> Int = { input -> x(x(input)) }

val applyXThousandfold: ((Int) -> Int) -> (Int) -> Int = { x -> applyXTenfold(applyXTenfold(applyXTenfold(x))) }

fun applyXTenfold(x: (Int) -> Int): (Int) -> Int = { input -> x(x(x(x(x(x(x(x(x(x(input)))))))))) }

fun f(a: ((Int) -> Int) -> ((Int) -> Int)): ((Int) -> Int) -> (Int) -> Int {
    val invocationCount = a(increment)(0)
    return if (invocationCount > 0) { x: (Int) -> Int -> { input -> applyXTimes(x, invocationCount - 1)(input) } }
    else { _ -> { input -> input } }
}

fun applyXTimes(x: (Int) -> Int, count: Int): (Int) -> Int
        = { argument -> (1..count).fold(argument, { acc, _ -> x(acc) }) }

val increment: (Int) -> Int = { counter -> counter + 1 }