Fun after an exam with kotlin extension functions to solve simple challenge for solving this challenge.

StringReduceChallenge.kt

fun main() {
    //abcabc => cba => aa = 2
    val two = stringReduceChallenge("abcabc")
    println(two)

    // aaaa = 4
    val four = stringReduceChallenge("aaaa")
    println(four)

    // abcab => cbb => ab => c = 1
    val one = stringReduceChallenge("abcab")
    println(one)
}

fun stringReduceChallenge(input: String): Int {
    var desiredString = input

    while (!desiredString.allStringCharactersAreTheSame()) {
        val mList = desiredString.toList()
        desiredString = ""
        mList.forEachTwo { s1, s2 ->
            when {
                s1 == s2 -> {
                    desiredString += "$s1$s2"
                }
                s2 == null -> {
                    desiredString += s1
                }
                else -> desiredString += flip(s1, s2)
            }
        }
    }

    return desiredString.length
}

fun flip(s1: Char, s2: Char): Char {
    return if (s1 == 'a' && s2 == 'b' || s1 == 'b' && s2 == 'a') {
        'c'
    } else if (s1 == 'b' && s2 == 'c' || s1 == 'c' && s2 == 'b') {
        'a'
    } else {
        'b'
    }
}


fun String.allStringCharactersAreTheSame(): Boolean {
    if (this.isEmpty()) return true
    // 1- Get the first char
    // 2- If there is any char in the string that doesn't
    // equal the first char
    // then not equal else equal
    // note that any returns (true) if any matches the given predicate
    // so here if any char doesn't equal the first it returns true
    // but from the name of the method it return true if all equals
    // very easy just negate the result :) XD
    return !any { this.first() != it }
}

/**
 * Performs the given action on each two elements.
 * if odd size collection is used it will be considered
 * as odd collection + 1 null item to make it even
 *
 * even ex: ['a','b','c','d'] will return:
 * => 'a','b'
 * => 'c','d'
 *
 * odd ex: ['a','b','c'] will return:
 * => 'a','b'
 * => 'c',null
 * */
inline fun <T> Collection<T>.forEachTwo(
    action: (T, T?) -> Unit
) {

    forEachIndexed { i, t ->
        if (size == 0) return
        if (!isEven(i) && i != 0) {
            action(this.elementAt(i - 1), t)
        } else if (i == size - 1) { // is last index
            action(this.elementAt(i), null)
        }
    }
}

fun isEven(int: Int) = int % 2 == 0