"Hello World!" Simplistic Tabu Search Algorithm. This sample is meant to be readable and is not efficient at all, so !!! Do not use in production !!!

ts.kt

import kotlin.math.abs

const val target = "Hello World!"
const val possibilities = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ123456789 .,!?"

val String.fitness: Int get() = // lower fitness better (for this case) fitness of 0 = solution
    indices.sumOf { index ->
        val indexFirst = possibilities.indexOf(target[index])
        val indexSecond = possibilities.indexOf(this[index])
        val high = maxOf(indexFirst, indexSecond)
        val low = minOf(indexFirst, indexSecond)
        if ((high - low) > possibilities.length / 2) {
            abs(high - possibilities.length - low)
        } else {
            high - low
        }
    }

var current = (target.indices).map { possibilities.random() }.joinToString("")

val tabu = mutableSetOf(current)

fun String.neighbors() : Set<String> {
    val result = mutableSetOf(this)
    for (i in this.indices) {
        val posIndex = possibilities.indexOf(current[i])
        var testPos1 = posIndex + 1
        var testPos2 = posIndex - 1
        if (testPos2 < 0) testPos2 = possibilities.length - 1
        if (testPos1 >= possibilities.length) testPos1 = 0

        val new1 = current.replaceRange(i, i + 1, possibilities[testPos2].toString())
        val new2 = current.replaceRange(i, i + 1, possibilities[testPos1].toString())

        result += new1
        result += new2
    }

    return result
}

fun main() {
    val startAt = System.currentTimeMillis()
    var iteration = 0
    while(current.fitness != 0) {
        if (iteration % 100 == 0) {
            println("\"$current\" score ${current.fitness} found in $iteration iterations")
        }
        val best = current.neighbors()
            .filter { it !in tabu }
            .minByOrNull { it.fitness - current.fitness } ?: break
        tabu.add(best)
        current = best
        iteration++
    }
    println("Solution \"$current\" found in $iteration iterations and ${System.currentTimeMillis() - startAt}ms")
}