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

sa.kt

import kotlin.math.abs
import kotlin.math.pow
import kotlin.random.Random

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

val random = Random(System.currentTimeMillis())

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("")

fun String.neighbor() : String {
    val pos = random.nextInt(this.length)
    val sign = if (random.nextBoolean()) 1 else -1
    val result = (possibilities.indexOf(this[pos]) + sign * 1) % possibilities.length
    val character = possibilities[if (result < 0) result + possibilities.length else result]
    return this.substring(0, pos) + character + this.substring(pos + 1)
}

fun accept(current: String, neighbor: String, temperature: Double) : Double {
    return Math.E.pow(- ((neighbor.fitness - current.fitness)/temperature))
}

fun main() {
    val startAt = System.currentTimeMillis()
    for (i in 0 until 10000) {
        if (i % 100 == 0) {
            println("Solution \"$current\" score ${current.fitness} found in $i iterations")
        }
        val temperature = 100.0 / (i + 100)
        val neighbor = current.neighbor()
        val diff = neighbor.fitness - current.fitness

        if (diff < 0 || random.nextDouble() < accept(current, neighbor, temperature)) {
            current = neighbor
        }

        if (current.fitness == 0) {
            println("Solution \"$current\" found in $i iterations and ${System.currentTimeMillis() - startAt}ms")
            return
        }
    }
}