N-queens problem solved using TabuSearch Algorithm

nqueens_tabusearch.kt

import kotlin.math.pow

const val boardSize = 10

fun cost(solution: Array<Int>): Int {
    var cost = 0
    for (i in 0 until boardSize) {
        for (j in (i + 1) until boardSize) {
            if (solution[i] == solution[j]) cost++
            if (solution[i] == solution[j] + (j - i)) cost++
            if (solution[i] == solution[j] - (j - i)) cost++
        }
    }
    return cost
}

fun bruteforce() {
    for (i in 0 until boardSize.toDouble().pow(boardSize).toInt()) {
        val solution = i.toString(boardSize).padStart(boardSize, '0').map { it.digitToInt() }.toTypedArray()
        if (cost(solution) == 0) {
            println("${solution.joinToString("-")}")
            return
        }
    }
}

fun tabuSearch() {
    var solution = Array(boardSize) { (0 until boardSize).random() }
    val tabu = mutableListOf<Array<Int>>()
    while (cost(solution) != 0) {
        val neighbors = (0 until boardSize).map { column ->
            solution.copyOf().apply { this[column] = (this[column] + 1) % boardSize }
        }
        solution = neighbors.filter { neighbor -> tabu.none { it.joinToString() == neighbor.joinToString() } }.minByOrNull { cost(it) }!!
        tabu.add(solution)
    }
    println("${solution.joinToString("-")}")
}

fun main() {
    val start = System.currentTimeMillis()
    print("tabu search - ")
    tabuSearch()
    println(System.currentTimeMillis() - start)

    val start2 = System.currentTimeMillis()
    print("bruteforce - ")
    bruteforce()
    println(System.currentTimeMillis() - start2)
}