8 Queens Puzzle

index.php

<?php

class Queen
{
    private int $row;

    public function __construct(private int $column, private ?Queen $neighbour)
    {
        $this->row = 1;
    }

    public function findSolution(): bool
    {
        while ($this->neighbour?->canAttack($this->row, $this->column)) {
            if (! $this->advance()) {
                return false;
            }
        }

        return true;
    }

    private function canAttack(int $row, int $column): bool
    {
        if ($this->row === $row) {
            return true;
        }

        if ($this->canAttackInDiagonal($row, $column)) {
            return true;
        }

        if (! $this->neighbour) {
            return false;
        }

        return $this->neighbour->canAttack($row, $column);
    }

    private function canAttackInDiagonal(int $row, int $column): bool
    {
        $columnDiff = $column - $this->column;

        return $this->row + $columnDiff === $row || $this->row - $columnDiff === $row;
    }

    private function advance(): bool
    {
        if ($this->row < 8) {
            $this->row++;
            return $this->findSolution();
        }

        if (! $this->neighbour) {
            return false;
        }

        if (! $this->neighbour->advance()) {
            return false;
        }

        if (! $this->neighbour->findSolution()) {
            return false;
        }

        $this->row = 1;
        return $this->findSolution();
    }

    public function print(): void
    {
        $this->neighbour?->print();

        foreach (range(1, 8) as $index) {
            echo ($index === $this->row
                ? "q"
                : ".");
        }

        echo PHP_EOL;
    }
}

$lastQueen = null;

for ($i = 1; $i <= 8; $i++) {
    $lastQueen = new Queen($i, $lastQueen);
    $lastQueen->findSolution();
}

$lastQueen->print();

/**
$ php index.php
q.......
....q...
.......q
.....q..
..q.....
......q.
.q......
...q....
*/