function solve($board)
{
// get empty spaces, find one with one choice, set it , rinse and repeat
while (!filled($board) && !validSolution($board)) {
for ($i = 0; $i < 9; $i++) {
for ($j = 0; $j < 9; $j++) {
if ($board[$i][$j] == 0) {
$valid = validValuesFor($board, [$i, $j]);
if (count($valid) == 1) {
$board[$i][$j] = $valid[0];
}
}
}
}
}
return $board;
}
public function test_validValuesFor()
{
$this->assertEquals([], validValuesFor([[5, 3, 0, 0, 7, 0, 0, 0, 0], [6, 0, 0, 1, 9, 5, 0, 0, 0], [0, 9, 8, 0, 0, 0, 0, 6, 0], [8, 0, 0, 0, 6, 0, 0, 0, 3], [4, 0, 0, 8, 0, 3, 0, 0, 1], [7, 0, 0, 0, 2, 0, 0, 0, 6], [0, 6, 0, 0, 0, 0, 2, 8, 0], [0, 0, 0, 4, 1, 9, 0, 0, 5], [0, 0, 0, 0, 8, 0, 0, 7, 9]], [0, 0]));
$this->assertEquals([1, 2, 4], validValuesFor([[5, 3, 0, 0, 7, 0, 0, 0, 0], [6, 0, 0, 1, 9, 5, 0, 0, 0], [0, 9, 8, 0, 0, 0, 0, 6, 0], [8, 0, 0, 0, 6, 0, 0, 0, 3], [4, 0, 0, 8, 0, 3, 0, 0, 1], [7, 0, 0, 0, 2, 0, 0, 0, 6], [0, 6, 0, 0, 0, 0, 2, 8, 0], [0, 0, 0, 4, 1, 9, 0, 0, 5], [0, 0, 0, 0, 8, 0, 0, 7, 9]], [0, 2]));
}
