perft method
Implementation
int perft(int? depth) {
var moves = generate_moves({
'legal': false
});
var nodes = 0;
var color = turn;
for (var i = 0,
len = moves.length; i < len; i++) {
make_move(moves[i]);
if (!king_attacked(color)) {
if (depth! - 1 > 0) {
var child_nodes = perft(depth - 1);
nodes += child_nodes;
} else {
nodes++;
}
}
undo_move();
}
return nodes;
}