}
    fn backup(self: @This()) @This() {
        var new_gs: GameState = undefined;
        for (self.bitboards) |bb, idx| {
            new_gs.bitboards[idx] = bb;
        }
        for (self.occupancies) |occ, idx| {
            new_gs.occupancies[idx] = occ;
        }
        new_gs.side = self.side;
        new_gs.enpassant = self.enpassant;
        new_gs.castling = self.castling;
        return new_gs;

    fn restore(self: *@This(), bck: @This()) void {
        for (bck.bitboards) |bb, idx| {
            self.bitboards[idx] = bb;
        }
        for (bck.occupancies) |occ, idx| {
            self.occupancies[idx] = occ;
        }
        self.side = bck.side;
        self.enpassant = bck.enpassant;
        self.castling = bck.castling;
    }

    var gs = try GameState.init("r3k2r/p11pqpbp/bn2pnp1/2pPN3/1p2P3/2N2Q1p/PPPBBPPP/R3K2R w Kkq c6 0 1");

    const tricky_position = "r3k2r/p1ppqpb1/bn2pnp1/3PN3/1p2P3/2N2Q1p/PPPBBPPP/R3K2R w KQkq c6 0 1";
    var gs = try GameState.init(tricky_position);

    gs.show();

    try std.testing.expectEqual(@as(usize, 49), gs.moves.len);
}
test "backup and restore" {
    const tricky_position = "r3k2r/p1ppqpb1/bn2pnp1/3PN3/1p2P3/2N2Q1p/PPPBBPPP/R3K2R w KQkq c6 0 1";
    var gs = try GameState.init(tricky_position);
    const backup = gs.backup();
    gs.bitboards[PE.P.int()].b8 = true;
    gs.enpassant = null;
    gs.castling = .{};
    gs.side = .black;
    gs.restore(backup);
    try std.testing.expectEqual(false, gs.bitboards[PE.P.int()].h8);
    try std.testing.expectEqual(Square.c6, gs.enpassant.?);
    try std.testing.expectEqual(@as(u4, 0b1111), @bitCast(u4, gs.castling));