isValid

Chess.sol

contract Chess {
    /// @notice Determines whether a move is valid or not (i.e. within bounds and not capturing
    /// same colored piece).
    /// @dev As mentioned above, the board representation has 2 sentinel rows and columns for
    /// efficient boundary validation as follows:
    ///                                           0 0 0 0 0 0 0 0
    ///                                           0 1 1 1 1 1 1 0
    ///                                           0 1 1 1 1 1 1 0
    ///                                           0 1 1 1 1 1 1 0
    ///                                           0 1 1 1 1 1 1 0
    ///                                           0 1 1 1 1 1 1 0
    ///                                           0 1 1 1 1 1 1 0
    ///                                           0 0 0 0 0 0 0 0,
    /// where 1 means a piece is within the board, and 0 means the piece is out of bounds. The bits
    /// are bitpacked into a uint256 (i.e. ``0x7E7E7E7E7E7E00 = 0 << 63 | ... | 0 << 0'') for
    /// efficiency.
    function isValid(uint256 _board, uint256 _toIndex) internal pure returns (bool) {
        unchecked {
            return (0x7E7E7E7E7E7E00 >> _toIndex) & 1 == 1 // Move is within bounds.
                && ((_board >> (_toIndex << 2)) & 0xF == 0 // Square is empty.
                    || (((_board >> (_toIndex << 2)) & 0xF) >> 3) != _board & 1); // Piece captured.
        }
    }
}