helper functions

Chess.sol

contract Chess {
    /// @notice Takes in a board position, and applies the move `_move` to it.
    /// @dev After applying the move, the board's perspective is updated (see {rotate}). Thus,
    /// engines with symmterical search algorithms -- like negamax search -- probably work best.
    /// @param _board The board to apply the move to.
    /// @param _move The move to apply.
    /// @return The reversed board after applying `_move` to `_board`.
    function applyMove(uint256 _board, uint256 _move) internal pure returns (uint256) {
        unchecked {
            // Get piece at the from index
            uint256 piece = (_board >> ((_move >> 6) << 2)) & 0xF;
            // Replace 4 bits at the from index with 0000
            _board &= type(uint256).max ^ (0xF << ((_move >> 6) << 2));
            // Replace 4 bits at the to index with 0000
            _board &= type(uint256).max ^ (0xF << ((_move & 0x3F) << 2));
            // Place the piece at the to index
            _board |= (piece << ((_move & 0x3F) << 2));

            return _board.rotate();
        }
    }

    /// @notice Switches the perspective of the board by reversing its 4-bit subdivisions (e.g.
    /// 1100-0011 would become 0011-1100).
    /// @dev Since the last bit exchanges positions with the 4th bit, the turn identifier is updated
    /// as well.
    /// @param _board The board to reverse the perspective on.
    /// @return `_board` reversed.
    function rotate(uint256 _board) internal pure returns (uint256) {
        uint256 rotatedBoard;

        unchecked {
            for (uint256 i; i < 64; ++i) {
                rotatedBoard = (rotatedBoard << 4) | (_board & 0xF);
                _board >>= 4;
            }
        }

        return rotatedBoard;
    }

    /// @notice Generates all possible pseudolegal moves for a given position and color.
    /// @dev The last bit denotes which color to generate the moves for (see {Chess}). Also, the
    /// function errors if more than 105 moves are found (see {Chess-MovesArray}). All moves are
    /// expressed in code as shifts respective to the board's 8x8 representation (see {Chess}).
    /// @param _board The board position to generate moves for.
    /// @return Bitpacked uint256(s) containing moves.
    function generateMoves(uint256 _board) internal pure returns (uint256[5] memory) {}

    /// @notice Determines whether a move is a legal move or not (includes checking whether king is
    /// checked or not after the move).
    /// @param _board The board to analyze.
    /// @param _move The move to check.
    /// @return Whether the move is legal or not.
    function isLegalMove(uint256 _board, uint256 _move) internal pure returns (bool) {}

    /// @notice Determines whether a move results in a capture or not.
    /// @param _board The board prior to the potential capture.
    /// @param _indexAdjustedBoard The board bitshifted to the to index to consider.
    /// @return Whether the move is a capture or not.
    function isCapture(uint256 _board, uint256 _indexAdjustedBoard) internal pure returns (bool) {}
}