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Move Generation: Other Pieces

Last time we looked at the pawn moves in detail which resulted in a fair amount of code due to all the special cases required by pawn moves (en-passant, double moves, can only capture diagonally). This time we will cover the moves of all the other pieces.

With the exception of castling, none of the other pieces have any ‘special’ moves or rules so we can handle them by using an offset table. The table contains a set of values for each piece, one for each possible direction the piece can move. To move the piece in a particular direction by one step you just add the value to the current cell location.

For example, looking at the values for the rook offset table:

This is a bit more obvious if we show the values for each direction on a chess board where CELLS_PER_RANK is 16 for an 0x88 board:

rook_moves

So, to go up 1 rank (go North) we add 16 to the current cell. To go down 1 rank we add -16 to the current cell.

This works well for the king as we can specify the offsets for all 8 directions it can move and that fully describes the possible locations for the king. But how do we handle sliding pieces that move until they bump into another piece or the edge of the board? Simple – we just keep adding the offset for the current direction in a loop until we reach a stopping condition:

Putting this all together we can write the get_valid_moves_table() function that handles the moves for all non-pawn pieces, sliding and otherwise.

The function is called with the cell_index containing the piece to find the valid moves for, a pointer to the relevant piece offset table and a flag that specifies whether the pieces is sliding (single == FALSE) or not (single == TRUE).

Looking back at the get_valid_moves() function we can see how we have now defined the moves for all the pieces with the exception of castling which we will cover next time.

 

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