a scheme for chess move flags

Design constraints

We want moves in a chess engine to have a small representation. One reason for this is because we want to minimize the size of entries in the transposition table; this entry size is 10 bytes in most chess engines. A common size for a move representation is 16 bits.

There is some amount of minimum required information we need to put into those 16 bits. These are:

• Source square
• Destination square
• Promotion piece, if this move is a promotion

Squares are generally represented by an integer from 0 to 63 (6 bits). There are four possible pieces a pawn can promote to (knight, bishop, rook, queen), and having an additional value for "not a promotion" would mean this field requires 3 bits. So, a simple implementation would use 15 bits for the move representation.

There is some remaining space we can use for additional information. Some additional information we would want to look up about a move would include:

• Is this move a capture?
• Is this move a castling move?
• Is this move a double pawn push? (i.e. Do we need to update en passant state?)
• Is this move an en passant capture?
• What is the piece type that is being moved?

By encoding this information in the move, we avoid the need to have to look up this information. By switching on move flags with more information, we also reduces the amount of branching we need to do in our makemove function.

If we use the obvious representation of 12 bits for source/destination squares, You can use less than 12 bits for encoding this, but encoding/decoding gets expensive. this leaves us 4 bits for move flags. This is enough to encode most of the above except for the piece type.

The Scheme

Rationale

“Make common operations fast”. Detecting capture or promotion is a bit test. Piece types were ordered NBRQ because this is the common existing relative ordering for piece types.

Example implementation of utility functions:

bool is_capture() const { return value & 0x8000; }
bool is_promo() const { return value & 0x4000; }
PieceType promo() const { return ((value & 0x3000) >> 12) + PieceType::knight; }
bool is_en_passant() const { return (value & 0xF000) == MoveFlags::en_passant; }
bool is_castle() const { return (value & 0xE000) == 0x2000; }