Programmer Puzzle: Encoding a chess board state throughout a game

Question

Not strictly a question, more of a puzzle...

Over the years, I\'ve been involved in a few technical interviews of new employees. Other than asking the standard \"do you

Answer 1

Most of the answers overlooked 3 fold repetition. unfortunately for 3 fold repetition you have to store all the positions played so far...

The question required us to store in space efficient manner so we really don't need to store position as long as we can construct it from the list of moves (provided we have standard starting position). We can optimize PGN and thats it we are done. Bellow is a simple scheme.

There are 64 squares on the board, 64 = 2^ 6. If we store only the initial and final square of each move that would take 12 bits (Promotion will be tackled later). Note that this scheme already covers player to move, emphassant, piece captured, castling etc; as of these can be constructed from just replaying the move list.

for promotion we can keep a separate array of vectors which would say "at move N promote to Piece XYZ". we can keep a vector of (int, byte).

It is tempting to optimize (To,From) vector also, Since many of these (To,From) vectors are not posible in chess. eg. there wont be a move from e1 to d8 etc. But I couldnt come up with any scheme. Any further ideas are most welcomed.