Command line chess program written in Ruby.
Standard Algebraic Notation with PGN standards is used for both display and input. A few clarifying notes regarding input:
- A move that comes with check or checkmate should have + or #, respectively, appended to the end of the input, but this is not required. The move list will show the symbols regardless.
- O-O and O-O-O are used for castling (the letter, not zero)
- En passant capture is entered as if capturing a pawn that had only moved 1 square (no e.p. at end)
- Disambiguation is required if more than one of the specified piece can move to the target square (e.g. Nbd2 to specify that the Knight on the b-file is meant to move to d2; R1e1 to specify that the Rook on the 1st rank is meant to move to e1)
- Promote a pawn to a Queen, Rook, Bishop, or Knight by appending to the move =Q, =R, =B, or =N, respectively
Enforces the rules of chess, including:
- King cannot move into or ignore check
- Castling is only legal if:
- King is not in check
- Neither piece has moved
- King will not move into check along the path to its destination square
- En passant is only legal immediately following opponent's two-square pawn move
- Checkmate if player to move has no legal moves and their King is in check
- Stalemate if player to move has no legal moves and their King is not in check
- Threefold repetition if position has occured three times, including the following considerations:
- Same player to move
- Same castling rights
- Same en passant rights
- Draw if during the last 50 moves no capture or pawn move has occurred
- Draw if insufficient mating material
User can enter any of the following commands instead of a move:
- help - show commands
- flip - flip board
- draw - offer opponent a draw (the engine will never accept a draw offer)
- resign (the engine will never resign)
- quit (alias: exit)
- save
- tutorial - shows instructions and examples on inputting moves
A note on display: the pieces are tragically small, but can get a bit larger depending on the font in terminal.
The chess engine uses a minimax algorithm with alpha beta pruning, but only very modest depths are searched within a reasonable amount of time. A collection of about 4,500 games by Victor Korchnoi is used as the opening database. (Note: The engine depth on Repl.it is by necessity set particularly low.)
There is an option to play through games from a handful of major chess events over the last half century. When in this mode, there are the following 4 controls:
- m - enter next move
- p - play all moves at a moderate tempo (every 2 seconds)
- z - zoom through moves at a brisk tempo (every 0.2 seconds)
- q - quit
-Andrew Hayhurst