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Maxwell Chess Engine

A Chess engine written from scratch in Rust.
If you use this code verbatim, or as a reference, please credit me!

Rating Lists featuring Maxwell: CCRL Blitz | MCERL
Play against Maxwell on Lichess! | Some of Maxwell's Games

Rough Roadmap

  • Tweak search values & thresholds
  • Internal Iterative Deepening
  • Static Exchange Evaluation
  • Late Move Pruning
  • Write an NNUE implementation! I've learned how Neural Networks work, so I'm really excited to get started on that
  • Multithreading

Features

Parameters

  • fen=<FEN STRING>: Sets up the board by a fen string (Doesn't work for UCI games) (default=STARTING_FEN)
  • debug_output=<BOOLEAN>: Toggle debug output that gets outputed per ply (default=true)
  • opening_book=<BOOLEAN>: Toggle built-in opening book (default=false)
  • time_management=<BOOLEAN>: Toggle time management, if false the bot will use all the remaining time (default=true)
  • hash_size=<INTEGER>: Sets the hash size in Megabytes, there's also a UCI option for this under the name "Hash" (default=256)

UCI Interface

  • uci, isready, ucinewgame, position, go, stop, and quit commands
  • "position" is only implemented for "position startpos", "position fen" is not yet implemented
  • "Hash" UCI option, which sets the hash / transposition table size in Megabytes

Board Representation

  • Purely bitboards
  • Supports loading from FEN strings

Move Generation

  • Basic handcrafted opening book
  • Magic bitboards for sliding pieces
  • Hardcoded pawn movement
  • Bitboard masks for other pieces calculated at startup
  • Calculates pseudo-legal moves, then skips illegal moves in move loop

Evaluation

  • Material count
  • Piece square tables
    • Separate middlegame and endgame tables for pawns and kings
  • Passed, isolated and doubled pawns
  • Attacked squares around kings

Move Ordering

  • Best move from the previous iteration, otherwise whatever move from the transposition table
  • MVV-LVA
  • 2 Killer Moves
  • History Heuristic
    • Indexed by side to move, move start square, move end square

Search

  • Single Threaded ~ For now ;)
  • Negamax
  • Iterative Deepening
  • Alpha-Beta Pruning
  • Late Move Reductions
  • Principal Variation Search
  • Reverse Futility Pruning (Static Null Move Pruning)
  • Null Move Pruning
  • Razoring
  • Internal Iterative Reductions
  • Quiescence Search
    • Delta Pruning
    • No TT Lookups
  • Transposition Table
    • UCI "Hash" option to change max size, default is 256 MB
    • Replacement scheme prefers higher depth and exact evaluation bound
  • Search Extensions
    • Checks
    • Pawn moves to the 2nd or 7th rank
  • Time management
    • If less than 7 moves have been played, it uses 2.5% of it's remaining time, otherwise 7%
    • This value is then also clamped between 0.05 and 30.0 seconds

Helpful Sources & References

Thanks to Sebastian Lague for making his YouTube series, which inspired me to make my own engine!

When I'm not sure what to do next, I like to read through other engine's code for ideas.
I try not to copy line for line, but in any case here are the engine's I've referenced:

And some other helpful resources