Simplex Chess Engine

Simplex is a (very) small mid-strength UCI chess engine written in Golang.

To play against it, you can challenge @SomeGoBot on Lichess.

Features

Move generation

The program uses the library github.com/dylhunn/dragontoothmg for move generation and board state handling.

Yes, I know, this is a bit of cheating.

Evaluation Function

By default, Simplex uses a small handcrafted evaluation function with:

• Material evaluation
• Piece-square tables
• Pawn structure evaluation
  • Bonus for passed pawns
  • Penalties for doubled and isolated pawns
• Bonus for rooks on open/semi-open files
• King safety
  • Attacks near the king
  • King virtual mobility (as a queen)
• Bishop pair bonus
• King-pawn tropism rewarding the king being close to friendly or ennemy pawns in the endgame
• Tapered interpolation between middlegame/endgame scores for all features

The eval parameters aren't tuned, so they are probably suboptimal.

The UCI option Use NNUE can be used to switch to the (very bad for the moment) NNUE evaluation. This is currently a lot too slow (~50x slower than the HCE), so still an experimental feature.

Search

• Negamax with Alpha-Beta pruning
• Principal variation search (PVS)
• Late move reductions (LMR)
• Quiescence search
• Transposition table
• Iterative deepening
• Aspiration windows
• Move ordering with:
  • Hash move first
  • MVV-LVA
  • Killer heuristic
  • History heuristic
• Reverse futility pruning
• Razoring
• Futility pruning
• Late move pruning
• Delta pruning (for the quiescence search)

UCI Interface

Simplex supports the UCI (Universal Chess Interface) protocol (at least the basic commands). It has only two UCI options:

• Use NNUE to activate the experimental neural network evaluation
• Hash to set the size of the transposition table (in MB)

Contribute

Any contribution to this small project is welcome and appreciated! If you find a bug, want to suggest or add a feature, feel free to open an issue or a pull request.