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 a very slightly modified version of the library github.com/dylhunn/dragontoothmg, github.com/Cubix1729/dragontoothmg (with added null move support) for move generation and board state handling.

Evaluation Function

By default, Simplex uses a small NNUE network (with architecture 768 -> 128 -> 1).

A HCE (handcrafted evaluation function) is also available, with the following features:

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

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 or deactivate the 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.