ChessML

Note

Personal Learning Project
This is a personal hobby project created for learning chess programming and OCaml. It has no specific roadmap, goals, or direction—just exploration and experimentation. Code quality and features evolve organically as I learn. Contributions and suggestions are welcome, but expect things to change arbitrarily as I try new ideas!

I will concider this project done when I can no longer beat it 😎

A bitboard chess engine written in OCaml with UCI & XBoard protocol support.

Play chess against ChessML using any UCI- or XBoard-compatible GUI! ♟️

Learn with me here: https://alexanderbrevig.github.io/ChessML/

✨ Features

• ⚡Fast Move Generation: Efficient bitboard-based move generation
• 🧠Alpha-Beta Search: Minimax search with alpha-beta pruning
• 💾Transposition Tables: Position caching for faster search
• 📚Opening Book: Support for Polyglot opening books generated from PGN
• 🏗️Book Generator: 1M+ position-move combinations from 9M+ games
• 🖥️UCI Protocol: Compatible with chess GUIs like Arena and ChessBase
• 🎮XBoard Protocol: Play via the xboard/winboard interface

🚀 Quick Start

Install Dependencies

# Ubuntu/Debian
sudo apt install opam stockfish

# Initialize OCaml environment
opam init
opam switch create 5.3.0
opam switch 5.3.0
eval (opam env)
opam install dune alcotest ocaml-lsp-server ocamlformat

Build the Engine

# Development build (debug mode, faster compilation)
dune build

# Release build (optimized, ~30-40% faster performance)
dune build --profile=release

⚠️ Performance Note: For benchmarks, testing, or actual gameplay, always use --profile=release! Release mode enables critical optimizations (inlining, flambda, bounds check elimination) that significantly improve search speed.

• Debug mode: ~86K NPS
• Release mode: ~120K NPS (+40% faster)

Opening Book Creation and Setup

![WARN] You will need 4.5 GiB RAM to run this. It takes ~15 minutes to complete the process and generates 18MB of data.

./scripts/download_openings.fish # about 6 minutes and 30 seconds
dune build --profile=release && CHESSML_PARALLEL=8 ./_build/default/bin/create_book.exe # about 8 minutes

You should the see something like

📊 Processed 9065131 games total (avg 7.9 plies per game)

📝 Building book entries (min 3 games)...
   • 1113046 unique position-move combinations
   • 682561 unique positions

💾 Writing book.bin...
   • 17808736 bytes
   • 1113046 moves

ChessML will automatically look for the opening book in the following locations (in order):

1. Current directory: ./book.bin
2. XDG data directory: $XDG_DATA_HOME/chessml/book.bin (typically ~/.local/share/chessml/book.bin)
3. User directory: ~/.chessml/book.bin
4. System-wide: /usr/local/share/chessml/book.bin
5. System-wide: /usr/share/chessml/book.bin

Cutechess will not find the book, so please install it. To install the opening book for your user:

# Create the directory
mkdir -p ~/.local/share/chessml

# Copy the book file
cp book.bin ~/.local/share/chessml/

🔧 Development

ChessML uses Just as a command runner see just list.

Quick Start

# Build the project
just

# Run all tests (includes quick search tests ~11s)
just test

# Run deep search validation (~2+ minutes)
just test-search

# Format code
just format

# Watch mode (rebuild on changes)
just watch

Using Dune Directly

You can also use dune commands directly:

# Development build (debug mode)
dune build

# Release build (optimized - use for performance testing!)
dune build --profile=release

# Run all tests (quick by default ~11s)
dune runtest

# Run quick search tests only
dune exec test/engine/test_search.exe -- -q

# Run slow/deep search tests
dune exec test/engine/test_search.exe -- test slow_tests

# Test UCI engine manually
echo "uci
setoption name MaxDepth value 3
position startpos
go depth 2
quit" | ./_build/default/bin/chessml_uci.exe

# Test XBoard engine manually
printf "xboard\nprotover 2\nnew\nusermove e2e4\nquit\n" | ./_build/default/bin/chessml_xboard.exe

# Performance benchmarks (ALWAYS use --profile=release!)
dune exec --profile=release examples/bench_breakdown.exe

⚙️ Configuration

The engine supports runtime configuration via both UCI and XBoard protocols:

UCI Configuration

# Set maximum search depth (1-50)
setoption name MaxDepth value 8

# Set quiescence search depth (1-20)
setoption name QuiescenceDepth value 6

# Enable/disable quiescence search
setoption name UseQuiescence value true

# Enable/disable transposition table
setoption name UseTranspositionTable value true

# Set hash table size in MB
setoption name Hash value 64

XBoard Configuration

# Configure via option commands
option MaxDepth=8
option UseQuiescence=true
option DebugOutput=false

💪 Strength Levels

• Depth 1-3: Beginner 🐣 (very fast)
• Depth 4-5: Intermediate 🎯 (default, ~1-10s per move)
• Depth 6-8: Advanced 🔥 (slower, stronger)
• Depth 9+: Expert 🏆 (very slow, tournament strength)

🧪 Testing

ChessML uses a comprehensive test suite with both quick and thorough validation:

Test Categories

• Core Tests: Data structures, move generation, position handling
• Engine Tests: Evaluation, game logic, zobrist hashing
• Search Tests:
  • Quick (~10s): Basic functionality, shallow search validation
  • Slow (~2+ min): Deep search validation, performance analysis
• Integration Tests: End-to-end functionality

Running Tests

# Quick development feedback (~11s total)
dune runtest
# or
just test

# Deep search validation when needed
just test-search

# Run specific test categories
just test-core          # Core data structures
just test-engine        # Engine functionality
just test-integration   # Integration tests

The test suite automatically runs quick tests by default to keep development cycles fast, with deep validation available when needed.

🎲 Play Against ChessML

Cutechess install

sudo apt install git build-essential cmake qtbase5-dev qtbase5-dev-tools libqt5svg5-dev
git clone git@github.com:cutechess/cutechess.git
cd cutechess
mkdir build
cd build
cmake ..
make
cp cutechess cutechess-cli /usr/local/bin # copy binaries to some directory in your path or add $PWD to $PATH

Launch cutechess, add the engine in settings (I recomment to use xboard version) and create a new game.

📖 Documentation

• docs/README.md - Intro to ChessML

🙏 Credits

• Chess Programming Wiki
• Stockfish - Reference implementation
• Real World OCaml
• OCaml Manual
• PGNMentor - Opening PGN database

🤝 Contributing

Contributions and suggestions are welcome! Please see CONTRIBUTING.md for guidelines.

📄 License

ChessML is licensed under the MIT License - see the LICENSE file for details.

📝 TODO

• Figure out if I can publish the book.bin generated from PGNMentor
• Tune evaluation parameters
• Add endgame tablebase support
• Improve parallel search performance

Running Engine Matches

# Run a match against Stockfish (skill level 0)
cutechess-cli \
   -engine name=ChessML cmd=./_build/default/bin/chessml_xboard.exe proto=xboard \
   -engine name=Stockfish cmd=stockfish proto=uci option."Skill Level"=0 \
   -each tc=40/300 \
   -rounds 2 \
   -repeat \
   -pgnout "games.pgn" \
   -recover