Rune Chess Engine

Rune is a high-performance open-source chess engine written in C++. It supports the Universal Chess Interface (UCI) protocol and is designed for speed, accuracy, and configurability.

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Table of Contents

• First Release Highlights
• Features
• Search Algorithm
• Evaluation Algorithm
• Usage
• UCI Commands
• Configuration
• Development
• License

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First Release Highlights

Feature	Description
UCI Protocol	Full support for GUI integration
Opening Book	Precomputed openings with filtering of weak lines
Move Generation	Advanced and accurate move generation
Evaluation	Handcrafted evaluation function taking into account multiple important factors
Search	Negamax framework with alpha-beta pruning using multiple techniques
Perft Testing	Debugging and move counting

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Features

• Full UCI protocol support
• Advanced move generation and evaluation
• Opening book support
• Multi-threaded search
• Perft testing for debugging
• Configurable evaluation parameters:
  • Material, piece-square tables, mobility, bishop pair, pawn structure, king safety
• Search options:
  • Quiescence, transposition table, iterative deepening, beta cutoff history
• Custom commands for debugging and board display

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Search Algorithm

Rune uses a Negamax search framework with alpha-beta pruning, enhanced with several modern techniques:

• Iterative Deepening: Searches incrementally from shallow to maximum depth to improve move ordering and time management.
• Principal Variation Search (PVS): Optimizes the alpha-beta search by searching the first move with a full window and others with a null window.
• Quiescence Search: Extends search at leaf nodes to capture tactical moves and avoid horizon effect.
• Transposition Table: Stores previously computed positions to avoid redundant calculations.
• Null-Move Pruning: Skips moves when safe to quickly identify pruning opportunities.
• Late-Move Reduction: Reduces the search depth of certain moves if they appear later in the movelist.
• Move Ordering: Prioritizes moves using TT move, captures (MVV-LVA), checks, promotions, and beta cutoff history.
• Static Exchange Evaluation (SEE): Estimates material gain for potential captures to improve move ordering.
• Time Management: Adjustable think time per move with monitoring for timeouts.

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Evaluation Algorithm

Rune's evaluation is modular and phase-aware, combining several heuristics:

• Material Evaluation: Counts pieces with standard values and adjusts for side-to-move perspective.
• Piece-Square Tables (PST): Scores piece placement differently for opening, middlegame, and endgame phases.
• Mobility: Rewards pieces with more legal moves; penalizes king mobility outside endgame.
• Pawn Structure: Rewards passed pawns, penalizes doubled or backward pawns.
• Bishop Pair: Bonus for possessing a pair of bishops, scaled by game phase.
• King Safety:
  • Evaluates pawn shield in front of castled king.
  • Penalizes open/semi-open files near king.
  • Considers enemy attacks and central king exposure.
• Game Phase Detection: Dynamically determines if the position is in opening, middlegame, or endgame to scale evaluation weights appropriately.

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Usage

Rune runs as a UCI-compatible engine and can be used with any modern GUI such as Arena, CuteChess, or lichess-bot.

Example (command line):

./rune_v1

Example (with CuteChess):

cutechess-cli -engine cmd=./rune_v1 name=Rune proto=uci -engine cmd=stockfish name=Stockfish proto=uci -games 10 -each tc=5+0.1

UCI Commands

Rune supports all standard UCI commands:

uci – initialize engine and return options

isready – check readiness

ucinewgame – start a new game

position – set up a position (FEN or moves from startpos)

go – start calculating with parameters (depth, movetime, wtime/btime)

stop – stop calculation

quit – exit engine

Configuration

Rune-Specific Options

These internal options (from config.cpp) allow you to fine-tune Rune’s behavior:

Option Name	Type	Description
movegenDoLegalMoveFiltering	bool	Enable/disable filtering to only legal moves
movegenDoOnlyCaptures	bool	Restrict move generation to captures only
evalDoMaterial	bool	Enable/disable material evaluation
evalDoPieceSquares	bool	Enable/disable piece-square table evaluation
evalDoMobility	bool	Enable/disable mobility evaluation
evalDoBishopPair	bool	Enable/disable bishop pair bonus
evalDoPawnStructure	bool	Enable/disable pawn structure evaluation
evalDoKingSafety	bool	Enable/disable king safety evaluation
searchDoQuiescense	bool	Enable/disable quiescence search
searchDoTranspositions	bool	Enable/disable transposition table usage
searchDoBetaCutoffHistory	bool	Enable/disable beta cutoff history
searchDoInfo	bool	Enable/disable detailed search info output
searchInitialDepth	int	Initial search depth
searchMaximumDepth	int	Maximum search depth
searchMaximumQuiescenseDepth	int	Maximum quiescence depth
searchDoOpeningBook	bool	Enable/disable opening book usage

These can be modified via UCI setoption commands, for example:

setoption name evalDoMobility value 0
setoption name searchMaximumDepth value 12

Development

Rune is an actively developed project. Contributions are welcome via pull requests and issues.

Development priorities include:

Improved evaluation (NNUE integration planned)

Endgame tablebase support (Syzygy)

Better time management heuristics

Search pruning and extensions

If you are interested in contributing, check out the issues page.

License

Rune is released under the MIT License.

You are free to use, modify, and distribute Rune in both commercial and non-commercial projects, provided proper attribution is given.