Reckless – Bitboard Chess Engine

Reckless is a UCI chess engine written in Rust as a personal project.

Guided by the insights from the chess programming community, it fearlessly combines established concepts, as the name suggests.

Features

Board representation

• Bitboards with Little-Endian Rank-File Mapping
• Magic Bitboards for sliding piece attacks
• Copy-Make approach

Move generation

• Pseudo legal move generator
• Pre-calculated attack maps using Fancy Magic Bitboards
• Magic numbers are pre-generated using Reckless Magics
• Compile time generation of move maps using a build script

Search

• Fail-Soft Alpha-Beta
• Principle Variation Search
• Quiescence Search
• Iterative Deepening
• Aspiration Windows
• Lockless Transposition Table
• Lazy SMP (Shared-Memory Parallel)

Selectivity

Pruning

• Reverse Futility Pruning
• Null Move Pruning
• Razoring
• Futility Pruning
• Late Move Pruning
• Delta Pruning
• Static Exchange Evaluation Pruning
• Mate Distance Pruning

Reductions

• Fractional Late Move Reductions
• Internal Iterative Reductions

Extensions

• Check Extensions

Move ordering

• Hash Move
• Killer Move Heuristic
• SEE with MVV-LVA as a tiebreaker for captures
• History Heuristic for quiet moves
  • Butterfly History
  • Counter Move History
  • Follow-Up History

Evaluation

• NNUE
• Architecture: (768 -> 384)x2 -> 1
• Activation Function: SCReLU (Squared Clipped Rectified Linear Unit)
• Quantization: i16 (256/64)
• Trained on original data generated entirely through self-play
• Handwritten SIMD for AVX2 instructions

Time management

• Soft and hard bounds
• Best move stability
• Distribution of root nodes

Rating

Version	CCRL Blitz	CCRL 40/15	Release Date
Reckless v0.6.0	3387 +/- 23 [#73]	3311 +/- 21 [#78]	Mar 22, 2024
Reckless v0.5.0	3243 +/- 19 [#94]	3213 +/- 21 [#94]	Feb 4, 2024
Reckless v0.4.0	2933 +/- 19 [#151]	2929 +/- 21 [#158]	Dec 13, 2023
Reckless v0.3.0	2617 +/- 20 [#229]	2615 +/- 21 [#251]	Nov 6, 2023
Reckless v0.2.0	2358 +/- 19 [#333]		Oct 7, 2023
Reckless v0.1.0	2020 +/- 20 [#471]		May 16, 2023

Getting started

Precompiled binaries

You can download precompiled builds from the GitHub Releases page.

• x86_64-v1: Slowest, compatible with any x86-64 CPU.
• x86_64-v2: Faster, requires support for POPCNT, SEE3, etc.
• x86_64-v3: Even faster, requires support for AVX2, etc.
• x86_64-v4: Fastest, requires support for AVX512.

For detailed information on the specific features needed for each level, refer to the x86-64 microarchitecture levels Wikipedia page.

Building from source

To build the engine from source, make sure you have Rust 1.65 or a later version installed. If you don't have Rust, follow the official Rust installation guide.

The source code doesn't include an NNUE model to keep the repository lightweight. Instead, models are stored in the Reckless Networks repository.

You can download the latest model using the fetch-nnue.sh (or fetch-nnue.ps1 on Windows) script, which will be placed in the networks directory and automatically detected during the build.

Alternatively, you can specify the path to the model file using the EVALFILE variable:

# Build the engine with a `./networks/model.nnue` model
make

# Build the engine with a specific model
make EVALFILE=networks/model.nnue

Keep in mind that older models may be incompatible with the current version of the engine.

Usage

Reckless is not a standalone chess program but a chess engine designed for use with UCI-compatible GUIs, such as Cute Chess or ChessBase.

Alternatively, you can communicate with the engine directly using the UCI protocol.

Custom commands

Along with the standard UCI commands, Reckless supports additional commands for testing and debugging:

Command	Description
perft <depth>	Run a perft test to count the number of leaf nodes at a given depth.
bench <depth>	Run a benchmark on a set of positions to measure the engine's performance.
eval	Print the static evaluation of the current position from white's perspective.

Acknowledgements

• OpenBench is the primary testing framework powered by Cute Chess.
• Open source chess engines, like Stockfish, Ethereal, Berserk, and numerous others, for serving as inspiration and providing ideas that fuel development.
• Stockfish Discord server for providing relevant insights and feedback.
• Chess Programming Wiki for contributing to the project's foundation.
• Many thanks to the CCRL team and all chess engine testers for their valuable contributions.

License

This project is licensed with the MIT license.