Elixir Chess Engine

Elixir is a UCI-compliant chess engine developed in C++. It is the successor of my previous engine, Shuffle, and is under development with the goal of achieving an ELO rating of 3500+

Ratings

	CCRL BLitz	CCRL 40/15
v3.0	-	-
v2.0	3018	2938
v1.0	2828	-

Instructions to build

Building Elixir requires Make and latest versions of clang++ or g++ with C++20 support

make CXX=<compiler>

• Replace <compiler> with your preferred compiler - either clang++ or g++
  • If not specified, the compiler defaults to clang++
• Upon successful compilation, the binary file will appear in the root directory as elixir or elixir.exe depending on your operating system
  • You can also give a custom name to the binary by specifying it as EXE=<name> during compilation

Alternatively, you can download pre-compiled binaries from the Releases page

Note

Elixir utilizes a third-party tool - INCBIN to embed NNUE weights into the binary. However, INCBIN is not fully compatible with clang++ on Windows. If you're using clang++ to compile Elixir on a Windows system, ensure that the nnue.bin file is located in the same directory as the compiled binary.

UCI Options

Name	Type	Default value	Valid values	Description
Hash	integer	64	[4, 1048576]	Memory allocated to the Transposition Table (in MB).
Threads	integer	1	[1, 1024]	Number of Threads used to search.

Features

• Search : Standard PVS with Quiescence Search and Iterative Deepening
  • Aspiration Windows Search
  • Check Extension
  • Repetition Draw Detection
  • Insufficient Material Draw Detection
  • Mate Distance Pruning
  • Transposition Table (TT) cutoff
  • Internal Iterative Reduction
  • Razoring
  • Reverse Futility Pruning
  • Null Move Pruning
  • Late Move Pruning
  • Futility Pruning
  • Static Exchange Evaluation (SEE) Pruning
  • Late Move Reductions
  • Improving Heuristic
• Move Ordering
  • TT Move ordering
  • Good and Bad Captures ordering using SEE
  • Killer Move Ordering
  • Counter Move History
  • Continuation History
    • 1-ply Continuation History
    • 2-ply Continuation History
  • Butterfly History
• Evaluation : Neural Network based evaluation (NNUE)
  • Architecture: (768 -> 1024)x2 -> 8
  • Material Output Buckets: 8
  • Input King Buckets: $${\color{yellow} \text{Not implemented yet}}$$
  • Data Size: 3.3B positions

Important

As of version 2.0, Elixir no longer utilizes Hand-Crafted Evaluation (HCE). The following information is retained here for reference purposes.

• Evaluation : Hand Crafted Evaluation (HCE) using texel-tuned values
  • Tapered Evaluation
  • Material + Piece Square Table Evaluation
  • Piece Mobility Evaluation
  • Supported Pawn Bonus
  • Pawn Duo Bonus
  • Pawn Bonus attacking opponent Majors & Minors
  • Pawn Bonus if opponent has no Majors
  • Stacked Pawn Penalty
  • Isolated Pawn Penalty
  • Knight Outpost Bonus
  • Bishop Pair Bonus
  • Passed Pawn Bonus
  • Rook Open & Semi-Open File Bonus
  • King Open & Semi-Open File Penalty
  • King Zone Attacker Bonus
  • Pawn Shelter Bonus
  • Pawn Storm Penalty
  • Tempo Bonus

Acknowledgements

• A special thanks to Ciecke, Zuppa, Yoshie, Shawn, A_randoomnoob, Gabe and everyone else on the StockFish Discord server for their continuous help in developing and bug-fixing Elixir.
• This project would never have been where it is right now, if it was not for the help of everyone on my OpenBench instance - Aron, Yoshie, Shawn, Mid. Massive CPU time donations by these guys is what enabled me to heavily test and tune Elixir.
• Andrew Grant for developing the OpenBench platform.