Chess 4K Engine

Overview

A c++ chess engine that fits into 4KiB

It supports the following subset of the UCI protocol:

• uci
• isready
• position startpos moves ...
• go wtime ... btime ...
• quit

Additional support for non-4K builds:

• ucinewgame
• position fen ... moves ...
• go infinite

The first message is always regarded as uci due to the space restriction

Building

Building c4ke requires g++ that supports c++23:

git clone https://github.com/citrus610/c4ke
cd c4ke
make c4ke

To build c4ke-mini on Linux, run:

make mini

Minification

The minifier isn't general-purpose and won't work on all c++ code

• Removing whitespaces
• Removing single-line comments
• Removing #ifdef blocks (usually for OpenBench compliance code)
• Finding and replacing #define constants
• Renaming identifiers

Features

Board representation

• Bitboard
• 8x8 Mailbox
• Hyperbola Quintessence move generation

Search

• Iterative deepening
• Aspiration windows
• Lazy SMP
• Principal variation search
• Quiescence search
  • Check evasions
• Transposition table
• Move ordering
  • TT move
  • MVV
  • Quiet history
  • Noisy history
  • Continuation history
    • 1-ply
    • 2-ply
  • Static exchange evaluation
• Selectivity
  • Pruning
    • Reverse futility pruning
    • Null move pruning
    • Late move pruning
    • Futility pruning
    • PVS SEE pruning
    • QS SEE pruning
  • Extensions
    • Singular extensions
      • Double extensions
      • Triple extensions
      • Multicut
  • Reductions
    • Late move reductions
• Static evaluation correction history
  • Pawn correction history
  • Non-pawn correction history
  • Continuation correction history
    • 1-ply
    • 2-ply

Evaluation

• Material
• Rank/file piece-square tables
• Bishop pair
• Mobility
• Tempo
• Rook on open file
• Rook on semi-open file
• Pawn threats
• Pawn push threats
• Pawn structure
  • Protected pawn
  • Doubled pawn
  • Passed pawn
  • Pawn phalanx
  • Passed pawn kings proximity
• King safety
  • King on open file
  • King on semi-open file
  • Pawn shield
  • King ring attack
  • Queenless attack
• Endgame scaling

Time management

• Soft limit
• Hard limit
• Best move's node count scaling

Thanks

• Members of the MinusKelvin discord server for helping me and allowing me into their OpenBench instance
• The following engines that I used as references:
  • ice4
  • 4ku