ice4

ice4 is a chess engine which fits in 4096 bytes. In particular, it is an executable shell script which automatically extracts C++ source code from itself using xz, compiles it using the default system C++ compiler, and executes the resulting binary.

It supports the following subset of UCI:

• uci
• ucinewgame
• isready
• position startpos moves ...
• go wtime <#> btime <#> ...
• quit

This subset is implemented in a space-efficient manner and so has some limitations. The first message is always interpreted as uci and none of the following messages may be interpreted as uci (following uci messages will be interpreted as ucinewgame instead). The wtime and btime parameters to the go command must be present immediately after go and in that order. This location requirement may limit GUI compatibility. Only Cute Chess is known to work.

OpenBench builds (make EXE=<name> or make ice4-ob) support some additional UCI features:

• position fen <fen> moves ...
• setoption name Hash value <mb>
• setoption name Threads value <threads>
• go infinite (stop is additionally supported for this form)
• wtime and btime are not subject to the location requirement mentioned above.

Features

High-level strategy

• 10x12 board representation
• Pseudolegal move generation
  • Underpromotions are not generated
• Twofold repetition detection
• Zobrist hashing
• Lazy SMP
  • Best move selected by depth tiebroken by score
• Iterative deepening
• Soft limit + hard limit time management
• Principal variation search
• Aspiration windows
  • Gradually widened
  • Results used if exact or fail-high
• Incremental evaluation

Search

• Transposition table cutoffs
• Quiescence search
• Internal Iterative Reductions
• Improving
• Reverse Futility Pruning
• Null Move Pruning
• Razoring
• Internal Iterative Deepening
• Pawn Protected Pruning
• Late Move Pruning
• Delta Pruning
• Check Extensions
• Late Move Reductions
  • Logarithmic base formula based on move index and depth
  • Increased reduction if TT move is capture
  • Adjusted based on move history
• Correction Histories
  • Updated using rolling average formula with squared depth
  • Pawn Correction History
  • Material Correction History
  • Countermove Correction History
  • Followup Correction History
• Move Ordering
  • TT move
  • MVV-LVA
  • Move Histories
    • Updated using gravity formula with squared depth
    • Plain History
    • Continuation History
    • Followup History

Evaluation

• King-relative pawn square table
• Piece rank tables for non-pawns
• Piece file tables for non-pawns
• Tempo
• Mobility (approximated)
• Bishop pair
• Rook on open file
• Rook on semi-open file
• Additional pawn evaluation
  • Isolated pawn
  • Protected pawn
  • Phalanx pawn rank table
  • Passed pawn rank table
  • Passed pawn distance to own king table
  • Passed pawn distance to enemy king table
• King safety
  • King on open file
  • King on semi-open file
  • Shield pawn count table
  • King ring attacks (approximated)

For performance reasons, mobility and king ring attack terms for the side not to move are approximated by using the value from the previous ply.

Acknowledgements

• This follows the TCEC 4K rules: https://wiki.chessdom.org/TCEC_4k_Rules
• Thanks to Analog for coming up with the name, logo, and various other contributions