FIDE & Google Efficient Chess AI Challenge Submission

Ethereal and Cfish Chess Engine - Memory Optimized Version

Overview

Ethereal Chess Engine is a competitive chess engine optimized for memory-constrained environments. This version has been specifically tuned to operate within strict memory limits while maintaining strong playing strength. We also optimized Cfish. For this competition, we noticed that C was the best programming language in terms of memory efficiency, and better than C++.

Competition Specifications

Time Controls

• 10 seconds per game
• 0.1 second Simple Delay (unused time is not banked)
• Games played in pairs with color swapping

Hardware Constraints

• 5 MiB RAM limit
• Single CPU core @ 2.20GHz
• 64KiB compressed submission size limit

Game Rules

• Standard chess rules apply
• Automatic draws:
  • Threefold repetition
  • 50-move rule (no captures or pawn moves)
• Random opening positions from a curated selection

Memory Optimizations

Transposition Table

• Reduced to 1 MB from original size
• Modified tt_init to work with smaller keysize
• Switched from 2 MB hugepages to standard 4096 byte memory pages

Search Parameters

• Reduced MAX_PLY and MAX_MOVES parameters in types.h by 50%
• Optimized for memory efficiency while maintaining search depth

Pawn King Cache

• Reduced PK cache size from 2^16 to 2^13 elements
• Updated PK cache mask accordingly
• Balanced between memory usage and evaluation accuracy

Competition System

Rating System

• Skill rating modeled as Gaussian N(μ,σ²)
  • μ: estimated skill
  • σ: uncertainty factor (decreases over time)
• Initial rating: μ₀ = 600
• Ratings adjusted based on game outcomes

Validation Process

• Self-play validation episode on submission
• Automatic error detection and logging
• Performance monitoring throughout competition

Technical Implementation Notes

• Memory allocations optimized for 5 MiB RAM constraint
• Critical attention to memory page alignment
• Efficient state management for time control handling

Development Focus

• Memory efficiency without sacrificing playing strength
• Robust time management for 10s games
• Optimization for single-core performance

Cfish

File Size Reductions

The following were removed:

• Openings poly book
• Endgame book
• Print statements
• Chess 960 support

Also, the file was compressed first with 7z, then gz.

Memory Reductions

The memory reductions were the same as Ethereal. Other things were tried, but they did not help or hurt the performance.

Testing

Local testing was done between different versions to check for improvements or degradation as kaggle submissions were limited per day. The matchups were tracked in an excel sheet along with the specific commit.