Tantabus

Tantabus is a WIP hobby Chess and Chess960 engine.
It is a "rewrite" of Lunatic.
The code is restructured and a bit cleaner, and it also uses my own cozy-chess library in place of chess.
Play me on lichess: https://lichess.org/@/TantabusEngine.

Features

Movegen

• Fixed shift fancy black magic bitboards using cozy-chess

Search

• Principal variation search
• Aspiration windows
• Transposition table
  • "Always replace" replacement scheme
• Quiescence search
• Extensions
  • Check extensions
• Reductions
  • Late move reductions
  • History reductions
  • Internal iterative reductions
• Pruning
  • Null move pruning
  • Futility pruning
  • Reverse futility pruning
  • Late move pruning
  • Negative SEE moves pruned in QSearch
• Move ordering
  • Hash move
  • Capture moves
    • Sorted with static exchange evaluation with capture history for tiebreaks
    • Losing captures delayed to last
  • Killer moves
  • History heuristic
• Lazy SMP
  • Extremely simple implementation where the TT is shared among N search threads
  • Lockless transposition table implemented using relaxed atomics and the XOR trick
  • 4 threads measured as approximately 148 Elo stronger than 1 thread in 10,000 games

Evaluation

• HCE
  • No longer exists, used to train NNUE
  • Automatically tuned with currently private tuner on the lichess-big3-resolved dataset
  • King relative symmetric piece-square tables
    • Dedicated passed pawn tables
  • Mobility evaluation (simple pseudo-legal counting)
  • Bishop pair bonus
  • Rook on open file bonus
  • Rook on semiopen file bonus
  • Basic king safety using "virtual queen mobility" and attacked squares around the king
  • Tapered/phased evaluation (using Fruit-like method)
• NNUE
  • Simple (768 -> 128)x2 -> 1 network architecture
  • Data generated using custom program
  • Network trained using MarlinFlow
  • Two halves ordered by side to move and side not to move
  • Network quantized into integers for inference
  • Inference accelerated by custom SIMD code

Time management

• Uses a fixed percentage of time left
• Aborts after an iteration completes when a "soft limit" is exceeded
• Aborts during an iteration when a "hard limit" is exceeded

Thanks

A (potentially incomplete) list of very useful resources:

• The CPW wiki - Extremely useful site for chess programming
• the NNUE document - Extremely useful for NNUE

Many engines have been very useful resources in the development of Tantabus.
A (potentially incomplete) list of citations is listed in the code, annotated with // CITE comments.
A (potentially incomplete) list of special thanks in no particular order:

• Pali (Black Marlin author), for assisting me with various things during the development of Tantabus on top of being like, cool and stuff.
• Jay (Berserk author) for having hosted the OpenBench instance that Tantabus developed on, as well as suggesting various improvements.
• Andrew (OpenBench and Ethereal author) for OpenBench. OpenBench has been an immensely helpful tool for engine development. Ethereal is also a very influential engine.
• MinusKelvin (Cold Clear and Frozenight author), for being like, really cool and stuff.
• Anyone who donated CPU time to instance that hosts Tantabus.
• Other people I probably forgot about.