Motors

This repository contains 4 crates related to board games:

• gears: Board representation and general utility
• motors: Engines
• monitors: a WIP UGI client, will eventually include a GUI (text-based for now)
• pliers: Tuner for HCE eval weights

Currently, the most interesting part is probably the superhuman UCI Chess960, DFRC and Chess Engine CAPS-LiTE.

Motors is both the name of this GitHub repo,

and of the motors crate, which contains engines.

An engine typically consists of two parts, the searcher and the evaluation function, such as caps-lite. These parts can be freely changed, including during a match.

Searchers

CAPS

A chess searcher estimated at > 3k elo when paired with the hand-crafted evaluation function LiTE.

Current features:

• Alpha-beta Pruning Negamax
• Quiescent Search
• Move Ordering:
  • TT Move
  • Killer Move
  • Various History Heuristics
  • MVV and Capture History
  • SEE to partition captures into good and bad captures
• Transposition Table
• Iterative Deepening
• Internal Iterative Reductions
• Aspiration Windows
• Principal Variation Search
• Check Extensions
• Null Move Pruning with Verification Search
• Reverse Futility Pruning
• Futility Pruning
• Late Move Pruning
• SEE pruning in quiescent search
• Adjusts pruning and reduction margins based on eval difference to previous move
• Time Management with a soft and hard bound, as well as support for fixed time, depth, nodes, mate and infinite, or any combination of these
  • Almost full UCI compliance, including searchmoves, ponder, multipv, etc. Notable missing features are endgame tablebases and a built-in opening book.
• Eval function can be changed at runtime (see below)

Other Searchers

In addition to the Chess-playing Alpha-beta Pruning Search (CAPS), there is also the General Alpha-beta Pruning Search (GAPS, a game-agnostic engine, currently still very basic), and Random (a random mover). Except for Random, those engines can be combined with any evaluation function supporting the current game.

They are currently being worked on and should become significantly stronger in the future. Further plans include additional engines, like Minimalistic Alpha-beta Pruning Search (MAPS), a simple alpha-beta pruning search without any further techniques, and an MCTS searcher.

Evaluation Functions

LiTE

Linear Tuned Eval, a chess eval using a linear combination of weights which have been tuned using the pliers tuner. It can be interpreted as a single layers perceptron, a neural net consisting of a single neuron. Such an eval functions is also often called a Hand-Crafted Eval function (HCE). This is the default eval for chess.

PiSTOn

Piece Square Table Only eval, a chess eval using only piece square tables, similar to the well-known PeSTO engine.

MateOnCE

Material Only Chess Eval, a material-only evaluation function for chess, using the classical piece values 1, 3, 3, 5, 9.

BAtE

Basic Ataxx Eval, a very simple material counting eval for Ataxx.

BasE

Basic m,n,k Eval, a simple hand-crafted eval for m,n,k games.

LUTE

Linear Ultimate Tic-tac-toe Eval, a simple hand-crafted eval for UTTT.

Random

Returns random values. Still stronger than the random engine when used as eval function for an actual engine like caps or gaps.

Games

Currently, 4 games are implemented:

• Chess, including Chess960 (a.k.a. Fischer Random Chess) and Double Fischer Random Chess (DFRC)
• Ataxx, a challenging board game where the goal is to convert your opponent's pieces
• m, n, k games, a generalization of Tic-Tac-Toe that can actually be difficult. The current implementation is somewhat limited and does not support boards larger than 128 squares, nor does it (yet) support rules specific to variants such as Connect 4 or Gomoku.
• Ultimate Tic-Tac-Toe, a much more challenging version of Tic-Tac-Toe where every square is itself a Tic-Tac-Toe board.

Usage

Building

To build the engines, it is enough to type make or use cargo. cargo can also be used to build other parts, such as the match manager monitors or the tuner pliers. Individual games and engines can be included or excluded from the build through cargo features, but the default is to build everything. Alternatively, I'm planning to do a GitHub release soon, which will contain at least the engines binary.

Running

Starting the motors executable without any command line options will start the default game, chess, with CAPS, the default engine for chess, and LiTE, the default eval for chess. Coincidentally, this is also the strongest and most developed engine and eval. This engine can be used out of the box with any UCI chess GUI.

Manual User Input

All engines use the UCI or the very similar and mostly compatible UGI protocol for communicating with the GUI.

But this interface has also been designed to be easy to use for a human. Incorrect commands will generally produce helpful error messages. Typing the start of a command will list context-dependent autocompletion options.

Use output <name> to change how the engine prints the current position.

The default is pretty, a human-readably diagram of the current position, but it's also possible to generate alternative ASCII or UTF-8 diagrams, or export the FEN or PGN of the current match:

For example, typing show pgn will keep the output unchanged but export a PGN of the current match. To select the game Chess (this is already the default), type play chess. To select the engine GAPS with eval PiSTON, type engine gaps-piston. Names are case-insensitive; leaving out the eval will use the default eval for the current game, which is lite for chess. Alternatively, it's also possible to change the eval of an engine during the game without resetting the engine using set_eval (the eval command instead prints the static eval of the current position). There are many more options, this document is too short to list them all in detail.

Command line flags

Command line flags are handled similarly to user input at runtime, but are a bit more restrictive in some cases. For example, to play Ataxx with GAPS and the BAtE eval, pass the following command-line flags: --game ataxx --engine gaps-bate. bate is already the default eval for Ataxx, and GAPS is the default engine for Ataxx, so this is equivalent to just --game ataxx.

Thanks

Huge thanks to everyone who helped me learn more about engine programming!