Carballo Chess Engine

Carballo (the galician word for Oak, it's all about search trees) is an Open Source Java chess engine with three interfaces:

• UCI: a text interface for chess GUIs: https://github.com/albertoruibal/carballo/raw/master/carballo-uci-1.8.tgz
• HTML5: developed with Google Web Toolkit (GWT) using the Vectomatic SVG library: http://www.mobialia.com/webchessgwt
• Desktop: a swing application

It is organized into modules:

• Core: the chess engine
• Jse: the Java Standard Edition version with the UCI interface and JUnit tests
• Gwt: components needed for the GWT GUI
• GwtGui: an HTML5 interface developed by Lukas Laag, it depends on Core and Gwt
• Swing: the swing application code, it depends on Core and Jse

It is licensed under GPLv3 and the source code is hosted at https://github.com/albertoruibal/carballo. You are free to use, distribute or modify the code, we ask for a mention to the original authors and/or a link to our pages.

Features

• UCI interface for chess GUIs like Arena or SCID
• It includes a great GWT interface by Lukas Laag and a Java Swing GUI
• Based on bitboards with a magic bitboard move generator, it also includes code for magic number generation
• Move iterator sorting moves with four killer move slots, Static Exchange Evaluation (SEE), Most Valuable Victim/Least Valuable Aggressor (MVV/LVA) and history heuristic
• PVS searcher
• Aspiration window, moves only one border of the window if it falls out
• Transposition Table (TT) with zobrist keys (it uses two zobrist keys per board to avoid collisions) and multiprobe
• Quiescent Search (QS) with only good or equal captures (according to SEE) and limited check generation
• Internal Iterative Deepening to improve sorting
• Extensions: Check (only with positive SEE), pawn push, mate threat and singular move
• Reductions: Late Move Reductions (LMR)
• Pruning: Null move pruning, static null move pruning, futility pruning and history pruning
• Pluggable evaluator function, distinct functions provided: the Simplified Evaluator Function, other Complete and other Experimental
• Selectable ELO level with an UCI parameter
• Supports Chess960
• Polyglot opening book support; in the code it includes Fruit's Small Book
• FEN notation import/export support, also EPD support for testing
• JUnit used for testing, multiple test suites provided (Perft, BS2830, BT2630, LCTII, WinAtChess, etc.)

Test results in a Intel Xeon CPU limited to 2.0GHz for consistency:

His real strength is about 2400 ELO points, you can check his tournament rankings at http://www.computerchess.org.uk/ccrl/

Authors

• Alberto Alonso Ruibal: http://www.alonsoruibal.com
• Lukas Laag, developer of a great GWT SVG library (http://www.vectomatic.org) and the Carballo GWT interface

Building

Carballo uses the Gradle build system and the gradle wrapper.

Build all the jars and install them to your local Maven repository:

./gradlew publishToMavenLocal

UCI Interface

Build the UCI interface (creates a carballo-${version}.jar in jse/):

cd jse
../gradlew proguard

Running the UCI interface:

cd jse
java -jar carballo-1.8.jar

HTML5 Interface

Build the GWT interface:

cd gwtgui
../gradlew compileGwt

Running the GWT interface:

cd gwtgui
../gradlew appRun

and access with your web browser to http://localhost:8080/chess/

Desktop Interface

Building the Swing interface (creates a carballo-swing-${version}.jar in swing/):

cd swing
../gradlew proguard

Running the Swing interface:

cd swing
java -jar carballo-swing-1.8.jar

Testing

Run tests:

./gradlew test

Run the "Win at Chess" test suite:

cd jse
../gradlew slowTest --tests=WinAtChessTest

Run the "Silent but Deadly" suite:

cd jse
../gradlew slowTest --tests=SilentButDeadlyTest

Changelog

Version 1.8: Java 11, new desktop application and source code improvements

• Change the source code level to Java 11
• Use JitPack for publishing the artifacts
• Changed artifact group to com.github.albertoruibal.carballo
• Use the gradle wrapper 7.5.1
• Update the tests to JUnit 5.9.1
• HTML5 app updated to GWT 2.10
• Converted the old Applet to a Swing application
• Implemented the standard "UCI_LimitStrength" and "UCI_Elo" UCI options instead of the old "Elo" option
• Implemented "searchmoves" and "go depth" in the UCI interface
• UCI Interface: Now "isready" may be called after "position"
• Add pawn blockade also to the CompleteEvaluator
• More small bug fixes

Version 1.7: A new Late Move Reductions (LMR) formula

• New LMR formula with a progressive reduction based on the node eval diff and the move history
• Reduce the LMR tables size saving memory
• Do history pruning before LMR
• Prune moves with negative SSEs taking reductions into account
• Penalty for pawns in [D,E] in the initial square blocked by our own pieces
• Remove the pawn push extension
• A new PGN parser supporting variations, comments, NAGs, etc.

Version 1.6: Refactoring, bug fixes, endgames, etc.

• Set the CompleteEvaluator as the new default evaluator
• Add history pruning
• Make the singular move margin depth dependent
• Don't do razoring in positions with known wins
• Add pawn push extension
• Fixes in the Opening-Endgame values to two-shorts-in-one-int logic that allows to separate the piece values from the square tables
• Add logic to detect draws in KQKQ, KRKR, KRPKR, KQKP, KBPKB, KBPKN and KRPPKP endgames
• Reduce the pawn value in the opening
• Add space evaluation
• Add rook trapped logic and improve bishop trapped evaluating pawn guard
• Change passer pawn logic adding a king distance bonus, remove unstoppable passer logic
• Improve the king shield and pawn storm logic
• Do not eval only forward mobility in bishops and knights
• Scale the contempt factor to 0 in the endgame
• Fix a bug returning from the excluded search for the singular move extension
• Add KBKB draw recognition (with same color bishops) https://en.wikipedia.org/wiki/Rules_of_chess#Draws
• Start to scale from midgame to endgame with 6 minor pieces (previously it was with 4 pieces)
• Fix some concurrence problems adding thread locks in the SearchEngine and SearchEngineThreaded class
• Fix seldepth reporting in the UCI interface (sometimes seldepth was lower than depth): if a TT move is returned, add the depth analyzed in the TT

Version 1.5: More search and evaluation tuning

• Prune non capture moves with SEE < 0 when depthRemaining < 3
• Change the outside passer detection, now only if the pawn is in the files a, b, c or f, g, h and there are opposite pawns
• Increase the passer pawn bonuses to match the "Two connected passers on 6th are better than a rook"
• Increase the attack bonuses
• Revise the absolute/relative pin logic, adding the bonus one time by each pinned piece
• Increase the bishop trapped penalty
• Increase the pawn center bonuses in the opening
• Fix a bug in the pawnCanAttack bitboards
• Added GNU's Octave scripts to generate the mobility/attack/passer bonuses and the Piece-Square tables under scripts/octave
• Added a GCJ compilation script to generate native binaries for Linux

Version 1.4: Another step in the engine strength

• Better understanding of pinned pieces, generating attacks only from legal moves
• New logarithmic piece mobility bonuses
• New set of pawn bonuses and changes in passer pawn evaluation adding an unstoppable passer bonus
• Remove evaluator section Config & UCI parameters speeding up things
• Changes in king safety evaluation taking into account three more squares in front of the king and modifying bonuses
• Improve pawn shield logic and add pawn storm evaluation
• Avoid negative values in the Opening-Endgame (O-E) arithmetic
• Make the bishop pair and the tempo bonuses O-E
• Reduce the tempo bonus in the endgame
• Merge piece values in the piece-square tables
• Simplify the rook on 5th, 6th, 7th rank logic replacing it by a bonus for each pawn attacked by the rook
• Remove the queen on 7th rank logic
• Use the PV value in the TT as the search starting score in each depth iteration
• Change time management to use more time
• New futility and razoring margins by depthRemaining, extend futility to more PLYs
• In quiescence search (QS), do futility also for PV nodes and generate checks at depth 0 also for non-PV nodes
• As fractional extensions are no longer used, now PLY is 1
• Use unicode figurines in the text board and in the GWT Gui SAN notation
• Fix engine crash analyzing positions already mate

Version 1.3: A lot of work in the evaluation function for a better positional play

• Now AttacksInfo holds the attacks information by piece type
• This allows to improve mobility and king safety evaluation detecting squares attacked by less valuable pieces
• Removed the king defense bonuses from the Experimental evaluator
• New MOBILITY array holding mobility bonuses by piece type and number of destiny squares
• Simplified the Bishop's Capablanca rule
• Removed some rook in 7th rank logic
• Improved the pawnCanAttack squares detection removing squares that cannot be reached due to opposite pawns
• Fixed the unsupported pawn penalty
• Modify the "rook attacks backward pawn logic" to detect real backward pawns
• Evaluation refactoring using the "W" and "B" constants and the "us" and "them" variables
• The Attacks evaluation is now done in a separated evalAttacks() method, unifying the attack bonuses in the PAWN_ATTACKS, MINOR_ATTACKS and MAJOR_ATTACKS arrays
• Piece value constants moved to the Config class and removed the PIECE_VALUES array
• Better midgame-to-endgame evaluation scaling with the new NON_PAWN_MATERIAL_MIDGAME_MAX and NON_PAWN_MATERIAL_ENDGAME_MIN constants
• Remove some UCI parameters and change them by constants for a better running optimization

Version 1.2: A new MoveIterator, Chess960 and lots of UCI improvements

• New MoveIterator that generates only legal moves with a check flag set
• Added support for Chess960 (Fischer Random Chess)
• Implemented UCI Ponder
• New replace strategy for the TT, taking into account the entry depth and the generation difference
• In the TT in QS, store the entries with checks generated with depth 1 and entries without checks with depth 0
• Four killer move slots
• Set default razoring margin to 400
• Fix the futility in QS and set the default futility in QS to 80
• Remove the null move margin and improve the null move reduction calculation
• Remove the recapture extension
• New time management strategy adding a "panic time" when the search in the root node fails low by a margin of 100
• Do not penalize pinned pawns in the evaluators, multiply the hung pieces bonus by the number of hung pieces, and pawn center opening corrections
• Fixed a bug multiplying the opening/ending values in the evaluators by negative factors
• Generate piece-square values in different classes
• Fixed an important bug in the SWAP algorithm for the SSE evaluation
• Now it uses 1" for the WinAtChess tests

Version 1.1: Urgent bug fix for 1.0

• The UCI interface was ignoring all the UCI options
• Do not allocate memory inside ExperimentalEvaluator.evaluate()
• Added the new pawn and endgame logic to the CompleteEvaluator (ExperimentalEvaluator continues as the default)

Version 1.0: Lots of fixes, small advances in test results: 294 in WAC (5") and 2520 in BT2630.

• New Transposition Table with a separated slot for the eval values
• Now uses the TT in quiescence search
• Fix mate values before inserting them in the TT, now the mate problems are solved with the right distance to the mate
• Fixes to futility pruning in quiescence search
• New pawn classification in the ExperimentalEvaluator
• Now the PV line is shown every time that a move is found in the root node
• Implemented UCI seldepth, lowerbound, upperbound and hashfull
• Implemented the depth and node limit for the search
• Enabled the endgame knowledge
• In the search, assume that pawn pushes are to the 6th, 7th or 8th rank
• Extend only checks with positive SSE
• Disable by default the recapture and pawn extensions, set the mate threat extension to one full PLY
• Fix recapture extension (now disabled by default)
• Tests migrated to the JUnit 4 format with annotations, and created a "fastTest" gradle task to run only the fast tests
• C# code separated in another GitHub project

Version 0.9: Fixes evaluator bugs

• Reordered some logic in the ExperimentalEvaluator
• Fixed a bug in the detection of the 2 bishop bonus
• Fixed another bug in the detection of candidate passer pawns
• Bug in the detection of the adjacent columns found by Yonathan Maalo
• Some improvements in the HTML5 interface

Version 0.8: Build system and GWT modifications

• Project build system migrated from Maven to Gradle
• Solved some evaluator bugs
• New option to do not use Magic Bitboard Attacks: optimizes start time in HTML5
• Removed specific Bitboard attacks code from GWT, can be simulated setting BitboardAttacks.USE_MAGIC = false
• New ArrayBufferBook for GWT, it can process any opening book loaded as a JS ArrayBuffer

Version 0.7: A small leap on the engine performance and a big code clean

• Code moved to Github
• Integrated ROOT, PV and NULL nodes search routine
• Activated singular movement extensions and changed default singular extension margin
• Do null move only when the remaining depth is > 3 PLY
• No not overwrite the value in the TT if there is no room
• Converted code to C# using sharpen. At the moment only the core of the engine
• Solved a big bug getting the move from the transposition table
• Also found another bug on the search getting the last captured piece value
• And the complete evaluator had a bug calculating the attacks value

Version 0.6: Source code reorganization, GWT and PGN improvements, no changes on the engine code

• Code splitted in carballo-core, carballo-jse and carballo-applet
• Carballo-core is GWT-friendly
• Integrated SAN notation on Board class
• Improved PGN export with SAN notation
• Added a GWT interface based on the one by Lukas Laag (http://vectomatic.org) code

Version 0.5: Improves about 150 ELO points over Carballo 0.4

• PVS searcher: SearchEngine completely changed
• Futility pruning now works!
• New TT algorithm, now also uses TT to store evaluation values
• Bug with draw detection with 3-fold repetition
• Bug with time management on tournament, was using the opponent's time amount
• Bug with history table overflow

Version 0.4: First version integrated with Mobialia Chess

• Parameterizable evaluator
• Evaluator changes