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Carballo Chess Engine

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

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:

Test suite Time per position Version 1.8 Version 1.7
WinAtChess (New) 1 second 293/300 293/300
SilentButDeadly 1 second 125/134 125/134
ECMGCP 1 second 113/183 112/183
ECMGCP 5 seconds 156/183 154/183
Arasan 19a 60 seconds 52/200 52/200

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

Authors

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

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Carballo Chess Engine

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