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Travis Appveyor

Daydreamer 2.0 (pre-release)

Daydreamer is a chess-playing program I have been writing in my spare time. I named it Daydreamer after a bug in an early version caused it to occasionally follow up very strong play with bizarre blunders, as though it had lost its focus on the game and its mind was wandering aimlessly.

Changes from 1.75 to 2.0

2.0 is a total rewrite in Rust. I took the opportunity to modernize Daydreamer's internals as well. The biggest differences are a move from 0x88 board representation to bitboards and the introduction of phased move generation. There are also big changes to input handling and time management, largely due to differences between C and Rust.

The evaluation function is mostly unchanged, but there are some tweaks due to board representation changes, and some terms that I haven't ported, including material balance calculations and endgame scaling. There are also a number of features that I haven't ported at all, notably support for books and endgame tablebases.

I've updated and tuned the search significantly. The biggest changes are better aspiration windowing and much more aggressive pruning.

Version 2.0 isn't complete yet; the current release is a preview.

Changes from 1.7 to 1.75

  • Several bug fixes for ctg book support. Move selection should now behave much more intuitively.
  • Support for version 0.2 of Gaviota tablebases, which allows on-the-fly bitbase construction.
  • Overall better behavior for threaded tablebase loading. The background thread will no longer consume resources when not in use, and background thread loading can be disabled. Background loading has been significantly simplified, and now uses only one thread, rather than a pool.
  • Windows threading support no longer requires a pthreads dll.
  • Fixed a bug in pawn analysis.
  • Small tweaks to search parameters.
  • Changed verbose output format to prevent incorrect score reports in Arena.

Special thanks to Werner Schüle and Carlos Pesce for their help in identifying and fixing bugs in my tablebase and opening book implementations, respectively.

Changes from 1.6 to 1.7

The biggest new features in 1.7 are support for Gaviota endgame tablebases and support for opening books in Polyglot (.bin) or Chessbase (.ctg) format. To use the opening book support, turn on the 'OwnBook' option and set the path to the book using the 'Book File' option. The type of the book will be inferred from its file extension.

Tablebase support uses a new approach in which background threads load relevant bases into cache, and the search only directly uses values that already exist in cache. This approach dramatically reduces the amount of time spent waiting for TBs to load, improving performance in many endgame positions. The number of background thread can be controlled using the 'Endgame database thread pool size' option. I recommend setting this value no higher than the number of available cores on your machine.

In addition to the new features, Daydreamer 1.7 is significantly stronger than 1.6. This isn't due to any single big change; it's an accumulation of lots of small optimizations and tweaks. The biggest single improvement was probably deferring the expensive static exchange evaluation for captures in quiescence search until after the futility condition is applied, which is a trick I picked up from Stockfish.

  • Gaviota tablebase support
  • Polyglot book support (.bin)
  • Chessbase book support (.ctg)
  • Revamped material imbalance code, move ordering, and king safety
  • Endgame scaling for draw-ish endgames
  • Heavily retuned search, with new pruning and depth reduction/extension schemes
  • Lots of new evaluation terms
  • Numerous bugfixes and optimizations
  • New debugging and support routines


I'm the only person who actually writes the code of Daydreamer, but the ideas and structure of the code owe a lot to the chess programming community. I read through the source of a lot of open source engines before starting this project, and they've all influenced my design for Daydreamer. In particular, Fruit by Fabian Letouzey and the Viper/Glaurung/Stockfish series of engines by Tord Romstad (and Marco Costalba in the case of Stockfish) have strongly influenced the way I approached this project, and Daydreamer would be much worse without them.

I also got ideas from lots of other engines and programmers, and use some libraries that others have graciously made available:

  • Komodo, by Don Dailey and Larry Kaufman (eval terms and values)
  • Brutus, by Stephan Vermeire (CTG support)
  • Bison, by Ivan Bonkin (floating point depth management)
  • Crafty, by Bob Hyatt (eval terms and testing methodology)
  • ZCT, by Zach Wegner (endgame bitbase handling)
  • Dann Corbit and Michael Sherwin (null move depth reduction strategy)
  • Gaviota endgame tablebases, by Miguel Ballicora
  • Scorpio endgame bitbases, by Danial Shawul

I also had access to a lot of good writing about the design, implementation, and testing of chess engines. Bruce Moreland's site and the blogs of Jonatan Pettersson (Mediocre) and Matt Gingell (Chesley) have been particularly interesting.

Thanks to Jim Ablett for supplying optimized Windows compiles. Thanks also to everyone who has used or tested Daydreamer, to the authors of the many engines I've tested against, and to composers of EPD testing suites.

I also owe thanks to the developers of several support programs that I use to develop Daydreamer. In addition to my basic toolkit of vim, git, and gcc, I rely on cutechess-cli by Ilari Pihlajisto and Arto Jonsson, bayeselo by Remi Coulom, and pgn-extract by David Barnes.

Using Daydreamer

Daydreamer uses the universal chess interface (UCI) to communicate with graphical front-end programs. You can use it directly on the command-line, but that's not something most people will probably want to do. Daydreamer supports the UCI specification pretty faithfully, and supports ponder and MultiPV modes. At startup, it looks for a file named daydreamer.rc, and automatically executes any UCI commands that it finds inside. You can specifiy a different file to read from by passing a file name as an argument to the Daydreamer executable. You can use the rc file to automatically set options every time Daydreamer starts. For example, in my daydreamer.rc I have the following lines, which automatically register my Scorpio bitbases:

setoption name Endgame bitbase path value /Users/abecker/src/chess_engines/egbb/
setoption name Use endgame bitbases value true


If you have a C compiler that can handle C99, this should be easy. Just edit the CC variable in the Makefile to point at your compiler, set the compile flags you want in CFLAGS, and you should be good to go. If you compile with -DCOMPILE_COMMAND, you can pass a string that will be reported when you start the engine up. I find this pretty helpful for remembering what version I'm working with. I've tested with gcc on Mac and Linux, clang on Mac, and the MinGW cross-compiler for Windows. As of version 1.7, Daydreamer requires the pthreads library. This should already be installed on Mac and Linux machines, but on Windows it requires a separate install.


The whole thing is a single executable, so there's nothing to install, really. Windows builds also require the file 'pthreadGC2.dll' in the same directory. Just put it wherever you want. I've included a polyglot.ini file for compatibility with Winboard interfaces. Daydreamer looks for a file named 'daydreamer.rc' at startup and attempts to read UCI commands out of it, but aside from that there aren't any configuration files.

Changes from 1.5 to 1.6

My goal with the 1.6 release was to cram in all the features I want and get them all stable before trying to tackle parallel searching. Therefore, the number of new features is quite long. The most notable are ponder, multipv, and Chess960 support.

Here's the list of changes:

  • Chess960 support
  • MultiPV support
  • Ponder support
  • Scorpio bitbase support
  • Improved futility pruning
  • Razoring
  • Aspiration in root search
  • Weak square identification
  • Open and half-open file identification
  • Pawn storm bonuses
  • Improved fidelity to uci specifications
  • Revamped move ordering
  • Improved time management, and improved behavior at very fast time controls
  • Improved search behavior in positions with short mates or obvious moves
  • Fixed a bug that caused losses on time in some sudden death situations
  • Fixed a bug that prevented a hash size over 2GB
  • Fixed a bug that caused crashes in very long games (over 250 moves)
  • Fixed a bug that allowed incorrect en passant moves while in check

Thanks to Olivier Deville and Dann Corbit for their help identifying the time control and hash size bugs, and to everyone who tested version 1.5.

Changes from 1.0 to 1.5

Replaced the standard simplified evaluation with a custom job. The simple version is still available for testing purposes. In addition to re-doing the material valuation and piece-square tables, I added pawn evaluation (passed pawn identification is probably the single largest improvement over version 1 in terms of playing strength) and simple measures of mobility and king safety.

The board representation has been modified for efficiency, and the move generator is more sophisticated now. I've also completely revamped the move ordering (probably the second largest improvement) and added futility pruning to the search. History counters are now used both for futility and move ordering.

The result is a pretty significant jump in strength. Early on I underestimated the importance of good move ordering relative to raw nodes per second. For this release I focused much more energy on reducing the size of the search tree, with good results. Here are results from some testing matches I ran at 10s per side to give a rough idea of how the new release of Daydreamer stacks up:

Diablo 0.5.1    +30-20=0    30.0/50
Dirty ACCA 3    +32-13=5    34.5/50
Greko 6.5       +29-10=11   34.5/50
Sungorus 1.2    +24-13=13   30.5/50
Romichess P3K   +17-26=7    20.5/50
Bison 9.6       +5-34=11    10.5/50

and here are scores on two common (relatively easy) suites of problems, solved at 10s per problem:

WAC     293/300
ECMGMP  120/182


A chess engine, focused on search, w̶r̶i̶t̶t̶e̶n̶ ̶i̶n̶ ̶C̶ written in Rust.






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