Crystal is a UCI chess engine derived from Stockfish.
The official Stockfish github repository may be found here.
Crystal seeks to address the following issues where chess engines often have trouble:
1) Positional or tactical blindness due to over reductions or over pruning.
2) Draw blindess due to the move horizon (50 move rule).
3) Displayed PV reliability.
A few examples of what Crystal can do:
1) (Ba4+) 3B4/1r2p3/r2p1p2/bkp1P1p1/1p1P1PPp/p1P4P/PP1K4/3B4 w - -
2) (Draw) 4K1bn/5p2/5PpB/6P1/3k4/8/8/7q w - -
3) (f6) rk6/pP1p2p1/B7/3K1P2/8/8/7b/8 w - -
4) (Rd3) 8/5K2/3p3p/3p3P/pp1P4/rkb1R3/p1p3P1/n1B2B2 w - -
5) (Nd7) 1N1K1b1r/P3pPp1/4k1P1/rp1pB1RN/q4RP1/8/p2pB1p1/1b6 w - -
6) (b4) 3K4/1p1B4/bB1k4/rpR1p3/2ppppp1/8/RPPPPP2/r1n5 w - -
7) (Nc8) 8/1p1pNpbk/1q1P4/pP2p2K/P3N3/4P1P1/3P4/8 w - -
This distribution of Crystal consists of the following files:
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README.md, the file you are currently reading.
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Copying.txt, a text file containing the GNU General Public License version 3.
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AUTHORS, a text file with the list of authors for the project.
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src, a subdirectory containing the full source code, including a Makefile that can be used to compile Crystal on Unix-like systems.
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a file with the .nnue extension, storing the neural network for the NNUE evaluation. Binary distributions will have this file embedded.
Stockfish is free and distributed under the GNU General Public License version 3 (GPL v3). Essentially, this means you are free to do almost exactly what you want with the program, including distributing it among your friends, making it available for download from your website, selling it (either by itself or as part of some bigger software package), or using it as the starting point for a software project of your own.
The only real limitation is that whenever you distribute Stockfish in some way, you MUST always include the license and the full source code (or a pointer to where the source code can be found) to generate the exact binary you are distributing. If you make any changes to the source code, these changes must also be made available under GPL v3.
Stockfish uses neural networks trained on data provided by the Leela Chess Zero project, which is made available under the Open Database License (ODbL).