Robert Nurnberg @ elitebookandDisservin
74a0a73715
This PR fixes the remaining corner cases in the treatment of MultiPV mated-in PVs, as well as an oversight in #6886. See the discussion in In particular: 1. `previousScore` and `previousPV` can only be trusted, if that rootmove was indeed fully searched in the previous iteration. 2. A move beyond `pvIdx` (that was hence not fully searched) may have an exact loss score that cannot be trusted. So if a MultiPV search gets aborted while searching `pvIdx`, we mark all the following loss scores as bounds. 3. The forgotten mate logic also got broken in #6886, because the `previousPV` of the forgotten mate's bestmove can only be trusted if that move was fully searched in the previous iteration, something that is not guaranteed. So we now store both `lastBestMoveScore` and `lastBestMovePV`. Here some scenarios for MultiPV = 8 that explain how master was broken: 1. Move A with an inexact mated-in-2 score from the previous iteration (so outside the top8 moves) gets flushed into the top8 moves for the current iteration, because the previous top8 move B is now scored as a mated-in-1. Hence we cannot trust `previousScore` or `previousPV` for move A, if the search gets aborted while it is being searched. 2. In the scenario above, move B has `Score != -VALUE_INFINITE` and a mated-in-1 score, which cannot be trusted as it was not fully searched. 3. Iteration N has bestmove A with mated-in-10, which gets recorded in `lastBestMoveScore` (renamed from `lastIterationScore`). Iteration 11 forgets the mate and has bestmove B with a cp score, move A may have an incomplete PV, and may even have a non-mate score. Iteration 12 gets aborted, and in trying to remember the forgotten mate, master recovers the `previousScore` and `previousPV` of move A, which may be neither mate nor complete. Passed STC non-reg: LLR: 2.94 (-2.94,2.94) <-1.75,0.25> Total: 69728 W: 17748 L: 17573 D: 34407 Ptnml(0-2): 143, 7571, 19274, 7720, 156 https://tests.stockfishchess.org/tests/view/6a2c40c60d5d4b19d08052f2 closes https://github.com/official-stockfish/Stockfish/pull/6906 No functional change
Stockfish
A free and strong UCI chess engine.
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Overview
Stockfish is a free and strong UCI chess engine derived from Glaurung 2.1 that analyzes chess positions and computes the optimal moves.
Stockfish does not include a graphical user interface (GUI) that is required to display a chessboard and to make it easy to input moves. These GUIs are developed independently from Stockfish and are available online. Read the documentation for your GUI of choice for information about how to use Stockfish with it.
See also the Stockfish documentation for further usage help.
Files
This distribution of Stockfish 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 Stockfish 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.
Contributing
See Contributing Guide.
Donating hardware
Improving Stockfish requires a massive amount of testing. You can donate your hardware resources by installing the Fishtest Worker and viewing the current tests on Fishtest.
Improving the code
In the chessprogramming wiki, many techniques used in Stockfish are explained with a lot of background information. The section on Stockfish describes many features and techniques used by Stockfish. However, it is generic rather than focused on Stockfish's precise implementation.
The engine testing is done on Fishtest. If you want to help improve Stockfish, please read this guideline first, where the basics of Stockfish development are explained.
Discussions about Stockfish take place these days mainly in the Stockfish Discord server. This is also the best place to ask questions about the codebase and how to improve it.
Compiling Stockfish
Stockfish has support for 32 or 64-bit CPUs, certain hardware instructions, big-endian machines such as Power PC, and other platforms.
On Unix-like systems, it should be easy to compile Stockfish directly from the
source code with the included Makefile in the folder src. In general, it is
recommended to run make help to see a list of make targets with corresponding
descriptions. An example suitable for most Intel and AMD chips:
cd src
make -j profile-build
Detailed compilation instructions for all platforms can be found in our documentation. Our wiki also has information about the UCI commands supported by Stockfish.
Terms of use
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.
Acknowledgements
Stockfish uses neural networks trained on data provided by the Leela Chess Zero project, which is made available under the Open Database License (ODbL).