158864270a
This patch replaces RKISS by a simpler and faster PRNG, xorshift64* proposed by S. Vigna (2014). It is extremely simple, has a large enough period for Stockfish's needs (2^64), requires no warming-up (allowing such code to be removed), and offers slightly better randomness than MT19937. Paper: http://xorshift.di.unimi.it/ Reference source code (public domain): http://xorshift.di.unimi.it/xorshift64star.c The patch also simplifies how init_magics() searches for magics: - Old logic: seed the PRNG always with the same seed, then use optimized bit rotations to tailor the RNG sequence per rank. - New logic: seed the PRNG with an optimized seed per rank. This has two advantages: 1. Less code and less computation to perform during magics search (not ROTL). 2. More choices for random sequence tuning. The old logic only let us choose from 4096 bit rotation pairs. With the new one, we can look for the best seeds among 2^64 values. Indeed, the set of seeds[][] provided in the patch reduces the effort needed to find the magics: 64-bit SF: Old logic -> 5,783,789 rand64() calls needed to find the magics New logic -> 4,420,086 calls 32-bit SF: Old logic -> 2,175,518 calls New logic -> 1,895,955 calls In the 64-bit case, init_magics() take 25 ms less to complete (Intel Core i5). Finally, when playing with strength handicap, non-determinism is achieved by setting the seed of the static RNG only once. Afterwards, there is no need to skip output values. The bench only changes because the Zobrist keys are now different (since they are random numbers straight out of the PRNG). The RNG seed has been carefully chosen so that the resulting Zobrist keys are particularly well-behaved: 1. All triplets of XORed keys are unique, implying that it would take at least 7 keys to find a 64-bit collision (test suggested by ceebo) 2. All pairs of XORed keys are unique modulo 2^32 3. The cardinality of { (key1 ^ key2) >> 48 } is as close as possible to the maximum (65536) Point 2 aims at ensuring a good distribution among the bits that determine an TT entry's cluster, likewise point 3 among the bits that form the TT entry's key16 inside a cluster. Details: Bitset card(key1^key2) ------ --------------- RKISS key16 64894 = 99.020% of theoretical maximum low18 180117 = 99.293% low32 305362 = 99.997% Xorshift64*, old seed key16 64918 = 99.057% low18 179994 = 99.225% low32 305350 = 99.993% Xorshift64*, new seed key16 65027 = 99.223% low18 181118 = 99.845% low32 305371 = 100.000% Bench: 9324905 Resolves #148 |
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Overview
Stockfish is a free UCI chess engine derived from Glaurung 2.1. It is not a complete chess program and requires some UCI-compatible GUI (e.g. XBoard with PolyGlot, eboard, Arena, Sigma Chess, Shredder, Chess Partner or Fritz) in order to be used comfortably. Read the documentation for your GUI of choice for information about how to use Stockfish with it.
This version of Stockfish supports up to 128 cores. The engine defaults to one search thread, so it is therefore recommended to inspect the value of the Threads UCI parameter, and to make sure it equals the number of CPU cores on your computer.
This version of Stockfish has support for Syzygybases.
Files
This distribution of Stockfish consists of the following files:
-
Readme.md, the file you are currently reading.
-
Copying.txt, a text file containing the GNU General Public License.
-
src, a subdirectory containing the full source code, including a Makefile that can be used to compile Stockfish on Unix-like systems.
Syzygybases
Configuration
Syzygybases are configured using the UCI options "SyzygyPath", "SyzygyProbeDepth", "Syzygy50MoveRule" and "SyzygyProbeLimit".
The option "SyzygyPath" should be set to the directory or directories that contain the .rtbw and .rtbz files. Multiple directories should be separated by ";" on Windows and by ":" on Unix-based operating systems. Do not use spaces around the ";" or ":".
Example: C:\tablebases\wdl345;C:\tablebases\wdl6;D:\tablebases\dtz345;D:\tablebases\dtz6
It is recommended to store .rtbw files on an SSD. There is no loss in storing the .rtbz files on a regular HD.
Increasing the "SyzygyProbeDepth" option lets the engine probe less aggressively. Set this option to a higher value if you experience too much slowdown (in terms of nps) due to TB probing.
Set the "Syzygy50MoveRule" option to false if you want tablebase positions that are drawn by the 50-move rule to count as win or loss. This may be useful for correspondence games (because of tablebase adjudication).
The "SyzygyProbeLimit" option should normally be left at its default value.
What to expect
If the engine is searching a position that is not in the tablebases (e.g.
a position with 7 pieces), it will access the tablebases during the search.
If the engine reports a very large score (typically 123.xx), this means
that it has found a winning line into a tablebase position.
If the engine is given a position to search that is in the tablebases, it will use the tablebases at the beginning of the search to preselect all good moves, i.e. all moves that preserve the win or preserve the draw while taking into account the 50-move rule. It will then perform a search only on those moves. The engine will not move immediately, unless there is only a single good move. The engine likely will not report a mate score even if the position is known to be won.
It is therefore clear that behaviour is not identical to what one might be used to with Nalimov tablebases. There are technical reasons for this difference, the main technical reason being that Nalimov tablebases use the DTM metric (distance-to-mate), while Syzygybases use a variation of the DTZ metric (distance-to-zero, zero meaning any move that resets the 50-move counter). This special metric is one of the reasons that Syzygybases are more compact than Nalimov tablebases, while still storing all information needed for optimal play and in addition being able to take into account the 50-move rule.
Compiling it yourself
On Unix-like systems, it should be possible to compile Stockfish directly from the source code with the included Makefile.
Stockfish has support for 32 or 64-bit CPUs, the hardware POPCNT instruction, big-endian machines such as Power PC, and other platforms.
In general it is recommended to run make help to see a list of make
targets with corresponding descriptions. When not using the Makefile to
compile (for instance with Microsoft MSVC) you need to manually
set/unset some switches in the compiler command line; see file types.h
for a quick reference.
Terms of use
Stockfish is free, and distributed under the GNU General Public License (GPL). Essentially, this means that 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 web site, 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 full source code, or a pointer to where the source code can be found. If you make any changes to the source code, these changes must also be made available under the GPL.
For full details, read the copy of the GPL found in the file named Copying.txt