The '- 1' subtrahend was introduced for guarding against null move
search at root, which would be nonsense. But this is actually already
guaranteed by the !PvNode condition. This followed from the discussion
in pull request 1609: https://github.com/official-stockfish/Stockfish/pull/1609
No functional change
Fix an error when compiling current master with MSVC due to the
ambiguity of which operator* overload was intended (reported by
Jarrod Torriero).
No functional change.
define Color us and use this instead of pos.side_to_move() and nmp_odd. The latter allows to clarify the nmp verification criterion.
Tested for no regression:
LLR: 2.95 (-2.94,2.94) [-3.00,1.00]
Total: 76713 W: 15303 L: 15284 D: 46126
http://tests.stockfishchess.org/tests/view/5b046a0d0ebc5914abc12971
No functional change.
A position which has a move which draws by repetition, or which could have
been reached from an earlier position in the game tree, is considered to be
at least a draw for the side to move.
Cycle detection algorithm by Marcel van Kervink:
https://marcelk.net/2013-04-06/paper/upcoming-rep-v2.pdf
----------------------------
How does the algorithm work in practice? The algorithm is an efficient
method to detect if the side to move has a drawing move, without doing any
move generation, thus possibly giving a cheap cutoffThe most interesting
conditions are both on line 1195:
```
if ( originalKey == (progressKey ^ stp->key)
|| progressKey == Zobrist::side)
```
This uses the position keys as a sort-of Bloom filter, to avoid the expensive
checks which follow. For "upcoming repetition" consider the opening Nf3 Nf6 Ng1.
The XOR of this position's key with the starting position gives their difference,
which can be used to look up black's repeating move (Ng8). But that look-up is
expensive, so line 1195 checks that the white pieces are on their original squares.
This is the subtlest part of the algorithm, but the basic idea in the above game
is there are 4 positions (starting position and the one after each move). An XOR
of the first pair (startpos and after Nf3) gives a key matching Nf3. An XOR of
the second pair (after Nf6 and after Ng1) gives a key matching the move Ng1. But
since the difference in each pair is the location of the white knight those keys
are "identical" (not quite because while there are 4 keys the the side to move
changed 3 times, so the keys differ by Zobrist::side). The loop containing line
1195 does this pair-wise XOR-ing.
Continuing the example, after line 1195 determines that the white pieces are
back where they started we still need to make sure the changes in the black
pieces represents a legal move. This is done by looking up the "moveKey" to
see if it corresponds to possible move, and that there are no pieces blocking
its way. There is the additional complication that, to match the behavior of
is_draw(), if the repetition is not inside the search tree then there must be
an additional repetition in the game history. Since a position can have more
than one upcoming repetition a simple count does not suffice. So there is a
search loop ending on line 1215.
On the other hand, the "no-progress' is the same thing but offset by 1 ply.
I like the concept but think it currently has minimal or negative benefit,
and I'd be happy to remove it if that would get the patch accepted. This
will not, however, save many lines of code.
-----------------------------
STC:
LLR: 2.95 (-2.94,2.94) [0.00,5.00]
Total: 36430 W: 7446 L: 7150 D: 21834
http://tests.stockfishchess.org/tests/view/5afc123f0ebc591fdf408dfc
LTC:
LLR: 2.96 (-2.94,2.94) [0.00,5.00]
Total: 12998 W: 2045 L: 1876 D: 9077
http://tests.stockfishchess.org/tests/view/5afc2c630ebc591fdf408e0c
How could we continue after the patch:
• The code in search() that checks for cycles has numerous possible variants.
Perhaps the check need could be done in qsearch() too.
• The biggest improvement would be to get "no progress" to be of actual benefit,
and it would be helpful understand why it (probably) isn't. Perhaps there is an
interaction with the transposition table or the (fantastically complex) tree
search. Perhaps this would be hard to fix, but there may be a simple oversight.
Closes https://github.com/official-stockfish/Stockfish/pull/1575
Bench: 4550412
At PvNodes allow bonus for prior counter move that caused a fail low
for depth 1 and 2. Note : I did a speculative LTC on yellow STC patch
since history stats tend to be highly TC sensitive
STC (Yellow):
LLR: -2.96 (-2.94,2.94) [0.00,5.00]
Total: 64295 W: 13042 L: 12873 D: 38380
http://tests.stockfishchess.org/tests/view/5af507c80ebc5968e6524153
LTC:
LLR: 2.96 (-2.94,2.94) [0.00,5.00]
Total: 22407 W: 3413 L: 3211 D: 15783
http://tests.stockfishchess.org/tests/view/5af85dd40ebc591fdf408b87
Also use local variable excludedMove in NMP (marotear)
Bench: 5294316
This patch simplifies the control flow in search(), removing an if
and a goto. A side effect of the patch is that Stockfish is now a
little bit more selective at low depths, because we allow razoring,
futility pruning and probcut pruning after a null move.
passed STC:
LLR: 2.95 (-2.94,2.94) [-3.00,1.00]
Total: 32035 W: 6523 L: 6422 D: 19090
http://tests.stockfishchess.org/tests/view/5af142ca0ebc597fb3d39bb6
passed LTC:
LLR: 2.95 (-2.94,2.94) [-3.00,1.00]
Total: 41431 W: 6187 L: 6097 D: 29147
http://tests.stockfishchess.org/tests/view/5af148770ebc597fb3d39bc1
Ideas for further work:
• Use the nodes credit opened by the patch (the increased selectivity)
to try somewhat higher razoring, futility or probcut margins at [0..4].
Bench: 4855031
Change the operators of the Option type in uci.h to accept floating
point numbers in double precision on input as the numerical type for
the "spin" values of the UCI protocol.
The output of Stockfish after the "uci" command is unaffected.
This change is compatible with all the existing GUI (as they will
continue sending integers that we can interpret as doubles in SF),
and allows us to pass double parameters to Stockfish in the console
via the "setoption" command. This will be useful if we implement
another tuner as an alternative for SPSA.
Closes https://github.com/official-stockfish/Stockfish/pull/1556
No functional change.
---------------------
A example of the new functionality in action in the branch `tune_float2'`:
876c322d0f
I have added the following lines in ucioptions.cpp:
```C++
void on_pi(const Option& o)
{
double x = Options["PI"]; // or double x = o;
std::cerr << "received value is x = " << x << std::endl;
}
...
o["PI"] << Option(3.1415926, -10000000, 10000000, on_pi);
```
Then I can change the value of Pi in Stockfish via the command line, and
check that Stockfish understands a floating point:
````
> ./stockfish
> setoption name PI value 2.7182818284
received value is x = 2.71828
````
On output, the default value of Pi is truncated to 3 (to remain compatible
with the UCI protocol and GUIs):
````
> uci
[...]
option name SyzygyProbeLimit type spin default 6 min 0 max 6
option name PI type spin default 3 min -10000000 max 10000000
uciok
````
This patch corrects both MultiPV behaviour and "go searchmoves" behaviour
for tablebases.
We change the logic of table base probing at root positions from filtering
to ranking. The ranking code is much more straightforward than the current
filtering code (this is a simplification), and also more versatile.
If the root is a TB position, each root move is probed and assigned a TB score
and a TB rank. The TB score is the Value to be displayed to the user for that
move (unless the search finds a mate score), while the TB rank determines which
moves should appear higher in a multi-pv search. In game play, the engine will
always pick a move with the highest rank.
Ranks run from -1000 to +1000:
901 to 1000 : TB win
900 : normally a TB win, in rare cases this could be a draw
1 to 899 : cursed TB wins
0 : draw
-1 to -899 : blessed TB losses
-900 : normally a TB loss, in rare cases this could be a draw
-901 to -1000 : TB loss
Normally all winning moves get rank 1000 (to let the search pick the best
among them). The exception is if there has been a first repetition. In that
case, moves are ranked strictly by DTZ so that the engine will play a move
that lowers DTZ (and therefore cannot repeat the position a second time).
Losing moves get rank -1000 unless they have relatively high DTZ, meaning
they have some drawing chances. Those get ranks towards -901 (when they
cross -900 the draw is certain).
Closes https://github.com/official-stockfish/Stockfish/pull/1467
No functional change (without tablebases).
This patch introduces an Analysis Contempt UCI combo box to control
the behaviour of contempt during analysis. The possible values are
Both, Off, White, Black. Technically, the engine is supposed to be in
analysis mode if UCI_AnalyseMode is set by the graphical user interface
or if the user has chosen infinite analysis mode ("go infinite").
Credits: the idea for the combo box is due to Michel Van den Bergh.
No functional change (outside analysis mode).
-----------------------------------------------------
The so-called "contempt" is an optimism value that the engine adds
to one color to avoid simplifications and keep tension in the position
during its search. It was introduced in Stockfish 9 and seemed to give
good results during the TCEC 11 tournament (Stockfish seemed to play a
little bit more actively than in previous seasons).
The patch does not change the play during match or blitz play, but gives
more options for correspondance players to decide for which color(s) they
would like to use contempt in analysis mode (infinite time). Here is a
description of the various options:
* Both : in analysis mode, use the contempt for both players (alternating)
* Off : in analysis mode, use the contempt for none of the players
* White : in analysis mode, White will play actively, Black will play passively
* Black : in analysis mode, Black will play actively, White will play passively
We now use per-thread dynamic contempt. This patch has the following
effects:
* for Threads=1: **non-functional**
* for Threads>1:
* with MultiPV=1: **no regression, little to no ELO gain**
* with MultiPV>1: **clear improvement over master**
First, I tried testing at standard MultiPV=1 play with [0,5] bounds.
This yielded 2 yellow and 1 red test:
5+0.05, Threads=5:
LLR: -2.96 (-2.94,2.94) [0.00,5.00]
Total: 82689 W: 16439 L: 16190 D: 50060
http://tests.stockfishchess.org/tests/view/5aa93a5a0ebc5902952892e6
5+0.05, Threads=8:
LLR: -2.96 (-2.94,2.94) [0.00,5.00]
Total: 27164 W: 4974 L: 4983 D: 17207
http://tests.stockfishchess.org/tests/view/5ab2639b0ebc5902a6fbefd5
5+0.5, Threads=16:
LLR: -2.97 (-2.94,2.94) [0.00,5.00]
Total: 41396 W: 7127 L: 7082 D: 27187
http://tests.stockfishchess.org/tests/view/5ab124220ebc59029516cb62
Then, I tested with Skill Level=17 (implicitly MutliPV=4), showing
a clear improvement:
5+0.05, Threads=5:
LLR: 2.96 (-2.94,2.94) [0.00,5.00]
Total: 3498 W: 1316 L: 1135 D: 1047
http://tests.stockfishchess.org/tests/view/5ab4b6580ebc5902932aeca2
Next, I tested the patch with MultiPV=1 again, this time checking for
non-regression ([-3, 1]):
5+0.5, Threads=5:
LLR: 2.96 (-2.94,2.94) [-3.00,1.00]
Total: 65575 W: 12786 L: 12745 D: 40044
http://tests.stockfishchess.org/tests/view/5ab4e8500ebc5902932aecb3
Finally, I ran some tests with fixed number of games, checking if
reverting dynamic contempt gains more elo with Skill Level=17 (i.e.
MultiPV) than applying the "prevScore" fix and this patch. These tests
showed, that this patch gains 15 ELO when playing with Skill Level=17:
5+0.05, Threads=3, "revert dynamic contempt" vs. "WITHOUT this patch":
ELO: -11.43 +-4.1 (95%) LOS: 0.0%
Total: 20000 W: 7085 L: 7743 D: 5172
http://tests.stockfishchess.org/tests/view/5ab636450ebc590295d88536
5+0.05, Threads=3, "revert dynamic contempt" vs. "WITH this patch":
ELO: -26.42 +-4.1 (95%) LOS: 0.0%
Total: 20000 W: 6661 L: 8179 D: 5160
http://tests.stockfishchess.org/tests/view/5ab62e680ebc590295d88524
---
***FAQ***
**Why should this be commited?**
I believe that the gain for multi-thread MultiPV search is a sufficient
justification for this otherwise neutral change. I also believe this
implementation of dynamic contempt is more logical, although this may
be just my opinion.
**Why is per-thread contempt better at MultiPV?**
A likely explanation for the gain in MultiPV mode is that during
search each thread independently switches between rootMoves and via
the shared contempt score skews each other's evaluation.
**Why were the tests done with Skill Level=17?**
This was originally suggested by @Hanamuke and the idea is that with
Skill Level Stockfish sometimes plays also moves it thinks are slightly
sub-optimal and thus the quality of all moves offered by the MultiPV
search is checked by the test.
**Why are the ELO differences so huge?**
This is most likely because of the nature of Skill Level mode --
since it slower and weaker than normal mode, bugs in evaluation have
much greater effect.
---
Closes https://github.com/official-stockfish/Stockfish/pull/1515.
No functional change -- in single thread mode.
Adjust criterion for applying extra reduction if not improving.
We now add an extra ply of reduction if r > 1.0, instead of the
previous condition Reductions[NonPV][imp][d][mc] >= 2.
Why does this work? Previously, reductions when not improving had
a discontinuity as the depth and/or move count increases due to the
Reductions[NonPV][imp][d][mc] >= 2 condition. Hence, values of r
such that 0.5 < r < 1.5 would be mapped to a reduction of 1, while
1.5 < r < 2.5 would be mapped to a reduction of 3. This patch allows
values of r satisfying 1.0 < r < 1.5 to be mapped to a reduction of 2,
making the reduction formula more continuous.
STC:
LLR: 2.96 (-2.94,2.94) [0.00,5.00]
Total: 35908 W: 7382 L: 7087 D: 21439
http://tests.stockfishchess.org/tests/view/5aba723a0ebc5902a4743e8f
LTC:
LLR: 2.96 (-2.94,2.94) [0.00,5.00]
Total: 23087 W: 3584 L: 3378 D: 16125
http://tests.stockfishchess.org/tests/view/5aba89070ebc5902a4743ea9
Ideas for future work:
- We could look at retuning the LMR formula.
- We could look at adjusting the reductions in PV nodes if not improving.
Bench: 5326261
Use rootMoves[PVIdx].previousScore instead of bestValue for
dynamic contempt. This is equivalent for MultiPV=1 (bench remained the
same, even for higher depths), but more correct for MultiPV.
STC (MultiPV=3):
LLR: 2.95 (-2.94,2.94) [0.00,5.00]
Total: 2657 W: 1079 L: 898 D: 680
http://tests.stockfishchess.org/tests/view/5aaa47cb0ebc590297330403
LTC (MultiPV=3):
LLR: 2.95 (-2.94,2.94) [0.00,5.00]
Total: 2390 W: 874 L: 706 D: 810
http://tests.stockfishchess.org/tests/view/5aaa593a0ebc59029733040b
VLTC 240+2.4 (MultiPV=3):
LLR: 2.96 (-2.94,2.94) [0.00,5.00]
Total: 2399 W: 861 L: 694 D: 844
http://tests.stockfishchess.org/tests/view/5aaf983e0ebc5902a182131f
LTC (MultiPV=4, Skill Level=17):
LLR: 2.95 (-2.94,2.94) [0.00,5.00]
Total: 747 W: 333 L: 175 D: 239
http://tests.stockfishchess.org/tests/view/5aabccee0ebc5902997ff006
Note: although the ELO differences seem huge, they are inflated by the
nature of Skill Level / MultiPV search, so I don't think they can be
reasonably compared with classic ELO strength.
See https://github.com/official-stockfish/Stockfish/pull/1491 for some
verifications searches with MultiPV = 10 at depths 12 and 24 from the
starting position and the position after 1.e4, comparing the outputs
of the full PV by the old master and by this patch.
No functional change for MultiPV=1
This is a patch to fix issue #1498, switching the time management variables
to 64 bits to avoid overflow of time variables after 25 days.
There was a bug in Stockfish 9 causing the output to be wrong after
2^31 milliseconds search. Here is a long run from the starting position:
info depth 64 seldepth 87 multipv 1 score cp 23 nodes 13928920239402
nps 0 tbhits 0 time -504995523 pv g1f3 d7d5 d2d4 g8f6 c2c4 d5c4 e2e3 e7e6 f1c4
c7c5 e1g1 b8c6 d4c5 d8d1 f1d1 f8c5 c4e2 e8g8 a2a3 c5e7 b2b4 f8d8 b1d2 b7b6 c1b2
c8b7 a1c1 a8c8 c1c2 c6e5 d1c1 c8c2 c1c2 e5f3 d2f3 a7a5 b4b5 e7c5 f3d4 d8c8 d4b3
c5d6 c2c8 b7c8 b3d2 c8b7 d2c4 d6c5 e2f3 b7d5 f3d5 e6d5 c4e5 a5a4 e5d3 f6e4 d3c5
e4c5 b2d4 c5e4 d4b6 e4d6 g2g4 d6b5 b6c5 b5c7 g1g2 c7e6 c5d6 g7g6
We check at compile time that the TimePoint type is exactly 64 bits long for
the compiler (TimePoint is our alias in Stockfish for std::chrono::milliseconds
-- it is a signed integer type of at least 45 bits according to the C++ standard,
but will most probably be implemented as a 64 bits signed integer on modern
compilers), and we use this TimePoint type consistently across the code.
Bug report by user "fischerandom" on the TCEC chat (thanks), and the
patch includes code and suggestions by user "WOnder93" and Ronald de Man.
Fixes issue: https://github.com/official-stockfish/Stockfish/issues/1498
Closes pull request: https://github.com/official-stockfish/Stockfish/pull/1510
No functional change.
To more clearly distinguish them from "const" local variables, this patch
defines compile-time local constants as constexpr. This is consistent with
the definition of PvNode as constexpr in search() and qsearch(). It also
makes the code more robust, since the compiler will now check that those
constants are indeed compile-time constants.
We can go even one step further and define all the evaluation and search
compile-time constants as constexpr.
In generate_castling() I replaced "K" with "step", since K was incorrectly
capitalised (in the Chess960 case).
In timeman.cpp I had to make the non-local constants MaxRatio and StealRatio
constepxr, since otherwise gcc would complain when calculating TMaxRatio and
TStealRatio. (Strangely, I did not have to make Is64Bit constexpr even though
it is used in ucioption.cpp in the calculation of constexpr MaxHashMB.)
I have renamed PieceCount to pieceCount in material.h, since the values of
the array are not compile-time constants.
Some compile-time constants in tbprobe.cpp were overlooked. Sides and MaxFile
are not compile-time constants, so were renamed to sides and maxFile.
Non-functional change.
Implements renaming suggestions by Marco Costalba, Günther Demetz,
Gontran Lemaire, Ronald de Man, Stéphane Nicolet, Alain Savard,
Joost VandeVondele, Jerry Donald Watson, Mike Whiteley, xoto10,
and I hope that I haven't forgotten anybody.
Perpetual renaming thread for suggestions:
https://github.com/official-stockfish/Stockfish/issues/1426
No functional change.
If search depth is less than ONE_PLY call qsearch(), no need to check the
depth condition at various call sites of search().
Passed STC:
LLR: 2.96 (-2.94,2.94) [-3.00,1.00]
Total: 14568 W: 3011 L: 2877 D: 8680
http://tests.stockfishchess.org/tests/view/5aa846190ebc59029781015b
Also helps gcc to find some optimizations (smaller binary, some speedup).
Thanks to Aram and Stefan for identifying an oversight in an early version.
Closes https://github.com/official-stockfish/Stockfish/pull/1487
No functional change.
Adjust ProbCut rBeta by whether the score is improving, and also
set improving to false when in check. More precisely, this patch
has two parts:
1) the increased beta threshold for ProbCut is now adjusted based
on whether the score is improving
2) when in check, improving is always set to false.
Co-authored by Joost VandeVondele (@vondele) and Bill Henry (@VoyagerOne).
STC:
LLR: 2.96 (-2.94,2.94) [0.00,5.00]
Total: 13480 W: 2840 L: 2648 D: 7992
http://tests.stockfishchess.org/tests/view/5aa693fe0ebc59029781004c
LTC:
LLR: 2.97 (-2.94,2.94) [0.00,5.00]
Total: 25895 W: 4099 L: 3880 D: 17916
http://tests.stockfishchess.org/tests/view/5aa6ac940ebc59029781006e
In terms of opportunities for future work opened up by this patch,
the ProbCut rBeta formula could probably be tuned to gain more Elo.
Closes https://github.com/official-stockfish/Stockfish/pull/1485
Bench: 5328254
Avoid duplicated code after recent commit "Use evaluation trend
to adjust futility margin". We initialize the improving variable
to true in the check case, which allows to avoid redundant code
in the general case.
Tested for speed by snicolet, patch seems about 0.4% faster.
No functional change.
Note: initializing the improving variable to false in the check
case was tested as a functional change, ending yellow in both STC
and LTC. This change is not included in the commit, but it is an
interesting result that could become part of a future patch about
improving or LMR. Reference of the LTC yellow test:
http://tests.stockfishchess.org/tests/view/5aa131560ebc590297cb636e
