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Eliminate ONE_PLY

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Simplification that eliminates ONE_PLY, based on a suggestion in the forum that
support for fractional plies has never been used, and @mcostalba's openness to
the idea of eliminating it. We lose a little bit of type safety by making Depth
an integer, but in return we simplify the code in search.cpp quite significantly.

No functional change

------------------------------------------

The argument favoring eliminating ONE_PLY:

* The term “ONE_PLY” comes up in a lot of forum posts (474 to date)
https://groups.google.com/forum/?fromgroups=#!searchin/fishcooking/ONE_PLY%7Csort:relevance

* There is occasionally a commit that breaks invariance of the code
with respect to ONE_PLY
https://groups.google.com/forum/?fromgroups=#!searchin/fishcooking/ONE_PLY%7Csort:date/fishcooking/ZIPdYj6k0fk/KdNGcPWeBgAJ

* To prevent such commits, there is a Travis CI hack that doubles ONE_PLY
and rechecks bench

* Sustaining ONE_PLY has, alas, not resulted in any improvements to the
  engine, despite many individuals testing many experiments over 5 years.

The strongest argument in favor of preserving ONE_PLY comes from @locutus:
“If we use par example ONE_PLY=256 the parameter space is increases by the
factor 256. So it seems very unlikely that the optimal setting is in the
subspace of ONE_PLY=1.”

There is a strong theoretical impediment to fractional depth systems: the
transposition table uses depth to determine when a stored result is good
enough to supply an answer for a current search. If you have fractional
depths, then different pathways to the position can be at fractionally
different depths.

In the end, there are three separate times when a proposal to remove ONE_PLY
was defeated by the suggestion to “give it a few more months.” So… it seems
like time to remove this distraction from the community.

See the pull request here:
https://github.com/official-stockfish/Stockfish/pull/2289
This commit is contained in:
Brian Sheppard
2019-09-28 16:27:23 -04:00
committed by Stéphane Nicolet
parent 328bdd0947
commit ca7d4e9ac7
7 changed files with 86 additions and 100 deletions
Download Patch File Download Diff File Expand all files Collapse all files

6
src/movepick.cpp
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@@ -61,7 +61,7 @@ MovePicker::MovePicker(const Position& p, Move ttm, Depth d, const ButterflyHist
: pos(p), mainHistory(mh), captureHistory(cph), continuationHistory(ch),
refutations{{killers[0], 0}, {killers[1], 0}, {cm, 0}}, depth(d) {
assert(d > DEPTH_ZERO);
assert(d > 0);
stage = pos.checkers() ? EVASION_TT : MAIN_TT;
ttMove = ttm && pos.pseudo_legal(ttm) ? ttm : MOVE_NONE;
@@ -73,7 +73,7 @@ MovePicker::MovePicker(const Position& p, Move ttm, Depth d, const ButterflyHist
const CapturePieceToHistory* cph, const PieceToHistory** ch, Square rs)
: pos(p), mainHistory(mh), captureHistory(cph), continuationHistory(ch), recaptureSquare(rs), depth(d) {
assert(d <= DEPTH_ZERO);
assert(d <= 0);
stage = pos.checkers() ? EVASION_TT : QSEARCH_TT;
ttMove = ttm
@@ -206,7 +206,7 @@ top:
endMoves = generate<QUIETS>(pos, cur);
score<QUIETS>();
partial_insertion_sort(cur, endMoves, -4000 * depth / ONE_PLY);
partial_insertion_sort(cur, endMoves, -4000 * depth);
}
++stage;