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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

17
src/types.h
View File

@@ -203,22 +203,18 @@ enum Piece {
extern Value PieceValue[PHASE_NB][PIECE_NB];
enum Depth : int {
typedef int Depth;
ONE_PLY = 1,
enum : int {
DEPTH_ZERO = 0 * ONE_PLY,
DEPTH_QS_CHECKS = 0 * ONE_PLY,
DEPTH_QS_NO_CHECKS = -1 * ONE_PLY,
DEPTH_QS_RECAPTURES = -5 * ONE_PLY,
DEPTH_QS_CHECKS = 0,
DEPTH_QS_NO_CHECKS = -1,
DEPTH_QS_RECAPTURES = -5,
DEPTH_NONE = -6 * ONE_PLY,
DEPTH_NONE = -6,
DEPTH_OFFSET = DEPTH_NONE,
DEPTH_MAX = MAX_PLY * ONE_PLY
};
static_assert(!(ONE_PLY & (ONE_PLY - 1)), "ONE_PLY is not a power of 2");
enum Square : int {
SQ_A1, SQ_B1, SQ_C1, SQ_D1, SQ_E1, SQ_F1, SQ_G1, SQ_H1,
SQ_A2, SQ_B2, SQ_C2, SQ_D2, SQ_E2, SQ_F2, SQ_G2, SQ_H2,
@@ -298,7 +294,6 @@ inline T& operator*=(T& d, int i) { return d = T(int(d) * i); } \
inline T& operator/=(T& d, int i) { return d = T(int(d) / i); }
ENABLE_FULL_OPERATORS_ON(Value)
ENABLE_FULL_OPERATORS_ON(Depth)
ENABLE_FULL_OPERATORS_ON(Direction)
ENABLE_INCR_OPERATORS_ON(PieceType)