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Simplify ThreatByKing to be a single Score. 

In the current master, ThreatByKing is an array of two Scores, one for
when we have a single attack and one for when we have many. The latter
case is very rarely called during bench and was recently given a strange
negative value during a tuning run, as pointed out by @candirufish on
commit efd4ca2.  Here, we simplify away this second case entirely, and
increase the remaining ThreatByKing to compensate.

Although I derived the parameter tweak independently, with the goal of
preserving the same average bonus, I later noticed that a very similar
Score had already been derived by an ongoing SPSA tuning session.
I therefore recognize @candirufish for first discovering these values.

I would also like to thank @Rocky640 for valuable feedback that pointed
me in the direction of ThreatByKing.

STC:
LLR: 2.95 (-2.94,2.94) [-3.00,1.00]
Total: 7677 W: 1772 L: 1623 D: 4282
http://tests.stockfishchess.org/tests/view/5b3db0320ebc5902b9ffe97a

LTC:
LLR: 2.95 (-2.94,2.94) [-3.00,1.00]
Total: 108031 W: 18329 L: 18350 D: 71352
http://tests.stockfishchess.org/tests/view/5b3dbf4b0ebc5902b9ffe9db

Closes https://github.com/official-stockfish/Stockfish/pull/1666

Bench: 4678861
pull/1667/head
31m059 2018-07-04 22:18:52 -04:00 committed by Stéphane Nicolet
parent 3279655f12
commit 0f48095759
1 changed files with 4 additions and 7 deletions
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11
src/evaluate.cpp
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@ -140,10 +140,6 @@ namespace {
S(0, 0), S(0, 24), S(38, 71), S(38, 61), S(0, 38), S(36, 38)
};
// ThreatByKing[on one/on many] contains bonuses for king attacks on
// pawns or pieces which are not pawn-defended.
constexpr Score ThreatByKing[] = { S(30, 62), S(-9, 160) };
// PassedRank[Rank] contains a bonus according to the rank of a passed pawn
constexpr Score PassedRank[RANK_NB] = {
S(0, 0), S(4, 17), S(7, 20), S(14, 36), S(42, 62), S(165, 171), S(279, 252)
@ -175,6 +171,7 @@ namespace {
constexpr Score PawnlessFlank = S( 20, 80);
constexpr Score RookOnPawn = S( 8, 24);
constexpr Score SliderOnQueen = S( 42, 21);
constexpr Score ThreatByKing = S( 31, 75);
constexpr Score ThreatByPawnPush = S( 49, 30);
constexpr Score ThreatByRank = S( 16, 3);
constexpr Score ThreatBySafePawn = S(165,133);
@ -558,9 +555,9 @@ namespace {
score += ThreatByRank * (int)relative_rank(Them, s);
}
b = weak & attackedBy[Us][KING];
if (b)
score += ThreatByKing[more_than_one(b)];
// Bonus for king attacks on pawns or pieces which are not pawn-defended
if (weak & attackedBy[Us][KING])
score += ThreatByKing;
score += Hanging * popcount(weak & ~attackedBy[Them][ALL_PIECES]);