Remove the Queen from the mobility area of minor pieces

In master, we already remove the King from the mobility area of minor pieces
because the King simply stands in the way of other pieces, and since opponent
cannot capture the King, any piece which "protects" the King cannot recapture.

Similarly, this patch introduces the idea that it is rarely a need for a Queen
to be "protected" by a minor (unless it is attacked only by a Queen, in fact).
We used to have a LoosePiece bonus, and in a similar vein the Queen was excluded
from that penalty.

Idea came when reviewing an old game of Kholmov. He was a very good midgame
player, but in the opening his misplace his Queen (and won in the end :-) :
     http://www.chessgames.com/perl/chessgame?gid=1134645

Both white queen moves 10.Qd3 and 13.Qb3 are in the way of some minor piece.
I would prefer to not give a bishop mobility bonus at move 10 for the square d3,
or later a knight mobility bonus at move 13 for the square b3. And the textbook
move is 19.Qe3! which prepares 20.Nb3. This short game sample shows how much a
queen can be "in the way" of minor pieces.

STC
http://tests.stockfishchess.org/tests/view/5ac2c15f0ebc591746423fa3
LLR: 2.96 (-2.94,2.94) [0.00,5.00]
Total: 22066 W: 4561 L: 4330 D: 13175

LTC
http://tests.stockfishchess.org/tests/view/5ac2d6500ebc591746423faf
LLR: 2.96 (-2.94,2.94) [0.00,5.00]
Total: 25871 W: 3953 L: 3738 D: 18180

Closes https://github.com/official-stockfish/Stockfish/pull/1532

Ideas for future work in this area:

• tweak some more mobility areas for other piece type.
• construct a notion of global mobility for the whole piece set.
• bad bishops.

Bench: 4989125

diff --git a/src/evaluate.cpp b/src/evaluate.cpp
index e39bcb4a8e99721820fa637151ace370d009f5d9..41c66812717e587dffeafbca3f8935cef8204861 100644 (file)
--- a/src/evaluate.cpp
+++ b/src/evaluate.cpp
@@ -300,6 +300,7 @@ namespace {
     Bitboard b, bb;
     Square s;
     Score score = SCORE_ZERO;
     Bitboard b, bb;
     Square s;
     Score score = SCORE_ZERO;

+    int mob;

 
     attackedBy[Us][Pt] = 0;
 
 
     attackedBy[Us][Pt] = 0;
 


@@ -324,7 +325,8 @@ namespace {
             kingAttacksCount[Us] += popcount(b & attackedBy[Them][KING]);
         }
 
             kingAttacksCount[Us] += popcount(b & attackedBy[Them][KING]);
         }
 

-        int mob = popcount(b & mobilityArea[Us]);
+        mob = (Pt == KNIGHT || Pt == BISHOP) ? popcount(b & mobilityArea[Us] & ~pos.pieces(Us, QUEEN))
+                                             : popcount(b & mobilityArea[Us]);

 
         mobility[Us] += MobilityBonus[Pt - 2][mob];
 
 
         mobility[Us] += MobilityBonus[Pt - 2][mob];