Count passed pawns in asymmetry measure
authorStéphane Nicolet <cassio@free.fr>
Sun, 25 Feb 2018 23:55:01 +0000 (00:55 +0100)
committerStéphane Nicolet <cassio@free.fr>
Mon, 26 Feb 2018 00:06:45 +0000 (01:06 +0100)
The previous asymmetry measure of the pawn structure only used to
consider the number of pawns on semi-opened files in the position.
With this patch we also increase the measure by the number of passed
pawns for both players.

Many thanks to the community for the nice feedback on the previous
version, with special mentions to Alain Savard and Marco Costalba
for clarity and speed suggestions.

STC:
LLR: 2.96 (-2.94,2.94) [0.00,5.00]
Total: 13146 W: 3038 L: 2840 D: 7268
http://tests.stockfishchess.org/tests/view/5a91dd0c0ebc590297cc877e

LTC:
LLR: 2.96 (-2.94,2.94) [0.00,5.00]
Total: 27776 W: 4771 L: 4536 D: 18469
http://tests.stockfishchess.org/tests/view/5a91fdd50ebc590297cc879b

How to continue after this patch?

Stockfish will now evaluate more positions with passed pawns, so
tuning the passed pawns values may bring Elo. The patch has also
consequences on the initiative term, where we might want to give
different weights to passed pawns and semi-openfiles (idea by
Stefano Cardanobile).

Bench: 5302866

src/pawns.cpp

index a1b5332..7e90089 100644 (file)
@@ -222,8 +222,9 @@ Entry* probe(const Position& pos) {
   e->key = key;
   e->scores[WHITE] = evaluate<WHITE>(pos, e);
   e->scores[BLACK] = evaluate<BLACK>(pos, e);
-  e->asymmetry = popcount(e->semiopenFiles[WHITE] ^ e->semiopenFiles[BLACK]);
   e->openFiles = popcount(e->semiopenFiles[WHITE] & e->semiopenFiles[BLACK]);
+  e->asymmetry = popcount(  (e->passedPawns[WHITE]   | e->passedPawns[BLACK])
+                          | (e->semiopenFiles[WHITE] ^ e->semiopenFiles[BLACK]));
 
   return e;
 }