From: Stéphane Nicolet <cassio@free.fr>
Date: Tue, 27 Feb 2018 18:10:40 +0000 (+0100)
Subject: Simplify tropism computation
X-Git-Url: https://git.sesse.net/?p=stockfish;a=commitdiff_plain;h=de642f16db0c443de2dc0ddf7b06c0a7d1c5b8b7

Simplify tropism computation

Simplification. Tests show that the "shift-and-superpose" trick is no longer
necessary. The speed benefit of avoiding a popcount is no longer relevant
on modern machines.

Passed STC:
LLR: 2.95 (-2.94,2.94) [-3.00,1.00]
Total: 41675 W: 9168 L: 9086 D: 23421
http://tests.stockfishchess.org/tests/view/5a840bcc0ebc590297cc80b5

Passed LTC:
LLR: 2.95 (-2.94,2.94) [-3.00,1.00]
Total: 117728 W: 19875 L: 19911 D: 77942
http://tests.stockfishchess.org/tests/view/5a8444800ebc590297cc80ca

No functional change.
---

diff --git a/src/evaluate.cpp b/src/evaluate.cpp
index 8e6d827e..0ee89e4d 100644
--- a/src/evaluate.cpp
+++ b/src/evaluate.cpp
@@ -482,22 +482,20 @@ namespace {
             score -= make_score(kingDanger * kingDanger / 4096, kingDanger / 16);
         }
     }
-    // Penalty when our king is on a pawnless flank
-    if (!(pos.pieces(PAWN) & KingFlank[file_of(ksq)]))
-        score -= PawnlessFlank;
 
-    // King tropism: firstly, find attacked squares in our king flank
-    b = attackedBy[Them][ALL_PIECES] & KingFlank[file_of(ksq)] & Camp;
+    Bitboard kf = KingFlank[file_of(ksq)];
 
-    assert(((Us == WHITE ? b << 4 : b >> 4) & b) == 0);
-    assert(popcount(Us == WHITE ? b << 4 : b >> 4) == popcount(b));
+    // Penalty when our king is on a pawnless flank
+    if (!(pos.pieces(PAWN) & kf))
+        score -= PawnlessFlank;
 
-    // Secondly, add the squares which are attacked twice in that flank and
-    // which are not defended by our pawns.
-    b =  (Us == WHITE ? b << 4 : b >> 4)
-       | (b & attackedBy2[Them] & ~attackedBy[Us][PAWN]);
+    // Find the squares that opponent attacks in our king flank, and the squares  
+    // which are attacked twice in that flank but not defended by our pawns.
+    b1 = attackedBy[Them][ALL_PIECES] & kf & Camp;
+    b2 = b1 & attackedBy2[Them] & ~attackedBy[Us][PAWN];
 
-    score -= CloseEnemies * popcount(b);
+    // King tropism, to anticipate slow motion attacks on our king
+    score -= CloseEnemies * (popcount(b1) + popcount(b2));
 
     if (T)
         Trace::add(KING, Us, score);