Introduce king infiltration bonus

Vizvezdenec · snicolet · commit de4e1cb88d9d · 2020-01-07T11:33:53.000+01:00
Add king infiltration bonus to initiative calculation. Idea is somewhat similar to outflanking - endgames are hard to win if each king is on it side of the board. So this adds extra bonus for one of kings crossing the middle line. STC LLR: 2.94 (-2.94,2.94) {-1.00,3.00} Total: 10533 W: 2372 L: 2242 D: 5919 Ptnml(0-2): 196, 1198, 2352, 1316, 202 http://tests.stockfishchess.org/tests/view/5e0e6fd1e97ea42ea89da9b3 LTC LLR: 2.96 (-2.94,2.94) {0.00,2.00} Total: 15074 W: 2563 L: 2381 D: 10130 Ptnml(0-2): 118, 1500, 4111, 1663, 129 http://tests.stockfishchess.org/tests/view/5e0e857ae97ea42ea89da9cc Closes #2471 Bench: 5146339
diff --git a/src/evaluate.cpp b/src/evaluate.cpp
@@ -705,6 +705,9 @@ namespace {
     int outflanking =  distance<File>(pos.square<KING>(WHITE), pos.square<KING>(BLACK))
                      - distance<Rank>(pos.square<KING>(WHITE), pos.square<KING>(BLACK));
 
+    bool infiltration =   rank_of(pos.square<KING>(WHITE)) > RANK_4
+                       || rank_of(pos.square<KING>(BLACK)) < RANK_5;
+
     bool pawnsOnBothFlanks =   (pos.pieces(PAWN) & QueenSide)
                             && (pos.pieces(PAWN) & KingSide);
 
@@ -716,10 +719,11 @@ namespace {
     int complexity =   9 * pe->passed_count()
                     + 11 * pos.count<PAWN>()
                     +  9 * outflanking
+                    + 12 * infiltration
                     + 21 * pawnsOnBothFlanks
                     + 51 * !pos.non_pawn_material()
                     - 43 * almostUnwinnable
-                    - 95 ;
+                    - 100 ;
 
     // Now apply the bonus: note that we find the attacking side by extracting the
     // sign of the midgame or endgame values, and that we carefully cap the bonus
