El Filja: Improve AI balance between mill formation and center control

Address user feedback where AI prioritized mill formation as white,
allowing black to exploit and gain advantage. Updated the evaluation
function to account for the difference in pieces forming mills during
the placing phase. This ensures the AI balances between controlling
the center and defending against early mill formations, resulting in
more challenging and realistic gameplay.

src/evaluate.cpp

@@ -43,6 +43,15 @@ Value Evaluation::value() const
         break;
 
     case Phase::placing:
+        if (rule.millFormationActionInPlacingPhase ==
+            MillFormationActionInPlacingPhase::removalBasedOnMillCounts) {
+            if (pos.get_action() == Action::remove) {
+                value += VALUE_EACH_PIECE_NEEDREMOVE * pieceToRemoveDiffCount;
+            } else {
+                value += pos.mills_pieces_count_difference();
+            }
+            break;
+        }
     case Phase::moving:
         if (pos.shouldConsiderMobility()) {
             value += pos.get_mobility_diff();

src/position.h

@@ -191,6 +191,7 @@ class Position
     bool move_piece(Square from, Square to);
 
     int total_mills_count(Color c);
+    int mills_pieces_count_difference() const;
     void calculate_removal_based_on_mill_counts();
     bool is_board_full_removal_at_placing_phase_end();
     bool is_adjacent_to(Square s, Color c);
@@ -404,6 +405,12 @@ inline int Position::get_mobility_diff() const
     return mobilityDiff;
 }
 
+inline int Position::mills_pieces_count_difference() const
+{
+    return popcount(formedMillsBB[WHITE]) -
+           popcount(formedMillsBB[BLACK]);
+}
+
 inline bool Position::shouldFocusOnBlockingPaths() const
 {
     if (get_phase() == Phase::placing) {