rule50: Detect rule50 when searching

src/position.cpp

@@ -405,15 +405,24 @@ void Position::do_move(Move m)
 
     switch (mt) {
     case MOVETYPE_REMOVE:
-        // Reset rule 50 counter
-        st.rule50 = 0;
         ret = remove_piece(to_sq(m));
+        if (ret) {
+            // Reset rule 50 counter
+            st.rule50 = 0;
+        }
         break;
     case MOVETYPE_MOVE:
         ret = move_piece(from_sq(m), to_sq(m));
+        if (ret) {
+            ++st.rule50;
+        }
         break;
     case MOVETYPE_PLACE:
         ret = put_piece(to_sq(m));
+        if (ret) {
+            // Reset rule 50 counter
+            st.rule50 = 0;
+        }
         break;
     default:
         break;
@@ -423,10 +432,8 @@ void Position::do_move(Move m)
         return;
     }
 
-    // Increment ply counters. In particular, rule50 will be reset to zero later on
-    // in case of a remove.
+    // Increment ply counters. In particular
     ++gamePly;
-    ++st.rule50;
     ++st.pliesFromNull;
     
     move = m;

src/search.cpp

@@ -111,6 +111,9 @@ int Thread::search()
 
     if (rootPos->get_phase() == Phase::placing) {
         posKeyHistory.clear();
+        rootPos->st.rule50 = 0;
+    } else if (rootPos->get_phase() == Phase::moving) {
+        rootPos->st.rule50 = (int)posKeyHistory.size();
     }
 
 
@@ -201,6 +204,13 @@ Value search(Position *pos, Sanmill::Stack<Position> &ss, Depth depth, Depth ori
 
     Depth epsilon;
 
+    if (pos->rule50_count() > 99) {
+        alpha = VALUE_DRAW;
+        if (alpha >= beta) {
+            return alpha;
+        }
+    }
+
 #ifdef THREEFOLD_REPETITION
     // Check if we have an upcoming move which draws by repetition, or
     // if the opponent had an alternative move earlier to this position.