Bench: 26587524

src/movegen.c

@@ -187,3 +187,10 @@ void GenLegalMoves(Position *pos, MoveList *list) {
     }
     pos->nodes = 0;
 }
+
+int LegalMoveCount(Position *pos) {
+    MoveList list;
+    list.count = list.next = 0;
+    GenLegalMoves(pos, &list);
+    return list.count;
+}

src/movegen.h

@@ -38,3 +38,4 @@ void GenNoisyMoves(const Position *pos, MoveList *list);
 void GenQuietMoves(const Position *pos, MoveList *list);
 void GenAllMoves(const Position *pos, MoveList *list);
 void GenLegalMoves(Position *pos, MoveList *list);
+int LegalMoveCount(Position *pos);

src/search.c

@@ -105,8 +105,12 @@ static int Quiescence(Thread *thread, Stack *ss, int alpha, int beta) {
             return alpha;
     }
 
-    // Position is drawn
-    if (IsRepetition(pos) || pos->rule50 >= 100)
+    // Position is drawn by repetition
+    if (IsRepetition(pos))
+        return DrawScore(pos);
+
+    // Position is drawn by 50 move rule
+    if (pos->rule50 >= 100 && (!pos->checkers || LegalMoveCount(pos) > 0))
         return DrawScore(pos);
 
     // If we are at max depth, return static eval
@@ -260,8 +264,12 @@ static int AlphaBeta(Thread *thread, Stack *ss, int alpha, int beta, Depth depth
                 return alpha;
         }
 
-        // Position is drawn
-        if (IsRepetition(pos) || pos->rule50 >= 100)
+        // Position is drawn by repetition
+        if (IsRepetition(pos))
+            return DrawScore(pos);
+
+        // Position is drawn by 50 move rule
+        if (pos->rule50 >= 100 && (!pos->checkers || LegalMoveCount(pos) > 0))
             return DrawScore(pos);
 
         // Max depth reached