Properly handle checkmate occuring exactly when reaching the 50 move …

…rule (#754)

Elo   | 0.18 +- 2.84 (95%)
Conf  | 8.0+0.08s Threads=1 Hash=32MB
Games | N: 21244 W: 5800 L: 5789 D: 9655
Penta | [369, 2456, 4950, 2489, 358]
http://chess.grantnet.us/test/38610/

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 && (!inCheck || LegalMoveCount(pos) > 0))
         return DrawScore(pos);
 
     // If we are at max depth, return static eval
@@ -244,7 +248,8 @@ static int AlphaBeta(Thread *thread, Stack *ss, int alpha, int beta, Depth depth
     ss->doubleExtensions = (ss-1)->doubleExtensions;
 
     const bool pvNode = alpha != beta - 1;
-    const bool root   = ss->ply == 0;
+    const bool root = ss->ply == 0;
+    const bool inCheck = pos->checkers;
 
     // Check time situation
     if (OutOfTime(thread) || loadRelaxed(ABORT_SIGNAL))
@@ -260,8 +265,12 @@ 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 && (!inCheck || LegalMoveCount(pos) > 0))
             return DrawScore(pos);
 
         // Max depth reached
@@ -325,8 +334,6 @@ static int AlphaBeta(Thread *thread, Stack *ss, int alpha, int beta, Depth depth
         }
     }
 
-    const bool inCheck = pos->checkers;
-
     // Do a static evaluation for pruning considerations
     int eval = ss->staticEval =  inCheck           ? NOSCORE
                                : lastMoveNullMove  ? -(ss-1)->staticEval + 2 * Tempo