Handle UCI command "mate in x moves"

Following a user request I added the handling of UCI:

go mate x

Currently we just return from a PV node if x moves have been
done. Probably not the best approach. I have looked at Fruit/Toga
sources and there is even simpler: engine falls back on a fixed
depth search.

No functional change.

src/search.cpp

@@ -194,7 +194,7 @@ void Search::think() {
       goto finalize;
   }
 
-  if (Options["OwnBook"] && !Limits.infinite)
+  if (Options["OwnBook"] && !Limits.infinite && !Limits.mate)
   {
       Move bookMove = book.probe(RootPos, Options["Book File"], Options["Best Book Move"]);
 
@@ -410,6 +410,12 @@ namespace {
         if (depth > 2 && BestMoveChanges)
             bestMoveNeverChanged = false;
 
+        // Do we have found a "mate in x"?
+        if (   Limits.mate
+            && bestValue >= VALUE_MATE_IN_MAX_PLY
+            && VALUE_MATE - bestValue <= 2 * Limits.mate)
+            Signals.stop = true;
+
         // Do we have time for the next iteration? Can we stop searching now?
         if (Limits.use_time_management() && !Signals.stopOnPonderhit)
         {
@@ -602,6 +608,11 @@ namespace {
                  ss->staticEval, ss->evalMargin);
     }
 
+    // Handling of UCI command 'mate in x moves'. We simply return if after
+    // 'x' moves we still have not checkmated the opponent.
+    if (PvNode && !RootNode && !inCheck && Limits.mate && ss->ply > 2 * Limits.mate)
+        return eval;
+
     // Update gain for the parent non-capture move given the static position
     // evaluation before and after the move.
     if (   (move = (ss-1)->currentMove) != MOVE_NULL

src/search.h

@@ -80,9 +80,9 @@ struct RootMove {
 struct LimitsType {
 
   LimitsType() { memset(this, 0, sizeof(LimitsType)); }
-  bool use_time_management() const { return !(movetime | depth | nodes | infinite); }
+  bool use_time_management() const { return !(mate | movetime | depth | nodes | infinite); }
 
-  int time[COLOR_NB], inc[COLOR_NB], movestogo, depth, nodes, movetime, infinite, ponder;
+  int time[COLOR_NB], inc[COLOR_NB], movestogo, depth, nodes, movetime, mate, infinite, ponder;
 };
 
 

src/uci.cpp

@@ -215,6 +215,7 @@ namespace {
         else if (token == "depth")     is >> limits.depth;
         else if (token == "nodes")     is >> limits.nodes;
         else if (token == "movetime")  is >> limits.movetime;
+        else if (token == "mate")      is >> limits.mate;
         else if (token == "infinite")  limits.infinite = true;
         else if (token == "ponder")    limits.ponder = true;
     }