experimental ponder support

issues:
* `lift` and other commands during ponder
* stability?

src/search.cpp

@@ -301,14 +301,17 @@ void MainThread::search() {
   if (Options["Protocol"] == "xboard")
   {
       // Send move only when not in analyze mode and not at game end
-      if (!Options["UCI_AnalyseMode"] && rootMoves[0].pv[0] != MOVE_NONE && !Threads.abort.exchange(true))
+      if (!Limits.infinite && !ponder && rootMoves[0].pv[0] != MOVE_NONE && !Threads.abort.exchange(true))
       {
           Move bestMove = bestThread->rootMoves[0].pv[0];
           sync_cout << "move " << UCI::move(rootPos, bestMove) << sync_endl;
           if (XBoard::stateMachine->moveAfterSearch)
           {
               XBoard::stateMachine->do_move(bestMove);
               XBoard::stateMachine->moveAfterSearch = false;
+              if (Options["Ponder"] && (   bestThread->rootMoves[0].pv.size() > 1
+                                        || bestThread->rootMoves[0].extract_ponder_from_tt(rootPos)))
+                  XBoard::stateMachine->ponderMove = bestThread->rootMoves[0].pv[1];
           }
       }
       return;

src/thread.cpp

@@ -25,6 +25,7 @@
 #include "uci.h"
 #include "syzygy/tbprobe.h"
 #include "tt.h"
+#include "xboard.h"
 
 ThreadPool Threads; // Global object
 
@@ -109,6 +110,12 @@ void Thread::idle_loop() {
       std::unique_lock<std::mutex> lk(mutex);
       searching = false;
       cv.notify_one(); // Wake up anyone waiting for search finished
+      // Start ponder search from separate thread to prevent deadlock
+      if (Threads.size() && this == Threads.main() && XBoard::stateMachine && XBoard::stateMachine->ponderMove)
+      {
+          NativeThread t(&XBoard::StateMachine::ponder, XBoard::stateMachine);
+          t.detach();
+      }
       cv.wait(lk, [&]{ return searching; });
 
       if (exit)

src/thread_win32_osx.h

@@ -55,6 +55,7 @@ class NativeThread {
     pthread_create(&thread, attr, start_routine<T>, new P(obj, fun));
   }
   void join() { pthread_join(thread, NULL); }
+  void detach() { pthread_detach(thread); }
 };
 
 #else // Default case: use STL classes

src/xboard.cpp

@@ -36,18 +36,31 @@ namespace {
     return limits;
   }();
 
+} // namespace
+
+namespace XBoard {
+
+  StateMachine* stateMachine = nullptr;
+
   // go() starts the search for game play, analysis, or perft.
 
-  void go(Position& pos, Search::LimitsType limits, StateListPtr& states) {
+  void StateMachine::go(Search::LimitsType searchLimits, bool ponder) {
+
+    searchLimits.startTime = now(); // As early as possible!
 
-    limits.startTime = now(); // As early as possible!
+    Threads.start_thinking(pos, states, searchLimits, ponder);
+  }
+
+  void StateMachine::ponder() {
 
-    Threads.start_thinking(pos, states, limits, false);
+    do_move(ponderMove);
+    ponderMove = MOVE_NONE;
+    go(limits, true);
   }
 
   // setboard() is called when engine receives the "setboard" XBoard command.
 
-  void setboard(Position& pos, std::deque<Move>& moveList, StateListPtr& states, std::string fen = "") {
+  void StateMachine::setboard(std::string fen) {
 
     if (fen.empty())
         fen = variants.find(Options["UCI_Variant"])->second->startFen;
@@ -57,12 +70,6 @@ namespace {
     pos.set(variants.find(Options["UCI_Variant"])->second, fen, Options["UCI_Chess960"], &states->back(), Threads.main());
   }
 
-} // namespace
-
-namespace XBoard {
-
-  StateMachine* stateMachine;
-
   // do_move() is called when engine needs to apply a move when using XBoard protocol.
 
   void StateMachine::do_move(Move m) {
@@ -94,13 +101,32 @@ namespace XBoard {
 /// StateMachine::process_command() processes commands of the XBoard protocol.
 
 void StateMachine::process_command(std::string token, std::istringstream& is) {
+  // TODO: move logic to respective commands?
   if (moveAfterSearch && token != "ptell")
   {
       // try to abort search in bughouse when receiving "holding" command
       if (token == "holding")
           Threads.abort = true;
       Threads.stop = true;
       Threads.main()->wait_for_search_finished();
+      Threads.main()->ponder = false;
+  }
+  // TODO: how to deal with other commands during ponder (such as lift -> consider other pos!)?
+  if (Threads.main()->ponder)
+  {
+      assert(moveList.size());
+      if ((token != "usermove" || is >> token) && token == UCI::move(pos, moveList.back()))
+      {
+          Threads.main()->ponder = false;
+          return;
+      }
+      else
+      {
+          Threads.stop = true;
+          Threads.main()->wait_for_search_finished();
+          undo_move();
+          Threads.main()->ponder = false;
+      }
   }
   if (token == "protover")
   {
@@ -172,7 +198,7 @@ void StateMachine::process_command(std::string token, std::istringstream& is) {
   else if (token == "new")
   {
       Search::clear();
-      setboard(pos, moveList, states);
+      setboard();
       // play second by default
       playColor = ~pos.side_to_move();
       Threads.sit = false;
@@ -182,14 +208,14 @@ void StateMachine::process_command(std::string token, std::istringstream& is) {
   {
       if (is >> token)
           Options["UCI_Variant"] = token;
-      setboard(pos, moveList, states);
+      setboard();
   }
   else if (token == "force")
       playColor = COLOR_NB;
   else if (token == "go")
   {
       playColor = pos.side_to_move();
-      go(pos, limits, states);
+      go(limits);
       moveAfterSearch = true;
   }
   else if (token == "level" || token == "st" || token == "sd" || token == "time" || token == "otim")
@@ -254,14 +280,14 @@ void StateMachine::process_command(std::string token, std::istringstream& is) {
               do_move(m);
           // apply setboard if passing does not lead to a match
           if (pos.key() != p.key())
-              setboard(pos, moveList, states, fen);
+              setboard(fen);
       }
       else
-          setboard(pos, moveList, states, fen);
+          setboard(fen);
       // Winboard sends setboard after passing moves
       if (pos.side_to_move() == playColor)
       {
-          go(pos, limits, states);
+          go(limits);
           moveAfterSearch = true;
       }
   }
@@ -293,7 +319,7 @@ void StateMachine::process_command(std::string token, std::istringstream& is) {
   else if (token == "analyze")
   {
       Options["UCI_AnalyseMode"] = std::string("true");
-      go(pos, analysisLimits, states);
+      go(analysisLimits);
   }
   else if (token == "exit")
   {
@@ -312,7 +338,7 @@ void StateMachine::process_command(std::string token, std::istringstream& is) {
           }
           undo_move();
           if (Options["UCI_AnalyseMode"])
-              go(pos, analysisLimits, states);
+              go(analysisLimits);
       }
   }
   // Bughouse commands
@@ -354,18 +380,18 @@ void StateMachine::process_command(std::string token, std::istringstream& is) {
               std::transform(black_holdings.begin(), black_holdings.end(), black_holdings.begin(), ::tolower);
               fen = pos.fen(false, false, 0, white_holdings + black_holdings);
           }
-          setboard(pos, moveList, states, fen);
+          setboard(fen);
       }
       // restart search
       if (moveAfterSearch)
-          go(pos, limits, states);
+          go(limits);
   }
   // Additional custom non-XBoard commands
   else if (token == "perft")
   {
       Search::LimitsType perft_limits;
       is >> perft_limits.perft;
-      go(pos, perft_limits, states);
+      go(perft_limits);
   }
   else if (token == "d")
       sync_cout << pos << sync_endl;
@@ -392,11 +418,11 @@ void StateMachine::process_command(std::string token, std::istringstream& is) {
       else
           sync_cout << (isMove ? "Illegal move: " : "Error (unknown command): ") << token << sync_endl;
       if (Options["UCI_AnalyseMode"])
-          go(pos, analysisLimits, states);
+          go(analysisLimits);
       else if (pos.side_to_move() == playColor)
       {
-          go(pos, limits, states);
           moveAfterSearch = true;
+          go(limits);
       }
   }
 }

src/xboard.h

@@ -36,11 +36,16 @@ class StateMachine {
     moveList = std::deque<Move>();
     moveAfterSearch = false;
     playColor = COLOR_NB;
+    ponderMove = MOVE_NONE;
   }
+  void go(Search::LimitsType searchLimits, bool ponder = false);
+  void ponder();
+  void setboard(std::string fen = "");
   void do_move(Move m);
   void undo_move();
   void process_command(std::string token, std::istringstream& is);
   bool moveAfterSearch;
+  Move ponderMove;
 
 private:
   Position& pos;