Add syzygy support

bench: 8080602

src/search.cpp

@@ -33,6 +33,7 @@
 #include "thread.h"
 #include "tt.h"
 #include "uci.h"
+#include "syzygy/tbprobe.h"
 
 namespace Search {
 
@@ -44,6 +45,18 @@ namespace Search {
   StateStackPtr SetupStates;
 }
 
+namespace Tablebases {
+
+  int Cardinality;
+  uint64_t Hits;
+  bool RootInTB;
+  bool UseRule50;
+  Depth ProbeDepth;
+  Value Score;
+}
+
+namespace TB = Tablebases;
+
 using std::string;
 using Eval::evaluate;
 using namespace Search;
@@ -186,6 +199,19 @@ void Search::think() {
   DrawValue[ RootPos.side_to_move()] = VALUE_DRAW - Value(contempt);
   DrawValue[~RootPos.side_to_move()] = VALUE_DRAW + Value(contempt);
 
+  TB::Hits = 0;
+  TB::RootInTB = false;
+  TB::UseRule50 = Options["Syzygy50MoveRule"];
+  TB::ProbeDepth = Options["SyzygyProbeDepth"] * ONE_PLY;
+  TB::Cardinality = Options["SyzygyProbeLimit"];
+
+  // Skip TB probing when no TB found: !TBLargest -> !TB::Cardinality
+  if (TB::Cardinality > TB::MaxCardinality)
+  {
+      TB::Cardinality = TB::MaxCardinality;
+      TB::ProbeDepth = DEPTH_ZERO;
+  }
+
   if (RootMoves.empty())
   {
       RootMoves.push_back(MOVE_NONE);
@@ -195,6 +221,37 @@ void Search::think() {
   }
   else
   {
+      if (TB::Cardinality >=  RootPos.count<ALL_PIECES>(WHITE)
+                            + RootPos.count<ALL_PIECES>(BLACK))
+      {
+          // If the current root position is in the tablebases then RootMoves
+          // contains only moves that preserve the draw or win.
+          TB::RootInTB = Tablebases::root_probe(RootPos, RootMoves, TB::Score);
+
+          if (TB::RootInTB)
+              TB::Cardinality = 0; // Do not probe tablebases during the search
+
+          else // If DTZ tables are missing, use WDL tables as a fallback
+          {
+              // Filter out moves that do not preserve a draw or win
+              TB::RootInTB = Tablebases::root_probe_wdl(RootPos, RootMoves, TB::Score);
+
+              // Only probe during search if winning
+              if (TB::Score <= VALUE_DRAW)
+                  TB::Cardinality = 0;
+          }
+
+          if (TB::RootInTB)
+          {
+              TB::Hits = RootMoves.size();
+
+              if (!TB::UseRule50)
+                  TB::Score =  TB::Score > VALUE_DRAW ?  VALUE_MATE - MAX_PLY - 1
+                             : TB::Score < VALUE_DRAW ? -VALUE_MATE + MAX_PLY + 1
+                                                      :  VALUE_DRAW;
+          }
+      }
+
       for (Thread* th : Threads)
           th->maxPly = 0;
 
@@ -487,6 +544,36 @@ namespace {
         return ttValue;
     }
 
+    // Step 4a. Tablebase probe
+    if (!RootNode && TB::Cardinality)
+    {
+        int piecesCnt = pos.count<ALL_PIECES>(WHITE) + pos.count<ALL_PIECES>(BLACK);
+
+        if (    piecesCnt <= TB::Cardinality
+            && (piecesCnt <  TB::Cardinality || depth >= TB::ProbeDepth)
+            &&  pos.rule50_count() == 0)
+        {
+            int found, v = Tablebases::probe_wdl(pos, &found);
+
+            if (found)
+            {
+                TB::Hits++;
+
+                int drawScore = TB::UseRule50 ? 1 : 0;
+
+                value =  v < -drawScore ? -VALUE_MATE + MAX_PLY + ss->ply
+                       : v >  drawScore ?  VALUE_MATE - MAX_PLY - ss->ply
+                                        :  VALUE_DRAW + 2 * v * drawScore;
+
+                tte->save(posKey, value_to_tt(value, ss->ply), BOUND_EXACT,
+                          std::min(DEPTH_MAX - ONE_PLY, depth + 6 * ONE_PLY),
+                          MOVE_NONE, VALUE_NONE, TT.generation());
+
+                return value;
+            }
+        }
+    }
+
     // Step 5. Evaluate the position statically and update parent's gain statistics
     if (inCheck)
     {
@@ -1347,6 +1434,9 @@ moves_loop: // When in check and at SpNode search starts from here
         Depth d = updated ? depth : depth - ONE_PLY;
         Value v = updated ? RootMoves[i].score : RootMoves[i].previousScore;
 
+        bool tb = TB::RootInTB && abs(v) < VALUE_MATE - MAX_PLY;
+        v = tb ? TB::Score : v;
+
         if (ss.rdbuf()->in_avail()) // Not at first line
             ss << "\n";
 
@@ -1355,11 +1445,12 @@ moves_loop: // When in check and at SpNode search starts from here
            << " multipv "   << i + 1
            << " score "     << UCI::value(v);
 
-        if (i == PVIdx)
+        if (!tb && i == PVIdx)
               ss << (v >= beta ? " lowerbound" : v <= alpha ? " upperbound" : "");
 
         ss << " nodes "     << pos.nodes_searched()
            << " nps "       << pos.nodes_searched() * 1000 / elapsed
+           << " tbhits "    << TB::Hits
            << " time "      << elapsed
            << " pv";