Added a new function build_pv(), which extends a PV by walking

[stockfish] / src / search.cpp

diff --git a/src/search.cpp b/src/search.cpp
index f0d2431487641dc765f5c56d1a7a029f7cf439ef..d922986b7fd7b3744441e92e352391925dd5d967 100644 (file)
--- a/src/search.cpp
+++ b/src/search.cpp
@@ -306,6 +306,7 @@ namespace {
   void update_history(const Position& pos, Move m, Depth depth, Move movesSearched[], int moveCount);
   void update_killers(Move m, SearchStack& ss);
   void slowdown(const Position& pos);
+  void build_pv(const Position& pos, Move pv[]);
 
   bool fail_high_ply_1();
   int current_search_time();
@@ -975,6 +976,7 @@ namespace {
             // Update PV
             rml.set_move_score(i, value);
             update_pv(ss, 0);
+            build_pv(pos, ss[0].pv);
             rml.set_move_pv(i, ss[0].pv);
 
             if (MultiPV == 1)
@@ -2430,7 +2432,7 @@ namespace {
   }
 
 
-  // slowdown() simply wastes CPU cycles doing nothing useful.  It's used
+  // slowdown() simply wastes CPU cycles doing nothing useful. It's used
   // in strength handicap mode.
 
   void slowdown(const Position &pos) {
@@ -2444,6 +2446,37 @@ namespace {
   }
 
 
+  // build_pv() extends a PV by adding moves from the transposition table at
+  // the end. This should ensure that the PV is almost always at least two
+  // plies long, which is important, because otherwise we will often get
+  // single-move PVs when the search stops while failing high, and a
+  // single-move PV means that we don't have a ponder move.
+
+  void build_pv(const Position& pos, Move pv[]) {
+    int ply;
+    Position p(pos);
+    StateInfo st[100];
+
+    for (ply = 0; pv[ply] != MOVE_NONE; ply++)
+        p.do_move(pv[ply], st[ply]);
+
+    bool stop;
+    const TTEntry* tte;
+    for (stop = false, tte = TT.retrieve(p.get_key());
+         tte && tte->move() != MOVE_NONE && !stop;
+         tte = TT.retrieve(p.get_key()), ply++)
+    {
+        if (!move_is_legal(p, tte->move(), p.pinned_pieces(p.side_to_move())))
+            break;
+        pv[ply] = tte->move();
+        p.do_move(pv[ply], st[ply]);
+        for (int j = 0; j < ply; j++)
+            if (st[j].key == p.get_key()) stop = true;
+    }
+    pv[ply] = MOVE_NONE;
+  }
+
+
   // fail_high_ply_1() checks if some thread is currently resolving a fail
   // high at ply 1 at the node below the first root node.  This information
   // is used for time managment.