Introduce refine_eval()

Try to get a position evaluation better then
the quick one with the help of the TT table.

This allows the null search conditions and
chosen reductions to be more accurate.

After 908 games at 1+0
Mod vs Orig +209 =526 -173 +14 ELO

Functionality Signature: 16627355

Signed-off-by: Marco Costalba <mcostalba@gmail.com>

diff --git a/src/search.cpp b/src/search.cpp
index 5fc9be3679aa1d265b71285ab82b69e4460936eb..09b483cfc4143ec056b1ced25620245f9a925672 100644 (file)
--- a/src/search.cpp
+++ b/src/search.cpp
@@ -288,6 +288,7 @@ namespace {
   bool ok_to_do_nullmove(const Position& pos);
   bool ok_to_prune(const Position& pos, Move m, Move threat);
   bool ok_to_use_TT(const TTEntry* tte, Depth depth, Value beta, int ply);
+  Value refine_eval(const TTEntry* tte, Value defaultEval, int ply);
   void update_history(const Position& pos, Move move, Depth depth, Move movesSearched[], int moveCount);
   void update_killers(Move m, SearchStack& ss);
 
@@ -1360,7 +1361,7 @@ namespace {
         return value_from_tt(tte->value(), ply);
     }
 
-    approximateEval = quick_evaluate(pos);
+    approximateEval = refine_eval(tte, quick_evaluate(pos), ply);
     isCheck = pos.is_check();
 
     // Null move search
@@ -2474,6 +2475,23 @@ namespace {
   }
 
 
+  // refine_eval() returns the transposition table score if
+  // possible otherwise falls back on static position evaluation.
+
+  Value refine_eval(const TTEntry* tte, Value defaultEval, int ply) {
+
+      if (!tte)
+          return defaultEval;
+
+      Value v = value_from_tt(tte->value(), ply);
+
+      if (   (is_lower_bound(tte->type()) && v >= defaultEval)
+          || (is_upper_bound(tte->type()) && v < defaultEval))
+          return v;
+
+      return defaultEval;
+  }
+
   // update_history() registers a good move that produced a beta-cutoff
   // in history and marks as failures all the other moves of that ply.