An VALUE_TYPE_EVAL score cannot overwrite an entry

If we want to store a value of type VALUE_TYPE_EVAL for
a given position and we found an already exsisting entry
for the same position then we skip.

We don't want to overwrite a more valuable score with a
lesser one. Note that also in case the exsisting entry is
of VALUE_TYPE_EVAL type the overwrite is unuseful because
we would store the same score again.

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

diff --git a/src/tt.cpp b/src/tt.cpp
index d7b3b7d5b099a7dd91b39808c780067c56baf86f..82d967cab9bb0e018eb4ea00f0912f1f6b8d4415 100644 (file)
--- a/src/tt.cpp
+++ b/src/tt.cpp
@@ -101,7 +101,8 @@ void TranspositionTable::clear() {
 /// the least valuable of the four entries in a cluster.  A TTEntry t1 is
 /// considered to be more valuable than a TTEntry t2 if t1 is from the
 /// current search and t2 is from a previous search, or if the depth of t1
-/// is bigger than the depth of t2.
+/// is bigger than the depth of t2. A TTEntry of type VALUE_TYPE_EVAL
+/// never replaces another entry for the same position.
 
 TTEntry* TranspositionTable::store(const Position &pos, Value v, Depth d,
                                Move m, ValueType type) {
@@ -112,6 +113,11 @@ TTEntry* TranspositionTable::store(const Position &pos, Value v, Depth d,
   {
     if (!tte->key() || tte->key() == pos.get_key()) // empty or overwrite old
     {
+        // Do not overwrite position entry when VALUE_TYPE_EVAL
+        if (   tte->key()
+            && type == VALUE_TYPE_EVAL)
+            return NULL;
+
         if (m == MOVE_NONE)
             m = tte->move();