Add moves from failed high nodes in PV

Considering only VALUE_TYPE_EXACT nodes is too
restrictive and has a number of side-effects, most
notably the truncation of PV line after a fail high
at root.

Note that in this way we are no more guaranteed that
PV line is built up with PV nodes only, because
it could happen that a side search overwrites with a
cut-off move a PV node and this cut-off move ends up
in PV.

Change should be almost not measurable, perhaps with
ponder on we could have some beneficial effect.

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

diff --git a/src/tt.cpp b/src/tt.cpp
index 5c5e10c16b720115ca4b20fabb9c97dbd21ffd99..bc26606721413eb90ecae8923c7d4238960892d2 100644 (file)
--- a/src/tt.cpp
+++ b/src/tt.cpp
@@ -176,11 +176,11 @@ void TranspositionTable::insert_pv(const Position& pos, Move pv[]) {
 }
 
 
 }
 
 

-/// TranspositionTable::extract_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.
+/// TranspositionTable::extract_pv() builds a PV by adding moves from the
+/// transposition table. We consider also failing high nodes and not only
+/// VALUE_TYPE_EXACT nodes. This allow to always have a ponder move even
+/// when we fail high at root and also a long PV to print that is important
+/// for position analysis.

 
 void TranspositionTable::extract_pv(const Position& pos, Move bestMove, Move pv[], const int PLY_MAX) {
 
 
 void TranspositionTable::extract_pv(const Position& pos, Move bestMove, Move pv[], const int PLY_MAX) {
 


@@ -194,11 +194,8 @@ void TranspositionTable::extract_pv(const Position& pos, Move bestMove, Move pv[
   pv[ply] = bestMove;
   p.do_move(pv[ply++], st);
 
   pv[ply] = bestMove;
   p.do_move(pv[ply++], st);
 

-  // Extract moves from TT when possible. We try hard to always
-  // get a ponder move, that's the reason of ply < 2 conditions.

   while (   (tte = retrieve(p.get_key())) != NULL
          && tte->move() != MOVE_NONE
   while (   (tte = retrieve(p.get_key())) != NULL
          && tte->move() != MOVE_NONE

-         && (tte->type() == VALUE_TYPE_EXACT || ply < 2)

          && move_is_legal(p, tte->move())
          && (!p.is_draw() || ply < 2)
          && ply < PLY_MAX)
          && move_is_legal(p, tte->move())
          && (!p.is_draw() || ply < 2)
          && ply < PLY_MAX)