Null capture pruning

Null move can fail low because of a capture artifact due
to the side to move change. Try to detect this condition
and fail high instead.

This pruning is very powerful, around 7% of nodes, but is
still experimental so is disabled by default.

Set UseNullCapturePruning to true to enable.

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

diff --git a/src/search.cpp b/src/search.cpp
index d02035183b07a262abd14de260b6baee824aa332..ddcc851eb8cfa1d014fe5ecd06d21075507500b4 100644 (file)
--- a/src/search.cpp
+++ b/src/search.cpp
@@ -133,6 +133,9 @@ namespace {
   // evaluation of the position is more than NullMoveMargin below beta.
   const Value NullMoveMargin = Value(0x300);
 
   // evaluation of the position is more than NullMoveMargin below beta.
   const Value NullMoveMargin = Value(0x300);
 

+  // Use null capture pruning?
+  const bool UseNullCapturePruning = false;
+

   // Pruning criterions.  See the code and comments in ok_to_prune() to
   // understand their precise meaning.
   const bool PruneEscapeMoves = false;
   // Pruning criterions.  See the code and comments in ok_to_prune() to
   // understand their precise meaning.
   const bool PruneEscapeMoves = false;


@@ -1130,6 +1133,7 @@ namespace {
 
     Value approximateEval = quick_evaluate(pos);
     bool mateThreat = false;
 
     Value approximateEval = quick_evaluate(pos);
     bool mateThreat = false;

+    bool nullCapturePruning = false;

     bool isCheck = pos.is_check();
 
     // Null move search
     bool isCheck = pos.is_check();
 
     // Null move search


@@ -1143,7 +1147,20 @@ namespace {
         UndoInfo u;
         pos.do_null_move(u);
         int R = (depth > 7 ? 4 : 3);
         UndoInfo u;
         pos.do_null_move(u);
         int R = (depth > 7 ? 4 : 3);

+

         Value nullValue = -search(pos, ss, -(beta-1), depth-R*OnePly, ply+1, false, threadID);
         Value nullValue = -search(pos, ss, -(beta-1), depth-R*OnePly, ply+1, false, threadID);

+
+        // Check for a null capture artifact, if the value without the null capture
+        // is above beta then mark the node as a suspicious failed low. We will verify
+        // later if we are really under threat.
+        if (   UseNullCapturePruning
+            && nullValue < beta
+            && depth < 5 * OnePly
+            && ss[ply + 1].currentMove != MOVE_NONE
+            && pos.move_is_capture(ss[ply + 1].currentMove)
+            && pos.see(ss[ply + 1].currentMove) * PawnValueMidgame + nullValue > beta)
+            nullCapturePruning = true;
+

         pos.undo_null_move(u);
 
         if (nullValue >= beta)
         pos.undo_null_move(u);
 
         if (nullValue >= beta)


@@ -1170,6 +1187,26 @@ namespace {
                 && ss[ply - 1].reduction
                 && connected_moves(pos, ss[ply - 1].currentMove, ss[ply].threatMove))
                 return beta - 1;
                 && ss[ply - 1].reduction
                 && connected_moves(pos, ss[ply - 1].currentMove, ss[ply].threatMove))
                 return beta - 1;

+
+            if (nullCapturePruning && !mateThreat)
+            {
+                // The null move failed low due to a suspicious capture. Verify if
+                // position is really dangerous or we are facing a null capture
+                // artifact due to the side to move change. So search this
+                // position with a reduced depth and see if we still fail low.
+                Move tm = ss[ply].threatMove;
+
+                assert(tm != MOVE_NONE);
+
+                Value v = search(pos, ss, beta, depth-3*OnePly, ply, false, threadID);
+                if (v >= beta)
+                    return beta;
+
+                // Restore stack and update ttMove if was empty
+                ss[ply].threatMove = tm;
+                if (ttMove == MOVE_NONE)
+                    ttMove = ss[ply].pv[ply];
+            }

         }
     }
     // Null move search not allowed, try razoring
         }
     }
     // Null move search not allowed, try razoring