+
+ // Null move search
+ if (move == MOVE_NULL)
+ {
+ ss[ply].currentMove = MOVE_NULL;
+
+ StateInfo st;
+ pos.do_null_move(st);
+ int R = (depth >= 5 * OnePly ? 4 : 3); // Null move dynamic reduction
+
+ Value nullValue = -search(pos, ss, -(beta-1), depth-R*OnePly, ply+1, false, threadID);
+
+ pos.undo_null_move();
+
+ if (nullValue >= beta)
+ {
+ if (depth < 6 * OnePly)
+ return beta;
+
+ // Do zugzwang verification search
+ Value v = search(pos, ss, beta, depth-5*OnePly, ply, false, threadID);
+ if (v >= beta)
+ return beta;
+ } else {
+ // The null move failed low, which means that we may be faced with
+ // some kind of threat. If the previous move was reduced, check if
+ // the move that refuted the null move was somehow connected to the
+ // move which was reduced. If a connection is found, return a fail
+ // low score (which will cause the reduced move to fail high in the
+ // parent node, which will trigger a re-search with full depth).
+ if (nullValue == value_mated_in(ply + 2))
+ mateThreat = true;
+
+ ss[ply].threatMove = ss[ply + 1].currentMove;
+ if ( depth < ThreatDepth
+ && ss[ply - 1].reduction
+ && connected_moves(pos, ss[ply - 1].currentMove, ss[ply].threatMove))
+ return beta - 1;
+ }
+ continue;
+ }
+