Fix issue official-stockfish#5023

src/search.cpp

@@ -105,6 +105,7 @@ struct Skill {
 
 Value value_to_tt(Value v, int ply);
 Value value_from_tt(Value v, int ply, int r50c);
+bool possibleFortress(Stack* ss);
 void update_pv(Move* pv, Move move, const Move* childPv);
 void update_continuation_histories(Stack* ss, Piece pc, Square to, int bonus);
 void update_quiet_stats(
@@ -1184,7 +1185,14 @@ Value Search::Worker::search(
  newDepth += doDeeperSearch - doShallowerSearch;
 
  if (newDepth > d)
+ {
+ if (possibleFortress(ss))
+ {
+ newDepth = 2 * newDepth / 3;
+ assert(newDepth >= 1);
+ }
  value = -search<NonPV>(pos, ss + 1, -(alpha + 1), -alpha, newDepth, !cutNode);
+ }
 
  // Post LMR continuation history updates (~1 Elo)
  int bonus = value <= alpha ? -stat_malus(newDepth)
@@ -1202,8 +1210,12 @@ Value Search::Worker::search(
  if (!ttMove)
  r += 2;
 
+ if (newDepth > 1 && possibleFortress(ss))
+ newDepth = 2 * newDepth / 3;
+
  // Note that if expected reduction is high, we reduce search depth by 1 here (~9 Elo)
- value = -search<NonPV>(pos, ss + 1, -(alpha + 1), -alpha, newDepth - (r > 3), !cutNode);
+ value = -search<NonPV>(pos, ss + 1, -(alpha + 1), -alpha,
+ newDepth - (r > 3 && !possibleFortress(ss)), !cutNode);
  }
 
  // For PV nodes only, do a full PV search on the first move or after a fail high,
@@ -1373,7 +1385,6 @@ Value Search::Worker::search(
  return bestValue;
 }
 
-
 // Quiescence search function, which is called by the main search
 // function with zero depth, or recursively with further decreasing depth per call.
 // (~155 Elo)
@@ -1812,6 +1823,24 @@ void update_quiet_stats(
 }
 }
 
+// Detects a possible fortress by checking weither any player can keep repeating moves
+// this fixes a fortress search explosion in case staticEval of the fortress is way off
+bool possibleFortress(Stack* ss) {
+ const bool fortressUs =
+ (ss - 4)->currentMove.is_ok()
+ && (ss - 6)->currentMove
+ == Move((ss - 4)->currentMove.to_sq(), (ss - 4)->currentMove.from_sq())
+ && (ss - 6)->currentMove == (ss - 2)->currentMove;
+
+ const bool fortressThem =
+ (ss - 5)->currentMove.is_ok()
+ && (ss - 7)->currentMove
+ == Move((ss - 5)->currentMove.to_sq(), (ss - 5)->currentMove.from_sq())
+ && (ss - 7)->currentMove == (ss - 3)->currentMove;
+
+ return fortressUs || fortressThem;
+}
+
 // When playing with strength handicap, choose the best move among a set of RootMoves
 // using a statistical rule dependent on 'level'. Idea by Heinz van Saanen.
 Move Skill::pick_best(const RootMoves& rootMoves, size_t multiPV) {