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Restore "fail-low of reduced" and close regression

This reverts "Threat Extensions" and is the last of
this revert series.

In single-thread tests we should now be on par with 2.3.1
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commit 9f94d2280110a1ba5eb9eb15b0756883037a817d 1 parent 0901e12
@mcostalba authored
Showing with 58 additions and 3 deletions.
  1. +58 −3 src/search.cpp
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61 src/search.cpp
@@ -99,6 +99,7 @@ namespace {
Value value_to_tt(Value v, int ply);
Value value_from_tt(Value v, int ply);
bool check_is_dangerous(Position& pos, Move move, Value futilityBase, Value beta);
+ bool allows_move(const Position& pos, Move first, Move second);
bool prevents_move(const Position& pos, Move first, Move second);
string uci_pv(const Position& pos, int depth, Value alpha, Value beta);
@@ -692,9 +693,21 @@ namespace {
return nullValue;
}
else
+ {
// The null move failed low, which means that we may be faced with
- // some kind of threat.
+ // 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).
threatMove = (ss+1)->currentMove;
+
+ if ( depth < 5 * ONE_PLY
+ && (ss-1)->reduction
+ && threatMove != MOVE_NONE
+ && allows_move(pos, (ss-1)->currentMove, threatMove))
+ return beta - 1;
+ }
}
// Step 9. ProbCut (is omitted in PV nodes)
@@ -847,12 +860,13 @@ namespace {
&& !inCheck
&& !dangerous
&& move != ttMove
- && (!threatMove || !prevents_move(pos, move, threatMove))
&& (bestValue > VALUE_MATED_IN_MAX_PLY || ( bestValue == -VALUE_INFINITE
&& alpha > VALUE_MATED_IN_MAX_PLY)))
{
// Move count based pruning
- if (depth < 16 * ONE_PLY && moveCount >= FutilityMoveCounts[depth])
+ if ( depth < 16 * ONE_PLY
+ && moveCount >= FutilityMoveCounts[depth]
+ && (!threatMove || !prevents_move(pos, move, threatMove)))
{
if (SpNode)
sp->mutex.lock();
@@ -1352,6 +1366,47 @@ namespace {
}
+ // allows_move() tests whether the move at previous ply (first) somehow makes a
+ // second move possible, for instance if the moving piece is the same in both
+ // moves. Normally the second move is the threat move (the best move returned
+ // from a null search that fails low).
+
+ bool allows_move(const Position& pos, Move first, Move second) {
+
+ assert(is_ok(first));
+ assert(is_ok(second));
+ assert(color_of(pos.piece_on(from_sq(second))) == ~pos.side_to_move());
+ assert(color_of(pos.piece_on(to_sq(first))) == ~pos.side_to_move());
+
+ Square m1from = from_sq(first);
+ Square m2from = from_sq(second);
+ Square m1to = to_sq(first);
+ Square m2to = to_sq(second);
+
+ // The piece is the same or second's destination was vacated by the first move
+ if (m1to == m2from || m2to == m1from)
+ return true;
+
+ // Second one moves through the square vacated by first one
+ if (between_bb(m2from, m2to) & m1from)
+ return true;
+
+ // Second's destination is defended by the first move's piece
+ Bitboard m1att = pos.attacks_from(pos.piece_on(m1to), m1to, pos.pieces() ^ m2from);
+ if (m1att & m2to)
+ return true;
+
+ // Second move gives a discovered check through the first's checking piece
+ if (m1att & pos.king_square(pos.side_to_move()))
+ {
+ assert(between_bb(m1to, pos.king_square(pos.side_to_move())) & m2from);
+ return true;
+ }
+
+ return false;
+ }
+
+
// prevents_move() tests whether a move (first) is able to defend against an
// opponent's move (second). In this case will not be pruned. Normally the
// second move is the threat move (the best move returned from a null search
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