X-Git-Url: https://git.sesse.net/?p=stockfish;a=blobdiff_plain;f=src%2Fposition.cpp;h=7c5ffdc5f6adcc905a2a98adb2e5b5cd60e1e4bc;hp=9f5816d70b1326919867959e077329cb29215bcc;hb=2ec626ddae9c81af6338f72296df8a1465b5e036;hpb=0fa80c9ba3b766c4d99ea06d4bc52eb33c28ff47

diff --git a/src/position.cpp b/src/position.cpp
index 9f5816d7..7c5ffdc5 100644
--- a/src/position.cpp
+++ b/src/position.cpp
@@ -45,7 +45,7 @@ namespace Zobrist {
   Key psq[PIECE_NB][SQUARE_NB];
   Key enpassant[FILE_NB];
   Key castling[CASTLING_RIGHT_NB];
-  Key side;
+  Key side, noPawns;
 }
 
 namespace {
@@ -145,6 +145,7 @@ void Position::init() {
   }
 
   Zobrist::side = rng.rand<Key>();
+  Zobrist::noPawns = rng.rand<Key>();
 }
 
 
@@ -176,8 +177,9 @@ Position& Position::set(const string& fenStr, bool isChess960, StateInfo* si, Th
 
    4) En passant target square (in algebraic notation). If there's no en passant
       target square, this is "-". If a pawn has just made a 2-square move, this
-      is the position "behind" the pawn. This is recorded regardless of whether
-      there is a pawn in position to make an en passant capture.
+      is the position "behind" the pawn. This is recorded only if there is a pawn
+      in position to make an en passant capture, and if there really is a pawn
+      that might have advanced two squares.
 
    5) Halfmove clock. This is the number of halfmoves since the last pawn advance
       or capture. This is used to determine if a draw can be claimed under the
@@ -254,7 +256,8 @@ Position& Position::set(const string& fenStr, bool isChess960, StateInfo* si, Th
   {
       st->epSquare = make_square(File(col - 'a'), Rank(row - '1'));
 
-      if (!(attackers_to(st->epSquare) & pieces(sideToMove, PAWN)))
+      if (   !(attackers_to(st->epSquare) & pieces(sideToMove, PAWN))
+          || !(pieces(~sideToMove, PAWN) & (st->epSquare + pawn_push(~sideToMove))))
           st->epSquare = SQ_NONE;
   }
   else
@@ -329,7 +332,8 @@ void Position::set_check_info(StateInfo* si) const {
 
 void Position::set_state(StateInfo* si) const {
 
-  si->key = si->pawnKey = si->materialKey = 0;
+  si->key = si->materialKey = 0;
+  si->pawnKey = Zobrist::noPawns;
   si->nonPawnMaterial[WHITE] = si->nonPawnMaterial[BLACK] = VALUE_ZERO;
   si->psq = SCORE_ZERO;
   si->checkersBB = attackers_to(square<KING>(sideToMove)) & pieces(~sideToMove);
@@ -1077,14 +1081,20 @@ bool Position::is_draw() const {
   if (st->rule50 > 99 && (!checkers() || MoveList<LEGAL>(*this).size()))
       return true;
 
-  StateInfo* stp = st;
-  for (int i = 2, e = std::min(st->rule50, st->pliesFromNull); i <= e; i += 2)
-  {
+  int e = std::min(st->rule50, st->pliesFromNull);
+
+  if (e < 4)
+    return false;
+
+  StateInfo* stp = st->previous->previous;
+
+  do {
       stp = stp->previous->previous;
 
       if (stp->key == st->key)
           return true; // Draw at first repetition
-  }
+
+  } while ((e -= 2) >= 4);
 
   return false;
 }