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Rethink En Passant Evasion Capture

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It now checks if it were a discovery attack instead of the attacking piece is the double-moved pawn.

As a side effect, certain illegal fens have different, and slightly more logical move generation.
There is no intend to maintain particular behavior for such non-reachable fens.

Passed STC:
LLR: 2.93 (-2.94,2.94) {-1.25,0.25}
Total: 47912 W: 4327 L: 4285 D: 39300
Ptnml(0-2): 144, 3312, 17012, 3334, 154
https://tests.stockfishchess.org/tests/view/5ff890946019e097de3ef0a5

closes https://github.com/official-stockfish/Stockfish/pull/3292

closes / fixes https://github.com/official-stockfish/Stockfish/issues/3270

No functional change
This commit is contained in:
BM123499
2021-01-08 14:03:26 -03:00
committed by Joost VandeVondele
parent 0266e70297
commit 5f222f1d98
1 changed files with 2 additions and 4 deletions
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6
src/movegen.cpp
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@@ -156,10 +156,8 @@ namespace {
{ {
assert(rank_of(pos.ep_square()) == relative_rank(Us, RANK_6)); assert(rank_of(pos.ep_square()) == relative_rank(Us, RANK_6));
// An en passant capture can be an evasion only if the checking piece // An en passant capture cannot resolve a discovered check.
// is the double pushed pawn and so is in the target. Otherwise this if (Type == EVASIONS && (target & (pos.ep_square() + Up)))
// is a discovery check and we are forced to do otherwise.
if (Type == EVASIONS && !(target & (pos.ep_square() - Up)))
return moveList; return moveList;
b1 = pawnsNotOn7 & pawn_attacks_bb(Them, pos.ep_square()); b1 = pawnsNotOn7 & pawn_attacks_bb(Them, pos.ep_square());