From: bmc4 <bmc4@cin.ufpe.br>
Date: Sun, 17 Jan 2021 18:21:45 +0000 (-0300)
Subject: Simplify Chess 960 castling
X-Git-Url: https://git.sesse.net/?p=stockfish;a=commitdiff_plain;h=dd9609521437dc839236c36d35bdb8cb633ba19f

Simplify Chess 960 castling

a little cleanup, and small speedup (about 0.3%) for Chess 960.

Verified with perft on a large set of chess960 positions.

Closes https://github.com/official-stockfish/Stockfish/pull/3317

No functional change
---

diff --git a/src/position.cpp b/src/position.cpp
index 954e5ffd..826e847f 100644
--- a/src/position.cpp
+++ b/src/position.cpp
@@ -533,11 +533,9 @@ bool Position::legal(Move m) const {
           if (attackers_to(s) & pieces(~us))
               return false;
 
-      // In case of Chess960, verify that when moving the castling rook we do
-      // not discover some hidden checker.
+      // In case of Chess960, verify if the Rook blocks some checks
       // For instance an enemy queen in SQ_A1 when castling rook is in SQ_B1.
-      return   !chess960
-            || !(attacks_bb<ROOK>(to, pieces() ^ to_sq(m)) & pieces(~us, ROOK, QUEEN));
+      return !chess960 || !(blockers_for_king(us) & to_sq(m));
   }
 
   // If the moving piece is a king, check whether the destination square is