From: pb00067 <pb00067@phxl0401.wp.lan>
Date: Thu, 17 Aug 2023 12:31:05 +0000 (+0200)
Subject: Simplify slider_blocker calculation
X-Git-Url: https://git.sesse.net/?p=stockfish;a=commitdiff_plain;h=1f7ff8406d323e634a2aa1e1264042340707cdd9

Simplify slider_blocker calculation

Now that classical evaluation was removed, we can adapt this method
to the needs of set_check_info.

STC:
2.95 (-2.94,2.94) <-1.75,0.25>
Total: 298176 W: 75802 L: 75868 D: 146506
Ptnml(0-2): 908, 33608, 80192, 33402, 978
https://tests.stockfishchess.org/tests/view/64e70b899009777747557b43

closes https://github.com/official-stockfish/Stockfish/pull/4753

no functional change
---

diff --git a/src/position.cpp b/src/position.cpp
index 0f15727d..12067743 100644
--- a/src/position.cpp
+++ b/src/position.cpp
@@ -321,8 +321,8 @@ void Position::set_castling_right(Color c, Square rfrom) {
 
 void Position::set_check_info() const {
 
-  st->blockersForKing[WHITE] = slider_blockers(pieces(BLACK), square<KING>(WHITE), st->pinners[BLACK]);
-  st->blockersForKing[BLACK] = slider_blockers(pieces(WHITE), square<KING>(BLACK), st->pinners[WHITE]);
+  update_slider_blockers(WHITE);
+  update_slider_blockers(BLACK);
 
   Square ksq = square<KING>(~sideToMove);
 
@@ -443,37 +443,33 @@ string Position::fen() const {
   return ss.str();
 }
 
+/// update_slider_blockers() calculates st->blockersForKing[c] and st->pinners[~c],
+/// which store respectively the pieces preventing king of color c from being in check
+/// and the slider pieces of color ~c pinning pieces of color c to the king.
+void Position::update_slider_blockers(Color c) const {
 
-/// Position::slider_blockers() returns a bitboard of all the pieces (both colors)
-/// that are blocking attacks on the square 's' from 'sliders'. A piece blocks a
-/// slider if removing that piece from the board would result in a position where
-/// square 's' is attacked. For example, a king-attack blocking piece can be either
-/// a pinned or a discovered check piece, according if its color is the opposite
-/// or the same of the color of the slider.
+  Square ksq =  square<KING>(c);
 
-Bitboard Position::slider_blockers(Bitboard sliders, Square s, Bitboard& pinners) const {
-
-  Bitboard blockers = 0;
-  pinners = 0;
+  st->blockersForKing[c] = 0;
+  st->pinners[~c] = 0;
 
   // Snipers are sliders that attack 's' when a piece and other snipers are removed
-  Bitboard snipers = (  (attacks_bb<  ROOK>(s) & pieces(QUEEN, ROOK))
-                      | (attacks_bb<BISHOP>(s) & pieces(QUEEN, BISHOP))) & sliders;
+  Bitboard snipers = (  (attacks_bb<  ROOK>(ksq) & pieces(QUEEN, ROOK))
+                      | (attacks_bb<BISHOP>(ksq) & pieces(QUEEN, BISHOP))) & pieces(~c);
   Bitboard occupancy = pieces() ^ snipers;
 
   while (snipers)
   {
     Square sniperSq = pop_lsb(snipers);
-    Bitboard b = between_bb(s, sniperSq) & occupancy;
+    Bitboard b = between_bb(ksq, sniperSq) & occupancy;
 
     if (b && !more_than_one(b))
     {
-        blockers |= b;
-        if (b & pieces(color_of(piece_on(s))))
-            pinners |= sniperSq;
+        st->blockersForKing[c] |= b;
+        if (b & pieces(c))
+            st->pinners[~c] |= sniperSq;
     }
   }
-  return blockers;
 }
 
 
diff --git a/src/position.h b/src/position.h
index f0546af3..ca7c3ace 100644
--- a/src/position.h
+++ b/src/position.h
@@ -114,7 +114,7 @@ public:
   // Attacks to/from a given square
   Bitboard attackers_to(Square s) const;
   Bitboard attackers_to(Square s, Bitboard occupied) const;
-  Bitboard slider_blockers(Bitboard sliders, Square s, Bitboard& pinners) const;
+  void update_slider_blockers(Color c) const;
   template<PieceType Pt> Bitboard attacks_by(Color c) const;
 
   // Properties of moves