From d232a4ae684eccd829fd703a1872c1e0e17aaee9 Mon Sep 17 00:00:00 2001
From: nickpelling <nickpelling@nickpelling.com>
Date: Sat, 21 Sep 2019 08:59:32 +0100
Subject: [PATCH 1/1] Clarify the mapping of files to queenside

This patch replaces the obscure expressions mapping files ABCDEFGH to ABCDDCBA
by explicite calls to an auxiliary function:

  old:   f = min(f, ~f)
  new:   f = map_to_queenside(f)

We used the Golbolt web site (https://godbolt.org) to check that the current
code for the auxiliary function is optimal.

STC:
LLR: 2.96 (-2.94,2.94) [-3.00,1.00]
Total: 30292 W: 6756 L: 6651 D: 16885
http://tests.stockfishchess.org/tests/view/5d8676720ebc5971531d6aa1

Achieved with a bit of help from Sopel97, snicolet and vondele, thanks everyone!
Closes https://github.com/official-stockfish/Stockfish/pull/2325

No functional change
---
 AUTHORS          | 1 +
 src/evaluate.cpp | 2 +-
 src/pawns.cpp    | 2 +-
 src/psqt.cpp     | 2 +-
 src/types.h      | 9 +++++----
 5 files changed, 9 insertions(+), 7 deletions(-)

diff --git a/AUTHORS b/AUTHORS
index 455cf6ca..aff6a6c9 100644
--- a/AUTHORS
+++ b/AUTHORS
@@ -99,6 +99,7 @@ Miroslav Fontán (Hexik)
 Moez Jellouli (MJZ1977)
 Mohammed Li (tthsqe12)
 Nathan Rugg (nmrugg)
+Nick Pelling (nickpelling)
 Nicklas Persson (NicklasPersson)
 Niklas Fiekas (niklasf)
 Ondrej Mosnáček (WOnder93)
diff --git a/src/evaluate.cpp b/src/evaluate.cpp
index 9521cd10..f37820af 100644
--- a/src/evaluate.cpp
+++ b/src/evaluate.cpp
@@ -662,7 +662,7 @@ namespace {
             || (pos.pieces(PAWN) & (s + Up)))
             bonus = bonus / 2;
 
-        score += bonus - PassedFile * std::min(f, ~f);
+        score += bonus - PassedFile * map_to_queenside(f);
     }
 
     if (T)
diff --git a/src/pawns.cpp b/src/pawns.cpp
index f6041199..ca4156da 100644
--- a/src/pawns.cpp
+++ b/src/pawns.cpp
@@ -198,7 +198,7 @@ Score Entry::evaluate_shelter(const Position& pos, Square ksq) {
       b = theirPawns & file_bb(f);
       int theirRank = b ? relative_rank(Us, frontmost_sq(Them, b)) : 0;
 
-      int d = std::min(f, ~f);
+      File d = map_to_queenside(f);
       bonus += make_score(ShelterStrength[d][ourRank], 0);
 
       if (ourRank && (ourRank == theirRank - 1))
diff --git a/src/psqt.cpp b/src/psqt.cpp
index 2b06931c..83d11cf1 100644
--- a/src/psqt.cpp
+++ b/src/psqt.cpp
@@ -119,7 +119,7 @@ void init() {
 
       for (Square s = SQ_A1; s <= SQ_H8; ++s)
       {
-          File f = std::min(file_of(s), ~file_of(s));
+          File f = map_to_queenside(file_of(s));
           psq[ pc][ s] = score + (type_of(pc) == PAWN ? PBonus[rank_of(s)][file_of(s)]
                                                       : Bonus[pc][rank_of(s)][f]);
           psq[~pc][~s] = -psq[pc][s];
diff --git a/src/types.h b/src/types.h
index c77d8040..6d2c09a6 100644
--- a/src/types.h
+++ b/src/types.h
@@ -43,6 +43,7 @@
 #include <climits>
 #include <cstdint>
 #include <cstdlib>
+#include <algorithm>
 
 #if defined(_MSC_VER)
 // Disable some silly and noisy warning from MSVC compiler
@@ -358,14 +359,14 @@ constexpr Square operator~(Square s) {
   return Square(s ^ SQ_A8); // Vertical flip SQ_A1 -> SQ_A8
 }
 
-constexpr File operator~(File f) {
-  return File(f ^ FILE_H); // Horizontal flip FILE_A -> FILE_H
-}
-
 constexpr Piece operator~(Piece pc) {
   return Piece(pc ^ 8); // Swap color of piece B_KNIGHT -> W_KNIGHT
 }
 
+inline File map_to_queenside(File f) {
+  return std::min(f, File(FILE_H - f)); // Map files ABCDEFGH to files ABCDDCBA
+}
+
 constexpr CastlingRights operator&(Color c, CastlingRights cr) {
   return CastlingRights((c == WHITE ? WHITE_CASTLING : BLACK_CASTLING) & cr);
 }
-- 
2.39.2