From 84b42b3ab328e3f1b9829d8f5d967e4d45a45a39 Mon Sep 17 00:00:00 2001
From: bmc4 <bmc4@cin.ufpe.br>
Date: Tue, 27 Apr 2021 19:59:46 -0300
Subject: [PATCH] Simplify pawn moves generator

This patch simplifies QUIET_CHECKS pawn move generator by merging discovery check
move generator with direct check move generator. It also simplifies emptySquares
instantiation. In addition, I added a comment in generate_moves() to clarify Check
branches.

STC:
LLR: 2.95 (-2.94,2.94) <-2.50,0.50>
Total: 112648 W: 9952 L: 9945 D: 92751
Ptnml(0-2): 369, 7682, 40195, 7729, 349
https://tests.stockfishchess.org/tests/view/6088226895e7f1852abd2978

LTC:
LLR: 2.93 (-2.94,2.94) <-2.50,0.50>
Total: 74656 W: 2797 L: 2765 D: 69094
Ptnml(0-2): 38, 2328, 32554, 2380, 28
https://tests.stockfishchess.org/tests/view/60884e5095e7f1852abd2994

closes https://github.com/official-stockfish/Stockfish/pull/3447

No functional change
---
 src/movegen.cpp | 43 ++++++++++++++-----------------------------
 1 file changed, 14 insertions(+), 29 deletions(-)

diff --git a/src/movegen.cpp b/src/movegen.cpp
index bd9d0b62..be168450 100644
--- a/src/movegen.cpp
+++ b/src/movegen.cpp
@@ -58,19 +58,16 @@ namespace {
     constexpr Direction UpLeft   = (Us == WHITE ? NORTH_WEST : SOUTH_EAST);
 
     const Square ksq = pos.square<KING>(Them);
-    Bitboard emptySquares;
+    const Bitboard emptySquares = Type == QUIETS || Type == QUIET_CHECKS ? target : ~pos.pieces();
+    const Bitboard enemies      = Type == EVASIONS ? pos.checkers()
+                                : Type == CAPTURES ? target : pos.pieces(Them);
 
     Bitboard pawnsOn7    = pos.pieces(Us, PAWN) &  TRank7BB;
     Bitboard pawnsNotOn7 = pos.pieces(Us, PAWN) & ~TRank7BB;
 
-    Bitboard enemies = (Type == EVASIONS ? pos.checkers()
-                     :  Type == CAPTURES ? target : pos.pieces(Them));
-
     // Single and double pawn pushes, no promotions
     if (Type != CAPTURES)
     {
-        emptySquares = (Type == QUIETS || Type == QUIET_CHECKS ? target : ~pos.pieces());
-
         Bitboard b1 = shift<Up>(pawnsNotOn7)   & emptySquares;
         Bitboard b2 = shift<Up>(b1 & TRank3BB) & emptySquares;
 
@@ -82,22 +79,12 @@ namespace {
 
         if (Type == QUIET_CHECKS)
         {
-            b1 &= pawn_attacks_bb(Them, ksq);
-            b2 &= pawn_attacks_bb(Them, ksq);
-
-            // Add pawn pushes which give discovered check. This is possible only
-            // if the pawn is not on the same file as the enemy king, because we
-            // don't generate captures. Note that a possible discovered check
-            // promotion has been already generated amongst the captures.
-            Bitboard dcCandidateQuiets = pos.blockers_for_king(Them) & pawnsNotOn7;
-            if (dcCandidateQuiets)
-            {
-                Bitboard dc1 = shift<Up>(dcCandidateQuiets) & emptySquares & ~file_bb(ksq);
-                Bitboard dc2 = shift<Up>(dc1 & TRank3BB) & emptySquares;
-
-                b1 |= dc1;
-                b2 |= dc2;
-            }
+            // To make a quiet check, you either make a direct check by pushing a pawn
+            // or push a blocker pawn that is not on the same file as the enemy king.
+            // Discovered check promotion has been already generated amongst the captures.
+            Bitboard dcCandidatePawns = pos.blockers_for_king(Them) & ~file_bb(ksq);
+            b1 &= pawn_attacks_bb(Them, ksq) | shift<   Up>(dcCandidatePawns);
+            b2 &= pawn_attacks_bb(Them, ksq) | shift<Up+Up>(dcCandidatePawns);
         }
 
         while (b1)
@@ -116,16 +103,13 @@ namespace {
     // Promotions and underpromotions
     if (pawnsOn7)
     {
-        if (Type == CAPTURES)
-            emptySquares = ~pos.pieces();
-
-        if (Type == EVASIONS)
-            emptySquares &= target;
-
         Bitboard b1 = shift<UpRight>(pawnsOn7) & enemies;
         Bitboard b2 = shift<UpLeft >(pawnsOn7) & enemies;
         Bitboard b3 = shift<Up     >(pawnsOn7) & emptySquares;
 
+        if (Type == EVASIONS)
+            b3 &= target;
+
         while (b1)
             moveList = make_promotions<Type, UpRight>(moveList, pop_lsb(b1), ksq);
 
@@ -185,8 +169,9 @@ namespace {
     while (bb)
     {
         Square from = pop_lsb(bb);
-
         Bitboard b = attacks_bb<Pt>(from, pos.pieces()) & target;
+
+        // To check, you either move freely a blocker or make a direct check.
         if (Checks && (Pt == QUEEN || !(pos.blockers_for_king(~Us) & from)))
             b &= pos.check_squares(Pt);
 
-- 
2.39.2