From 3df09c04d7081d341cb0c5bcc3adc498ba877f9a Mon Sep 17 00:00:00 2001
From: Shawn Xu <xu107288696@gmail.com>
Date: Tue, 9 Jul 2024 13:04:47 -0500
Subject: [PATCH] Simplify Away Refutation Stage

Simplify away killer stage to a constant bonus given to the killer move during
quiet move scoring.

Passed Non-regression STC (Against then-pending PR #5472):
LLR: 2.93 (-2.94,2.94) <-1.75,0.25>
Total: 106176 W: 27685 L: 27539 D: 50952
Ptnml(0-2): 410, 12765, 26637, 12821, 455
https://tests.stockfishchess.org/tests/view/668dd0835034141ae5999e8f

Passed Non-regression LTC:
LLR: 2.95 (-2.94,2.94) <-1.75,0.25>
Total: 92472 W: 23426 L: 23276 D: 45770
Ptnml(0-2): 55, 10376, 25215, 10544, 46
https://tests.stockfishchess.org/tests/view/669019e45034141ae5999fd2

closes https://github.com/official-stockfish/Stockfish/pull/5476

Bench 1459677
---
 src/movepick.cpp | 19 +++++--------------
 src/movepick.h   | 12 ++++++------
 2 files changed, 11 insertions(+), 20 deletions(-)

diff --git a/src/movepick.cpp b/src/movepick.cpp
index d54bcbc7..7619471f 100644
--- a/src/movepick.cpp
+++ b/src/movepick.cpp
@@ -34,7 +34,6 @@ enum Stages {
     MAIN_TT,
     CAPTURE_INIT,
     GOOD_CAPTURE,
-    KILLER,
     QUIET_INIT,
     GOOD_QUIET,
     BAD_CAPTURE,
@@ -97,7 +96,7 @@ MovePicker::MovePicker(const Position&              p,
     continuationHistory(ch),
     pawnHistory(ph),
     ttMove(ttm),
-    killer{km, 0},
+    killer(km),
     depth(d) {
     assert(d > 0);
 
@@ -184,6 +183,8 @@ void MovePicker::score() {
             m.value += (*continuationHistory[3])[pc][to];
             m.value += (*continuationHistory[5])[pc][to];
 
+            m.value += (m == killer) * 65536;
+
             // bonus for checks
             m.value += bool(pos.check_squares(pt) & to) * 16384;
 
@@ -270,16 +271,6 @@ top:
         ++stage;
         [[fallthrough]];
 
-    case KILLER :
-        // increment it before so if we aren't stuck here indefinitely
-        ++stage;
-
-        if (killer != ttMove && killer != Move::none() && !pos.capture_stage(killer)
-            && pos.pseudo_legal(killer))
-            return killer;
-
-        [[fallthrough]];
-
     case QUIET_INIT :
         if (!skipQuiets)
         {
@@ -294,7 +285,7 @@ top:
         [[fallthrough]];
 
     case GOOD_QUIET :
-        if (!skipQuiets && select<Next>([&]() { return *cur != killer; }))
+        if (!skipQuiets && select<Next>([]() { return true; }))
         {
             if ((cur - 1)->value > -7998 || (cur - 1)->value <= quiet_threshold(depth))
                 return *(cur - 1);
@@ -323,7 +314,7 @@ top:
 
     case BAD_QUIET :
         if (!skipQuiets)
-            return select<Next>([&]() { return *cur != killer; });
+            return select<Next>([]() { return true; });
 
         return Move::none();
 
diff --git a/src/movepick.h b/src/movepick.h
index 86a2a583..c6a5d25a 100644
--- a/src/movepick.h
+++ b/src/movepick.h
@@ -184,12 +184,12 @@ class MovePicker {
     const CapturePieceToHistory* captureHistory;
     const PieceToHistory**       continuationHistory;
     const PawnHistory*           pawnHistory;
-    Move                         ttMove;
-    ExtMove killer, *cur, *endMoves, *endBadCaptures, *beginBadQuiets, *endBadQuiets;
-    int     stage;
-    int     threshold;
-    Depth   depth;
-    ExtMove moves[MAX_MOVES];
+    Move                         ttMove, killer;
+    ExtMove *                    cur, *endMoves, *endBadCaptures, *beginBadQuiets, *endBadQuiets;
+    int                          stage;
+    int                          threshold;
+    Depth                        depth;
+    ExtMove                      moves[MAX_MOVES];
 };
 
 }  // namespace Stockfish