From ead7e650da1dd07a2614ba4d8207470fe921a87b Mon Sep 17 00:00:00 2001
From: anematode <timothy.herchen@gmail.com>
Date: Thu, 9 Apr 2026 21:43:44 +0200
Subject: [PATCH] Fix weird indexing bug
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

[Passed
STC](https://tests.stockfishchess.org/tests/live_elo/69cf6a5d1668971c9da23ae1)
LLR: 2.93 (-2.94,2.94) <-1.75,0.25>
Total: 51104 W: 13376 L: 13180 D: 24548
Ptnml(0-2): 134, 5555, 13963, 5781, 119

[Passed
LTC](https://tests.stockfishchess.org/tests/live_elo/69d00a7ee2b443cb2670b5c6)
LLR: 2.95 (-2.94,2.94) <-1.75,0.25>
Total: 101256 W: 25867 L: 25732 D: 49657
Ptnml(0-2): 59, 10423, 29520, 10576, 50

When processing a promotion we add/remove nonsensical threats, namely,
piece threats of pawns on the 1st or 8th rank. This is not only
inefficient but also seems to lead to an obscure bug during a
promotion–capture + `double_inc_update` that causes a lingering invalid
feature. Currently – as an artifact of the training process – these
parts of the net are filled with random values in [-1,1], which is why
there is a bench change.

@ces42 did a helpful analysis:

> checking bench with depth 17 actually shows that the bug in has been
there all the time since 8e5392d7 (when threat inputs was merged into
master)
> i.e. double_inc_update for ThreatFeatureSet has always been slightly
bugged

After this patch is applied we'll be able to better compress the net by
filling the invalid features with 0, w/o changing bench. I expect a
15%-ish size savings.

closes https://github.com/official-stockfish/Stockfish/pull/6701

Bench: 2700393
---
 src/position.cpp | 15 +++++++++++----
 1 file changed, 11 insertions(+), 4 deletions(-)

diff --git a/src/position.cpp b/src/position.cpp
index d9d57c0b3..fa732836c 100644
--- a/src/position.cpp
+++ b/src/position.cpp
@@ -966,13 +966,22 @@ void Position::do_move(Move                      m,
     // Move the piece. The tricky Chess960 castling is handled earlier
     if (m.type_of() != CASTLING)
     {
+        Piece toPc = pc;
+        if (m.type_of() == PROMOTION)
+            toPc = make_piece(us, m.promotion_type());
+
         if (captured && m.type_of() != EN_PASSANT)
         {
             remove_piece(from, &dts);
-            swap_piece(to, pc, &dts);
+            swap_piece(to, toPc, &dts);
         }
-        else
+        else if (pc == toPc)
             move_piece(from, to, &dts);
+        else
+        {
+            remove_piece(from, &dts);
+            put_piece(toPc, to, &dts);
+        }
     }
 
     // If the moving piece is a pawn do some special extra work
@@ -1011,8 +1020,6 @@ void Position::do_move(Move                      m,
             assert(relative_rank(us, to) == RANK_8);
             assert(type_of(promotion) >= KNIGHT && type_of(promotion) <= QUEEN);
 
-            swap_piece(to, promotion, &dts);
-
             dp.add_pc = promotion;
             dp.add_sq = to;
             dp.to     = SQ_NONE;