From e57fba7fc9be461cbb97c063b269a1e231cdd284 Mon Sep 17 00:00:00 2001
From: Joost VandeVondele <Joost.VandeVondele@gmail.com>
Date: Sun, 21 Jul 2024 13:15:12 +0200
Subject: [PATCH] Fix TB PV extension and MultiPV

in the case of MultiPV, the first move of the Nth multiPV could actually turn a
winning position in a losing one, so don't attempt to correct it. Instead,
always perform the first move without correction.

Fixes #5505

Closes https://github.com/official-stockfish/Stockfish/pull/5506

No functional change
---
 src/search.cpp | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/src/search.cpp b/src/search.cpp
index 945f8b40..435af4b2 100644
--- a/src/search.cpp
+++ b/src/search.cpp
@@ -1948,8 +1948,12 @@ void syzygy_extend_pv(const OptionsMap&         options,
 
     std::list<StateInfo> sts;
 
+    // Step 0, do the rootMove, no correction allowed, as needed for MultiPV in TB.
+    auto& stRoot = sts.emplace_back();
+    pos.do_move(rootMove.pv[0], stRoot);
+    int ply = 1;
+
     // Step 1, walk the PV to the last position in TB with correct decisive score
-    int ply = 0;
     while (size_t(ply) < rootMove.pv.size())
     {
         Move& pvMove = rootMove.pv[ply];