From b722955f7ebf1bf7f01637642e358042592bf5b1 Mon Sep 17 00:00:00 2001
From: Malte Langkabel <malte.langkabel@gmail.com>
Date: Sun, 2 Aug 2026 20:22:39 +0200
Subject: [PATCH] fix splitter filters being lost when rotating in place
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rotateInPlace re-implemented the belt-tile re-registration switch inline
instead of calling reregisterBeltTile, and its splitter branch omitted the
setSplitterFilters call the canonical version has. Since an operational
splitter keeps its filters only in BeltSystem, removeTile discarded them and
rotating a configured splitter silently reset it to "accept all".

Replace the duplicated switch with a call to reregisterBeltTile, capturing
the filters beforehand via getSplitterInfo — the same idiom deconstruct
already uses. This removes the second copy of the switch that allowed the
two to drift apart in the first place.

Add a regression test; the existing [rotate-in-place] cases covered belt
tiles only, which is why this went unnoticed.

Co-Authored-By: Claude <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01GH8ZMRY3vhxxXcaUxBqxkk
---
 src/lib/sim/BuildingSystem.cpp | 38 +++++++++++++++-------------------
 src/test/BuildingTest.cpp      | 30 +++++++++++++++++++++++++++
 2 files changed, 47 insertions(+), 21 deletions(-)

diff --git a/src/lib/sim/BuildingSystem.cpp b/src/lib/sim/BuildingSystem.cpp
index cf07d1c..f9f9954 100644
--- a/src/lib/sim/BuildingSystem.cpp
+++ b/src/lib/sim/BuildingSystem.cpp
@@ -1719,29 +1719,25 @@ void BuildingSystem::rotateInPlace(BuildingId id, Rotation newRotation)
         // the new port set (REQ-MAT-INPUT-INTAKE).
         b.incomingItems.assign(b.inputPorts.size(), {});
 
-        // Re-register with BeltSystem (items on tile are discarded).
-        if (b.type == BuildingType::Belt)
+        // Re-register with BeltSystem (items on tile are discarded). A splitter's
+        // filters live in BeltSystem and would be lost by removeTile, so capture
+        // them first and hand them back to reregisterBeltTile (REQ-BLD-SPLITTER).
+        if (isBeltSubsystemType(b.type))
         {
+            std::vector<ItemType> splitterFilterA;
+            std::vector<ItemType> splitterFilterB;
+            if (b.type == BuildingType::Splitter)
+            {
+                if (const std::optional<BeltSystem::SplitterInfo> info =
+                        m_belts.getSplitterInfo(b.anchor))
+                {
+                    splitterFilterA = info->filterA;
+                    splitterFilterB = info->filterB;
+                }
+            }
+
             m_belts.removeTile(b.anchor);
-            m_belts.placeBelt(b.anchor, newRotation);
-        }
-        else if (b.type == BuildingType::Splitter)
-        {
-            m_belts.removeTile(b.anchor);
-            assert(mask.outputPorts.size() >= 2);
-            m_belts.placeSplitter(b.anchor,
-                                  mask.outputPorts[0].direction,
-                                  mask.outputPorts[1].direction);
-        }
-        else if (b.type == BuildingType::TunnelEntry)
-        {
-            m_belts.removeTile(b.anchor);
-            m_belts.placeTunnelEntry(b.anchor, newRotation, m_config.world.tunnelMaxDistance_tiles);
-        }
-        else if (b.type == BuildingType::TunnelExit)
-        {
-            m_belts.removeTile(b.anchor);
-            m_belts.placeTunnelExit(b.anchor, newRotation);
+            reregisterBeltTile(b, splitterFilterA, splitterFilterB);
         }
 
         return;
diff --git a/src/test/BuildingTest.cpp b/src/test/BuildingTest.cpp
index a2cbfa0..8ca6df9 100644
--- a/src/test/BuildingTest.cpp
+++ b/src/test/BuildingTest.cpp
@@ -1427,6 +1427,36 @@ TEST_CASE("BuildingSystem: rotateInPlace re-registers a belt tile with BeltSyste
     REQUIRE(belts.tryPutItem(QPoint(0, 0), makeItem("iron_ore")));
 }
 
+TEST_CASE("BuildingSystem: rotateInPlace preserves the output filters of a splitter "
+          "(REQ-BLD-SPLITTER)", "[building][rotate-in-place]")
+{
+    PlacementFixture f;
+
+    const QPoint tile(5, 5);
+    const BuildingId id = f.bs.place(BuildingType::Splitter, tile, Rotation::East, 0).value();
+
+    // Run until construction completes, so the splitter is registered with BeltSystem.
+    Tick tick = 0;
+    while (f.bs.getAllBuildings().empty() && tick < 100000)
+    {
+        runTicks(f.bs, f.belts, 1, tick);
+    }
+    REQUIRE(f.bs.getAllBuildings().size() == 1);
+
+    const std::vector<ItemType> filterA{ ItemType{"iron_ore"} };
+    const std::vector<ItemType> filterB{ ItemType{"copper_ore"} };
+    f.belts.setSplitterFilters(tile, filterA, filterB);
+
+    f.bs.rotateInPlace(id, Rotation::North);
+
+    // The tile is re-registered with BeltSystem carrying the filters it had before
+    // the rotation — rotating must not reset a configured splitter to "accept all".
+    const std::optional<BeltSystem::SplitterInfo> info = f.belts.getSplitterInfo(tile);
+    REQUIRE(info.has_value());
+    REQUIRE(info->filterA == filterA);
+    REQUIRE(info->filterB == filterB);
+}
+
 TEST_CASE("BuildingSystem: splitter filters configured on a construction site carry over "
           "to the built splitter (REQ-BLD-SITE-CONFIG)", "[building]")
 {