From 559dde96cf591a51a0124e153b9c410bf4d71f69 Mon Sep 17 00:00:00 2001
From: mlangkabel <malte.langkabel@gmail.com>
Date: Mon, 27 Apr 2026 21:38:11 +0200
Subject: [PATCH] fix bug where buildings could not output directly on
 splitters

---
 src/lib/sim/BeltSystem.cpp     | 15 ++++++++++++++-
 src/lib/sim/BeltSystem.h       |  5 +++--
 src/lib/sim/BuildingSystem.cpp |  2 +-
 src/test/BeltSystemTest.cpp    | 18 ++++++++++++++++++
 4 files changed, 36 insertions(+), 4 deletions(-)

diff --git a/src/lib/sim/BeltSystem.cpp b/src/lib/sim/BeltSystem.cpp
index 9d18416..78985ad 100644
--- a/src/lib/sim/BeltSystem.cpp
+++ b/src/lib/sim/BeltSystem.cpp
@@ -199,7 +199,7 @@ void BeltSystem::reevaluateTunnelPairing()
 // Port interface
 // ---------------------------------------------------------------------------
 
-bool BeltSystem::tryPutItem(QPoint tile, Item item)
+bool BeltSystem::tryPutItem(QPoint tile, Item item, Rotation fromDir)
 {
     const std::map<std::pair<int, int>, BeltTile>::iterator bIt = m_belts.find(key(tile));
     if (bIt != m_belts.end())
@@ -207,6 +207,19 @@ bool BeltSystem::tryPutItem(QPoint tile, Item item)
         return tryPlaceOnBelt(tile, item);
     }
 
+    const std::map<std::pair<int, int>, SplitterTile>::iterator splIt =
+        m_splitters.find(key(tile));
+    if (splIt != m_splitters.end())
+    {
+        if (!splIt->second.back)
+        {
+            splIt->second.back    = BeltItemSlot{item, 0.0};
+            splIt->second.backDir = fromDir;
+            return true;
+        }
+        return false;
+    }
+
     const std::map<std::pair<int, int>, TunnelEntryTile>::iterator teIt =
         m_tunnelEntries.find(key(tile));
     if (teIt != m_tunnelEntries.end())
diff --git a/src/lib/sim/BeltSystem.h b/src/lib/sim/BeltSystem.h
index 854ed89..126907f 100644
--- a/src/lib/sim/BeltSystem.h
+++ b/src/lib/sim/BeltSystem.h
@@ -72,8 +72,9 @@ public:
     // port.direction = direction items flow on that tile
     //
     // tryPutItem: place item onto tile.
-    //   Returns false if the tile is not a belt, or tile full.
-    bool tryPutItem(QPoint tile, Item item);
+    //   Returns false if the tile is not a belt/splitter, or tile full.
+    //   fromDir: travel direction of the item (used for splitter animation).
+    bool tryPutItem(QPoint tile, Item item, Rotation fromDir = Rotation::West);
 
     // tryTakeItem: remove and return the leading item from port.tile.
     //   Returns nullopt if tile is not a belt, direction mismatches, or tile empty.
diff --git a/src/lib/sim/BuildingSystem.cpp b/src/lib/sim/BuildingSystem.cpp
index b2ff9ff..1c8d8e3 100644
--- a/src/lib/sim/BuildingSystem.cpp
+++ b/src/lib/sim/BuildingSystem.cpp
@@ -713,7 +713,7 @@ void BuildingSystem::tickBeltPush()
                 break;
             }
             const Item item = building.outputBuffer.items.front();
-            if (m_belts.tryPutItem(outputPort.tile, item))
+            if (m_belts.tryPutItem(outputPort.tile, item, outputPort.direction))
             {
                 building.outputBuffer.items.erase(building.outputBuffer.items.begin());
             }
diff --git a/src/test/BeltSystemTest.cpp b/src/test/BeltSystemTest.cpp
index a01006d..7c1360a 100644
--- a/src/test/BeltSystemTest.cpp
+++ b/src/test/BeltSystemTest.cpp
@@ -629,6 +629,24 @@ TEST_CASE("BeltSystem: splitter accepts new items after building pulls from fron
     REQUIRE(bs.peekItem(Port{tileSpl, Rotation::South}).has_value());
 }
 
+TEST_CASE("BeltSystem: tryPutItem succeeds directly on a splitter tile", "[belt]")
+{
+    // Regression: buildings outputting onto a splitter tile were silently dropped
+    // because tryPutItem had no splitter case and returned false.
+    BeltSystem bs(kFastBeltSpeed);
+
+    const QPoint tileSpl(0, 0);
+    bs.placeSplitter(tileSpl, Rotation::North, Rotation::South);
+
+    REQUIRE(bs.tryPutItem(tileSpl, makeItem("iron_ore"), Rotation::East));
+
+    // Item should arrive at one of the output fronts after the splitter ticks through.
+    bs.tick();   // back advances to 0.5, routes to frontA
+    bs.tick();   // frontA reaches 1.0
+
+    REQUIRE(bs.peekItem(Port{tileSpl, Rotation::North}).has_value());
+}
+
 TEST_CASE("BeltSystem: splitter alternates between two unregistered outputs (building inputs)", "[belt]")
 {
     BeltSystem bs(kFastBeltSpeed);