From cce3bb1845581182c6ad22b171d83006261a62a4 Mon Sep 17 00:00:00 2001
From: Duncan <duncans_pumpkin@hotmail.co.uk>
Date: Wed, 29 Sep 2021 17:48:50 +0100
Subject: [PATCH] Fix #15136: Exported SV6 hangs/crashes vanilla RCT2

---
 distribution/changelog.txt       |  1 +
 src/openrct2/rct2/S6Exporter.cpp | 39 +++++++++++++++++++++-----------
 src/openrct2/rct2/S6Exporter.h   |  1 +
 3 files changed, 28 insertions(+), 13 deletions(-)

diff --git a/distribution/changelog.txt b/distribution/changelog.txt
index 160dcffa53..e8ac49de2f 100644
--- a/distribution/changelog.txt
+++ b/distribution/changelog.txt
@@ -16,6 +16,7 @@
 - Fix: [#14649] ImageImporter incorrectly remaps colours outside the RCT2 palette.
 - Fix: [#14667] “Extreme Hawaiian Island” has unpurchaseable land tiles (original bug).
 - Fix: [#15096] Crash when placing entrances in the scenario editor near the map corner.
+- Fix: [#15136] Exported SV6 files cause vanilla RCT2 to hang.
 - Fix: [#15142] ToonTowner's mine roofs were moved into the pirate theme scenery group instead of the mine theme scenery group.
 - Fix: [#15148] Track Designs Manager delete confirmation window doesn't display properly.
 - Fix: [#15170] Plugin: incorrect label text alignment.
diff --git a/src/openrct2/rct2/S6Exporter.cpp b/src/openrct2/rct2/S6Exporter.cpp
index 7015d66994..f4ea1932d8 100644
--- a/src/openrct2/rct2/S6Exporter.cpp
+++ b/src/openrct2/rct2/S6Exporter.cpp
@@ -1090,6 +1090,23 @@ void S6Exporter::ExportMarketingCampaigns()
     }
 }
 
+void S6Exporter::RebuildEntitySpatialLocation(const TileCoordsXY& loc)
+{
+    uint16_t previous = SPRITE_INDEX_NULL;
+    for (auto* entity : EntityTileList(loc.ToCoordsXY()))
+    {
+        if (previous != SPRITE_INDEX_NULL)
+        {
+            _s6.sprites[previous].unknown.next_in_quadrant = entity->sprite_index;
+        }
+        previous = entity->sprite_index;
+    }
+    if (previous != SPRITE_INDEX_NULL)
+    {
+        _s6.sprites[previous].unknown.next_in_quadrant = SPRITE_INDEX_NULL;
+    }
+}
+
 void S6Exporter::RebuildEntityLinks()
 {
     // Rebuild next/previous linked list entity indexs
@@ -1098,6 +1115,9 @@ void S6Exporter::RebuildEntityLinks()
            RCT12EntityLinkListOffset::Peep, RCT12EntityLinkListOffset::TrainHead, RCT12EntityLinkListOffset::Vehicle })
     {
         uint16_t previous = SPRITE_INDEX_NULL;
+        // Intialise Head to NULL for situations where there are no entities of this type.
+        _s6.sprite_lists_head[EnumValue(list) >> 1] = SPRITE_INDEX_NULL;
+
         for (auto& entity : _s6.sprites)
         {
             if (entity.unknown.linked_list_type_offset == list)
@@ -1119,25 +1139,18 @@ void S6Exporter::RebuildEntityLinks()
     }
 
     // Rebuild next_in_quadrant linked list entity indexs
+    // This in theory is not required but to be on the safe side we are rebuilding it.
     for (auto x = 0; x < 255; ++x)
     {
         for (auto y = 0; y < 255; ++y)
         {
-            uint16_t previous = SPRITE_INDEX_NULL;
-            for (auto* entity : EntityTileList(TileCoordsXY{ x, y }.ToCoordsXY()))
-            {
-                if (previous != SPRITE_INDEX_NULL)
-                {
-                    _s6.sprites[previous].unknown.next_in_quadrant = entity->sprite_index;
-                }
-                previous = entity->sprite_index;
-            }
-            if (previous != SPRITE_INDEX_NULL)
-            {
-                _s6.sprites[previous].unknown.next_in_quadrant = SPRITE_INDEX_NULL;
-            }
+            RebuildEntitySpatialLocation(TileCoordsXY{ x, y });
         }
     }
+    // Rebuild next_in_quadrant linked list for LOCATION_NULL
+    TileCoordsXY invalid;
+    invalid.SetNull();
+    RebuildEntitySpatialLocation(invalid);
 }
 
 constexpr RCT12EntityLinkListOffset GetRCT2LinkListOffset(const EntityBase* src)
diff --git a/src/openrct2/rct2/S6Exporter.h b/src/openrct2/rct2/S6Exporter.h
index b5a69db52d..42cc586019 100644
--- a/src/openrct2/rct2/S6Exporter.h
+++ b/src/openrct2/rct2/S6Exporter.h
@@ -79,4 +79,5 @@ private:
     std::optional<uint16_t> AllocateUserString(std::string_view value);
     void ExportUserStrings();
     void RebuildEntityLinks();
+    void RebuildEntitySpatialLocation(const TileCoordsXY& loc);
 };