Add CoreModeIceTileOverlay map metadata property

Celeste.Mod.mm/Mod/Meta/MapMeta.cs

@@ -336,6 +336,7 @@ public MapMetaModeProperties(BinaryPacker.Element meta) {
         public bool? HeartIsEnd { get; set; }
         public bool? SeekerSlowdown { get; set; }
         public bool? TheoInBubble { get; set; }
+        public bool? CoreModeIceTileOverlay { get; set; }
 
         public patch_ModeProperties Convert()
             => new patch_ModeProperties() {
@@ -356,6 +357,7 @@ public void Parse(BinaryPacker.Element meta) {
             meta.AttrIfBool("HeartIsEnd", v => HeartIsEnd = v);
             meta.AttrIfBool("SeekerSlowdown", v => SeekerSlowdown = v);
             meta.AttrIfBool("TheoInBubble", v => TheoInBubble = v);
+            meta.AttrIfBool("CoreModeIceTileOverlay", v => CoreModeIceTileOverlay = v);
 
             BinaryPacker.Element child;
 

Celeste.Mod.mm/Patches/LevelLoader.cs

@@ -250,6 +250,7 @@ private XmlDocument getModdedSpritesXml(string path) {
         [MonoModIgnore] // We don't want to change anything about the method...
         [PatchLoadingThreadAddEvent] // ... except for manually manipulating the method via MonoModRules
         [PatchLoadingThreadAddSubHudRenderer]
+        [PatchLoadingThreadForMapMetadata]
         private extern void LoadingThread();
 
         private void LoadingThread_Safe() {
@@ -325,6 +326,9 @@ public override void Update() {
             }
         }
 
+        private static bool _ShouldAddIceTileOverlay(MapData mapData)
+            => ((patch_MapData) mapData).Meta?.CoreModeIceTileOverlay ?? false;
+
     }
 }
 
@@ -366,6 +370,12 @@ class PatchLoadingThreadAddSubHudRendererAttribute : Attribute { }
     [MonoModCustomMethodAttribute(nameof(MonoModRules.PatchLoadingThreadAddEvent))]
     class PatchLoadingThreadAddEventAttribute : Attribute { }
 
+    /// <summary>
+    /// Handles map metadata properties in some places throughout the loading thread.
+    /// </summary>
+    [MonoModCustomMethodAttribute(nameof(MonoModRules.PatchLoadingThreadForMapMetadata))]
+    class PatchLoadingThreadForMapMetadataAttribute : Attribute { }
+
     static partial class MonoModRules {
 
         public static void PatchLevelLoaderOrigCtor(ILContext context, CustomAttribute attrib) {
@@ -431,5 +441,40 @@ public static void PatchLoadingThreadAddEvent(ILContext context, CustomAttribute
             cursor.Emit(OpCodes.Call, m_Everest_Events_LevelLoader_LoadingThread);
         }
 
+        public static void PatchLoadingThreadForMapMetadata(ILContext context, CustomAttribute attrib) {
+            MethodDefinition m_shouldAddIceTileOverlay = context.Method.DeclaringType.FindMethod("_ShouldAddIceTileOverlay");
+
+            ILCursor cursor = new(context);
+
+            // first, get the local for MapData
+            int loc_mapData = -1;
+            cursor.GotoNext(
+                    instr => instr.MatchCallvirt("Celeste.Session", "get_MapData"),
+                    instr => instr.MatchStloc(out loc_mapData)
+                );
+
+            /*
+             now let's change this bit of code
+             if (session.Area.ID == 9) { ... }
+             to this
+             if (session.Area.ID == 9 || _ShouldAddIceTileOverlay(mapData)) { ... }
+             */
+            ILLabel addIceTileOverlaybel = cursor.DefineLabel();
+            ILLabel branchOutLabel = null;
+            cursor.GotoNext(instr => instr.MatchLdcI4(9));
+            cursor.GotoNext(MoveType.Before, instr => instr.MatchBneUn(out branchOutLabel));
+
+            // for simplicity, we'll remove the bne.un.s, and replace it with a different branch instruction
+            cursor.Remove();
+
+            cursor.Emit(OpCodes.Beq_S, addIceTileOverlaybel); // beq.s <=> bne.un.s
+
+            cursor.EmitLdloc(loc_mapData);
+            cursor.Emit(OpCodes.Call, m_shouldAddIceTileOverlay);
+            cursor.Emit(OpCodes.Brfalse_S, branchOutLabel);
+
+            cursor.MarkLabel(addIceTileOverlaybel);
+        }
+
     }
 }