Fix type for offset in MapConnection

include/global.fieldmap.h

@@ -131,7 +131,7 @@ struct MapEvents
 struct MapConnection
 {
     u8 direction;
-    u32 offset;
+    s32 offset;
     u8 mapGroup;
     u8 mapNum;
 };

src/fieldmap.c

@@ -144,7 +144,7 @@ static void InitBackupMapLayoutConnections(struct MapHeader *mapHeader)
         for (i = 0; i < count; i++, connection++)
         {
             struct MapHeader const *cMap = GetMapHeaderFromConnection(connection);
-            u32 offset = connection->offset;
+            s32 offset = connection->offset;
             switch (connection->direction)
             {
             case CONNECTION_SOUTH:

src/item_use.c

@@ -393,46 +393,43 @@ static bool8 IsHiddenItemPresentAtCoords(const struct MapEvents *events, s16 x,
 
 static bool8 IsHiddenItemPresentInConnection(const struct MapConnection *connection, int x, int y)
 {
-
-    u16 localX, localY;
-    u32 localOffset;
-    s32 localLength;
-
-    struct MapHeader const *const mapHeader = GetMapHeaderFromConnection(connection);
-
+    s16 connectionX, connectionY;
+    struct MapHeader const *const connectionHeader = GetMapHeaderFromConnection(connection);
+
+// To convert our x/y into coordinates that are relative to the connected map, we must:
+//  - Subtract the virtual offset used for the border buffer (MAP_OFFSET).
+//  - Subtract the horizontal offset between North/South connections, or the vertical offset for East/West
+//  - Account for map size. (0,0) is in the NW corner of our map, so when looking North/West we have to add the height/width of the connected map,
+//     and when looking South/East we have to subtract the height/width of our current map.
+#define localX (x - MAP_OFFSET)
+#define localY (y - MAP_OFFSET)
     switch (connection->direction)
     {
-    // same weird temp variable behavior seen in IsHiddenItemPresentAtCoords
     case CONNECTION_NORTH:
-        localOffset = connection->offset + MAP_OFFSET;
-        localX = x - localOffset;
-        localLength = mapHeader->mapLayout->height - MAP_OFFSET;
-        localY = localLength + y; // additions are reversed for some reason
+        connectionX = localX - connection->offset;
+        connectionY = connectionHeader->mapLayout->height + localY;
         break;
     case CONNECTION_SOUTH:
-        localOffset = connection->offset + MAP_OFFSET;
-        localX = x - localOffset;
-        localLength = gMapHeader.mapLayout->height + MAP_OFFSET;
-        localY = y - localLength;
+        connectionX = localX - connection->offset;
+        connectionY = localY - gMapHeader.mapLayout->height;
         break;
     case CONNECTION_WEST:
-        localLength = mapHeader->mapLayout->width - MAP_OFFSET;
-        localX = localLength + x; // additions are reversed for some reason
-        localOffset = connection->offset + MAP_OFFSET;
-        localY = y - localOffset;
+        connectionX = connectionHeader->mapLayout->width + localX;
+        connectionY = localY - connection->offset;
         break;
     case CONNECTION_EAST:
-        localLength = gMapHeader.mapLayout->width + MAP_OFFSET;
-        localX = x - localLength;
-        localOffset = connection->offset + MAP_OFFSET;
-        localY = y - localOffset;
+        connectionX = localX - gMapHeader.mapLayout->width;
+        connectionY = localY - connection->offset;
         break;
     default:
         return FALSE;
     }
-    return IsHiddenItemPresentAtCoords(mapHeader->events, localX, localY);
+    return IsHiddenItemPresentAtCoords(connectionHeader->events, connectionX, connectionY);
 }
 
+#undef localX
+#undef localY
+
 static void CheckForHiddenItemsInMapConnection(u8 taskId)
 {
     s16 playerX, playerY;