add change_tile_ids() function

src/game.rs

@@ -456,9 +456,7 @@ impl Game {
                 }
             }
 
-            room.tiles = room.tiles.iter().map(|tile_id|
+            room.change_tile_ids(&tile_id_changes);
-                tile_id_changes.get(tile_id).unwrap_or(tile_id).clone()
-            ).collect();
 
             room.items = room.items.iter().map(|instance|
                 if item_id_changes.contains_key(&instance.id) {

src/room.rs

@@ -2,6 +2,7 @@ use crate::{optional_data_line, Exit, ExitInstance, Instance, Position};
 use crate::game::{RoomType, RoomFormat};
 use crate::exit::Transition;
 use std::str::FromStr;
+use std::collections::HashMap;
 
 #[derive(Clone, Debug, Eq, PartialEq)]
 pub struct Room {
@@ -223,6 +224,14 @@ impl Room {
             self.palette_line()
         )
     }
+
+    /// "changes" is a hash of old -> new tile IDs
+    #[inline]
+    pub fn change_tile_ids(&mut self, changes: &HashMap<String, String>) {
+        self.tiles = self.tiles.iter().map(|tile_id|
+            changes.get(tile_id).unwrap_or(tile_id).clone()
+        ).collect();
+    }
 }
 
 #[cfg(test)]