diff --git a/src/game.rs b/src/game.rs
index 0ac5943..c37d014 100644
--- a/src/game.rs
+++ b/src/game.rs
@@ -163,6 +163,42 @@ impl ToString for Game {
     }
 }
 
+pub trait TileIds {
+    fn tile_ids(&self) -> Vec<u64>;
+}
+
+impl TileIds for Game {
+    fn tile_ids(&self) -> Vec<u64> {
+        self.tiles.iter().map(|tile| {tile.id}).collect()
+    }
+}
+
+/// first available tile ID.
+/// e.g. if current tile IDs are [0, 2, 3] the result will be `1`
+///      if current tile IDs are [0, 1, 2] the result will be `3`
+pub trait NewTileId {
+    fn new_tile_id(&self) -> u64;
+}
+
+impl NewTileId for Game {
+    fn new_tile_id(&self) -> u64 {
+        let mut new_id = 0;
+
+        let mut ids = self.tile_ids();
+        ids.sort();
+
+        for id in ids {
+            if new_id == id {
+                new_id += 1;
+            } else {
+                return new_id;
+            }
+        }
+
+        new_id + 1
+    }
+}
+
 #[test]
 fn test_game_from_string() {
     let output = Game::from(
@@ -180,3 +216,37 @@ fn test_game_to_string() {
     let expected = include_str!["../test/resources/default.bitsy"].to_string();
     assert_eq!(output, expected);
 }
+
+#[test]
+fn test_tile_ids() {
+    assert_eq!(mock::game_default().tile_ids(), vec![10]);
+}
+
+#[test]
+fn test_new_tile_id() {
+    // default tile has an id of 10 ("a"), so 0 is available
+    assert_eq!(mock::game_default().new_tile_id(), 0);
+
+    let mut game = mock::game_default();
+    let mut tiles : Vec<Tile> = Vec::new();
+
+    for n in 0..9 {
+        if n != 4 {
+            let mut new_tile = mock::tile_default();
+            new_tile.id = n;
+            tiles.push(new_tile);
+        }
+    }
+
+    game.tiles = tiles;
+
+    assert_eq!(game.new_tile_id(), 4);
+
+    // fill in the space created above, and test that tile IDs get sorted
+
+    let mut new_tile = mock::tile_default();
+    new_tile.id = 4;
+    game.tiles.push(new_tile);
+
+    assert_eq!(game.new_tile_id(), 10);
+}
diff --git a/src/mock.rs b/src/mock.rs
index 7914b4a..f6e3c8b 100644
--- a/src/mock.rs
+++ b/src/mock.rs
@@ -67,6 +67,28 @@ pub fn avatar() -> Avatar {
     }
 }
 
+pub fn tile_default() -> Tile {
+    Tile {
+        id: 10,
+        name: None,
+        wall: false,
+        animation_frames: vec![
+            Image {
+                pixels: vec![
+                    1,1,1,1,1,1,1,1,
+                    1,0,0,0,0,0,0,1,
+                    1,0,0,0,0,0,0,1,
+                    1,0,0,1,1,0,0,1,
+                    1,0,0,1,1,0,0,1,
+                    1,0,0,0,0,0,0,1,
+                    1,0,0,0,0,0,0,1,
+                    1,1,1,1,1,1,1,1,
+                ]
+            }
+        ]
+    }
+}
+
 pub fn sprite() -> Sprite {
     Sprite {
         id: 10,
@@ -207,25 +229,7 @@ pub fn game_default() -> Game {
             }
         ],
         tiles: vec![
-            Tile {
-                id: 10,
-                name: None,
-                wall: false,
-                animation_frames: vec![
-                    Image {
-                        pixels: vec![
-                            1,1,1,1,1,1,1,1,
-                            1,0,0,0,0,0,0,1,
-                            1,0,0,0,0,0,0,1,
-                            1,0,0,1,1,0,0,1,
-                            1,0,0,1,1,0,0,1,
-                            1,0,0,0,0,0,0,1,
-                            1,0,0,0,0,0,0,1,
-                            1,1,1,1,1,1,1,1,
-                        ]
-                    }
-                ]
-            }
+            self::tile_default(),
         ],
         avatar: Avatar {
             animation_frames: vec![