diff --git a/backup/src/delta.rs b/backup/src/delta.rs
index cc831ac..993dd3a 100644
--- a/backup/src/delta.rs
+++ b/backup/src/delta.rs
@@ -242,4 +242,56 @@ mod tests {
         let expected = Delta::default().with_added("dir_1", ["file1", "file2"]);
         assert_eq!(expected, b.union(&a));
     }
+
+    #[test]
+    fn test_difference() {
+        let a = Delta::default()
+            .with_added("dir1", ["file1", "file2"])
+            .with_removed("dir1", ["file3", "file4"]);
+        let b = Delta::default()
+            .with_added("dir1", ["file1"])
+            .with_removed("dir1", ["file3"]);
+        let expected = Delta::default()
+            .with_added("dir1", ["file2"])
+            .with_removed("dir1", ["file4"]);
+
+        assert_eq!(a.difference(&b), expected);
+        assert_eq!(b.difference(&a), Delta::default());
+    }
+
+    #[test]
+    fn test_strict_difference() {
+        let a = Delta::default()
+            .with_added("dir1", ["file1", "file2"])
+            .with_removed("dir1", ["file3", "file4"]);
+        let b = Delta::default()
+            .with_added("dir1", ["file1", "file4"])
+            .with_removed("dir1", ["file3"]);
+        let expected = Delta::default().with_added("dir1", ["file2"]);
+
+        assert_eq!(a.strict_difference(&b), expected);
+        assert_eq!(b.strict_difference(&a), Delta::default());
+    }
+
+    #[test]
+    fn test_contributions() {
+        let deltas = [
+            Delta::default().with_added("dir1", ["file4"]),
+            Delta::default().with_added("dir1", ["file1", "file2"]),
+            Delta::default()
+                .with_added("dir1", ["file1"])
+                .with_added("dir2", ["file3"]),
+            Delta::default()
+                .with_added("dir1", ["file2"])
+                .with_removed("dir2", ["file3"]),
+        ];
+        let expected = [
+            State::default().with_dir("dir1", ["file2"]),
+            State::default().with_dir("dir1", ["file1"]),
+            State::default(),
+            State::default().with_dir("dir1", ["file4"]),
+        ];
+
+        assert_eq!(Delta::contributions(deltas.iter().rev()), expected);
+    }
 }