diff --git a/backup/src/delta.rs b/backup/src/delta.rs
index cc831ac..95ec391 100644
--- a/backup/src/delta.rs
+++ b/backup/src/delta.rs
@@ -110,20 +110,19 @@ impl Delta {
         out
     }
 
-    /// Given a chain of deltas, ordered from last to first, calculate the "contribution" for each
-    /// state.
+    /// Given a chain of deltas, calculate the "contribution" for each state.
     ///
-    /// The contribution of a delta in a given chain is defined as the parts of the state produced
-    /// by this chain that are actually provided by this delta. This comes down to calculating the
-    /// strict difference of this delta and all of its successive deltas.
+    /// For each delta, its contribution is the part of its added and removed files that isn't
+    /// overwritten by any of its following deltas.
     pub fn contributions<I>(deltas: I) -> Vec<State>
     where
         I: IntoIterator,
+        I::IntoIter: DoubleEndedIterator,
         I::Item: Borrow<Delta>,
     {
         let mut contributions: Vec<State> = Vec::new();
 
-        let mut deltas = deltas.into_iter();
+        let mut deltas = deltas.into_iter().rev();
 
         if let Some(first_delta) = deltas.next() {
             // From last to first, we calculate the strict difference of the delta with the union of all its
@@ -138,8 +137,7 @@ impl Delta {
             }
         }
 
-        // contributions.reverse();
-
+        contributions.reverse();
         contributions
     }
 
@@ -242,4 +240,56 @@ 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", ["file4"]),
+            State::default(),
+            State::default().with_dir("dir1", ["file1"]),
+            State::default().with_dir("dir1", ["file2"]),
+        ];
+
+        assert_eq!(Delta::contributions(deltas), expected);
+    }
 }
diff --git a/backup/src/manager/mod.rs b/backup/src/manager/mod.rs
index f8bbc73..f13c69f 100644
--- a/backup/src/manager/mod.rs
+++ b/backup/src/manager/mod.rs
@@ -229,9 +229,8 @@ where
                     .map(|_| ()),
                     // Incremental backups are exported one by one according to their contribution
                     BackupType::Incremental => {
-                        let contributions = Delta::contributions(
-                            chain.iter().take(index + 1).map(|b| &b.delta).rev(),
-                        );
+                        let contributions =
+                            Delta::contributions(chain.iter().take(index + 1).map(|b| &b.delta));
 
                         let tar_gz = OpenOptions::new()
                             .write(true)
@@ -245,7 +244,6 @@ where
                         // overwritten by their successors anyways.
                         for (contribution, backup) in contributions
                             .iter()
-                            .rev()
                             .zip(chain.iter().take(index + 1))
                             .filter(|(contribution, _)| !contribution.is_empty())
                         {