fix(linked chunk): in LinkedChunk::ritems_from, skip as long as we're…

… on the right chunk

The previous code would skip based on the position's index, but not the
position's chunk. It could be that the position's chunk is different
from the first items chunk, as shown in the example, where the linked
chunk ends with a gap; in this case, the position's index would be 0,
while the first chunk found while iterating backwards had 3 items. As a
result, items 'd' and 'e' would be skipped incorrectly.

The fix is to take into account the chunk id when skipping over items.

crates/matrix-sdk-common/src/linked_chunk/mod.rs

@@ -816,8 +816,9 @@ impl<const CAP: usize, Item, Gap> LinkedChunk<CAP, Item, Gap> {
             .skip_while({
                 let expected_index = position.index();
 
-                move |(Position(_chunk_identifier, item_index), _item)| {
-                    *item_index != expected_index
+                move |(Position(chunk_identifier, item_index), _item)| {
+                    *chunk_identifier == position.chunk_identifier()
+                        && *item_index != expected_index
                 }
             }))
     }
@@ -1813,6 +1814,26 @@ mod tests {
         assert_matches!(iterator.next(), None);
     }
 
+    #[test]
+    fn test_ritems_with_final_gap() -> Result<(), Error> {
+        let mut linked_chunk = LinkedChunk::<3, char, ()>::new();
+        linked_chunk.push_items_back(['a', 'b']);
+        linked_chunk.push_gap_back(());
+        linked_chunk.push_items_back(['c', 'd', 'e']);
+        linked_chunk.push_gap_back(());
+
+        let mut iterator = linked_chunk.ritems();
+
+        assert_matches!(iterator.next(), Some((Position(ChunkIdentifier(2), 2), 'e')));
+        assert_matches!(iterator.next(), Some((Position(ChunkIdentifier(2), 1), 'd')));
+        assert_matches!(iterator.next(), Some((Position(ChunkIdentifier(2), 0), 'c')));
+        assert_matches!(iterator.next(), Some((Position(ChunkIdentifier(0), 1), 'b')));
+        assert_matches!(iterator.next(), Some((Position(ChunkIdentifier(0), 0), 'a')));
+        assert_matches!(iterator.next(), None);
+
+        Ok(())
+    }
+
     #[test]
     fn test_ritems_empty() {
         let linked_chunk = LinkedChunk::<2, char, ()>::new();