fix: another case where we need to use both offsets when only one interval remains

Author: Dr-Emann

roaring/src/bitmap/store/interval_store.rs

@@ -829,9 +829,14 @@ impl<I: SliceIterator<Interval>> Iterator for RunIter<I> {
         if let Some(skip) = n.checked_sub(1) {
             let mut to_skip = skip as u64;
             loop {
-                let to_remove = (self.intervals.as_slice().first()?.run_len()
-                    - self.forward_offset as u64)
-                    .min(to_skip);
+                let full_first_interval_len = self.intervals.as_slice().first()?.run_len();
+                let consumed_len = u64::from(self.forward_offset)
+                    + if self.intervals.as_slice().len() == 1 {
+                        u64::from(self.backward_offset)
+                    } else {
+                        0
+                    };
+                let to_remove = (full_first_interval_len - consumed_len).min(to_skip);
                 to_skip -= to_remove;
                 self.forward_offset += to_remove as u16;
                 self.move_next();

roaring/tests/iter_advance_to.rs

@@ -278,6 +278,16 @@ fn advance_to_with_next_len() {
     assert_eq!(iter.nth_back(0), None);
 }
 
+#[test]
+fn tmp() {
+    let mut bitmap = RoaringBitmap::new();
+    bitmap.insert_range(196363..=262143);
+    let mut iter = bitmap.iter();
+    assert_eq!(iter.next_back(), Some(262143));
+    iter.advance_to(228960);
+    assert_eq!(iter.nth(36643), None);
+}
+
 #[test]
 fn advance_bitset_front_and_back_past_each_other() {
     let mut bitmap = RoaringBitmap::new();