fix: when advancing a run iter, check if we've reached the other size of the only interval if present

Author: Dr-Emann

roaring/src/bitmap/store/interval_store.rs

@@ -712,7 +712,18 @@ impl<I: SliceIterator<Interval>> RunIter<I> {
                 if let Some(value) = index.checked_sub(1) {
                     self.intervals.nth(value);
                 }
-                self.forward_offset = n - self.intervals.as_slice().first().unwrap().start;
+                let first_interval = self.intervals.as_slice().first().unwrap();
+                self.forward_offset = n - first_interval.start;
+                if self.intervals.as_slice().len() == 1
+                    && u64::from(self.forward_offset) + u64::from(self.backward_offset)
+                        >= first_interval.run_len()
+                {
+                    // If we are now the only interval, and we've now met the forward offset,
+                    // consume the final interval
+                    _ = self.intervals.next();
+                    self.forward_offset = 0;
+                    self.backward_offset = 0;
+                }
             }
             Err(index) => {
                 if index == self.intervals.as_slice().len() {
@@ -749,7 +760,18 @@ impl<I: SliceIterator<Interval>> RunIter<I> {
                 if let Some(value) = backward_index.checked_sub(1) {
                     self.intervals.nth_back(value);
                 }
-                self.backward_offset = self.intervals.as_slice().last().unwrap().end - n;
+                let last_interval = self.intervals.as_slice().last().unwrap();
+                self.backward_offset = last_interval.end - n;
+                if self.intervals.as_slice().len() == 1
+                    && u64::from(self.forward_offset) + u64::from(self.backward_offset)
+                        >= last_interval.run_len()
+                {
+                    // If we are now the only interval, and we've now met the forward offset,
+                    // consume the final interval
+                    _ = self.intervals.next_back();
+                    self.forward_offset = 0;
+                    self.backward_offset = 0;
+                }
             }
             Err(index) => {
                 if index == 0 {

roaring/tests/iter_advance_to.rs

@@ -236,3 +236,13 @@ fn advance_run_back_reduces_forward_iter() {
     assert_eq!(iter.next(), Some(1));
     assert_eq!(iter.next(), None);
 }
+
+#[test]
+fn advance_run_front_and_back_past_each_other() {
+    let mut bitmap = RoaringBitmap::new();
+    bitmap.insert_range(0..=0x4000);
+    let mut iter = bitmap.iter();
+    iter.advance_back_to(100);
+    iter.advance_to(300);
+    assert_eq!(iter.next(), None);
+}