fix: run iterators need to respect the back_offset when only one interval remains

Author: Dr-Emann

roaring/src/bitmap/store/interval_store.rs

@@ -659,9 +659,13 @@ impl<I: SliceIterator<Interval>> RunIter<I> {
             self.intervals.next();
             return;
         }
-        if Some(self.forward_offset as u64)
-            >= self.intervals.as_slice().first().map(|f| f.run_len())
-        {
+        let total_offset = u64::from(self.forward_offset)
+            + if self.intervals.as_slice().len() == 1 {
+                u64::from(self.backward_offset)
+            } else {
+                0
+            };
+        if Some(total_offset) >= self.intervals.as_slice().first().map(|f| f.run_len()) {
             self.intervals.next();
             self.forward_offset = 0;
         }
@@ -674,9 +678,9 @@ impl<I: SliceIterator<Interval>> RunIter<I> {
             self.intervals.next_back();
             return;
         }
-        if Some(self.backward_offset as u64)
-            >= self.intervals.as_slice().last().map(|f| f.run_len())
-        {
+        let total_offset = u64::from(self.backward_offset)
+            + if self.intervals.as_slice().len() == 1 { u64::from(self.forward_offset) } else { 0 };
+        if Some(total_offset) >= self.intervals.as_slice().last().map(|f| f.run_len()) {
             self.intervals.next_back();
             self.backward_offset = 0;
         }

roaring/tests/iter_advance_to.rs

@@ -224,3 +224,15 @@ fn advance_run_back_before_start() {
     iter.advance_back_to(499);
     assert_eq!(iter.next_back(), None);
 }
+
+#[test]
+fn advance_run_back_reduces_forward_iter() {
+    let mut bitmap = RoaringBitmap::new();
+    bitmap.insert_range(0..=0x4000);
+    let mut iter = bitmap.iter();
+    iter.advance_back_to(1);
+
+    assert_eq!(iter.next(), Some(0));
+    assert_eq!(iter.next(), Some(1));
+    assert_eq!(iter.next(), None);
+}