refactor advance_by and advance_back_by. Add back cfg for with_critical_section

Author: Owen-CH-Leung

src/types/list.rs

@@ -640,13 +640,14 @@ impl<'py> BoundListIterator<'py> {
                     list.get_item(target_index).expect("get-item failed")
                 }
             };
-            *length = Length(target_index);
+            length.0 = target_index;
             Some(item)
         } else {
             None
         }
     }
 
+    #[cfg(not(Py_LIMITED_API))]
     fn with_critical_section<R>(
         &mut self,
         f: impl FnOnce(&mut Index, &mut Length, &Bound<'py, PyList>) -> R,
@@ -818,12 +819,25 @@ impl<'py> Iterator for BoundListIterator<'py> {
     #[cfg(feature = "nightly")]
     fn advance_by(&mut self, n: usize) -> Result<(), NonZero<usize>> {
         self.with_critical_section(|index, length, list| {
-            for i in 0..n {
-                if unsafe { Self::next_unchecked(index, length, list).is_none() } {
-                    return Err(unsafe { NonZero::new_unchecked(n - i) });
+            let max_len = length.0.min(list.len());
+            let currently_at = index.0;
+            if currently_at >= max_len {
+                if n == 0 {
+                    return Ok(());
+                } else {
+                    return Err(unsafe { NonZero::new_unchecked(n) });
                 }
             }
-            Ok(())
+
+            let items_left = max_len - currently_at;
+            if n <= items_left {
+                index.0 += n;
+                Ok(())
+            } else {
+                index.0 = max_len;
+                let remainder = n - items_left;
+                Err(unsafe { NonZero::new_unchecked(remainder) })
+            }
         })
     }
 }
@@ -891,12 +905,25 @@ impl DoubleEndedIterator for BoundListIterator<'_> {
     #[cfg(feature = "nightly")]
     fn advance_back_by(&mut self, n: usize) -> Result<(), NonZero<usize>> {
         self.with_critical_section(|index, length, list| {
-            for i in 0..n {
-                if unsafe { Self::next_back_unchecked(index, length, list).is_none() } {
-                    return Err(unsafe { NonZero::new_unchecked(n - i) });
+            let max_len = length.0.min(list.len());
+            let currently_at = index.0;
+            if currently_at >= max_len {
+                if n == 0 {
+                    return Ok(());
+                } else {
+                    return Err(unsafe { NonZero::new_unchecked(n) });
                 }
             }
-            Ok(())
+
+            let items_left = max_len - currently_at;
+            if n <= items_left {
+                length.0 = max_len - n;
+                Ok(())
+            } else {
+                length.0 = max_len;
+                let remainder = n - items_left;
+                Err(unsafe { NonZero::new_unchecked(remainder) })
+            }
         })
     }
 }
@@ -1637,8 +1664,7 @@ mod tests {
             assert_eq!(iter.next().unwrap().extract::<i32>().unwrap(), 10);
 
             let mut iter = list.iter();
-            println!("iter.nth_back(1) = {:?}", iter.nth_back(1));
-            // assert_eq!(iter.nth_back(1).unwrap().extract::<i32>().unwrap(), 9);
+            assert_eq!(iter.nth_back(1).unwrap().extract::<i32>().unwrap(), 9);
             assert_eq!(iter.nth(2).unwrap().extract::<i32>().unwrap(), 8);
             assert!(iter.next().is_none());
         });