sundy-li commented on a change in pull request #9602:
URL: https://github.com/apache/arrow/pull/9602#discussion_r593899532
##########
File path: rust/arrow/src/compute/kernels/sort.rs
##########
@@ -686,90 +815,124 @@ pub fn lexsort_to_indices(columns: &[SortColumn]) ->
Result<UInt32Array> {
};
let mut value_indices = (0..row_count).collect::<Vec<usize>>();
- value_indices.sort_by(lex_comparator);
+ let mut len = value_indices.len();
+
+ if let Some(limit) = limit {
+ len = limit.min(len);
+ }
+ sort_by(&mut value_indices, len, lex_comparator);
Ok(UInt32Array::from(
- value_indices
- .into_iter()
- .map(|i| i as u32)
+ (&value_indices)[0..len]
+ .iter()
+ .map(|i| *i as u32)
.collect::<Vec<u32>>(),
))
}
+pub fn partial_sort<T, F>(v: &mut [T], limit: usize, mut is_less: F)
+where
+ F: FnMut(&T, &T) -> Ordering,
+{
+ let (before, _mid, _after) = v.select_nth_unstable_by(limit, &mut is_less);
+ before.sort_unstable_by(is_less);
Review comment:
There are some reasons for using unstable_sort.
1. `select_nth_unstable_by` is unstable, so using `before.sort_stable_by`
don't ensure it's stable. Currently we do not have select_nth_stable_by
2. `sort_unstable` is faster than sort_stable, refer to
[doc](https://doc.rust-lang.org/std/primitive.slice.html#method.sort_unstable)
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