refactor lexico sort (#423) (#442)

Co-authored-by: Jiayu Liu <Jimexist@users.noreply.github.com>

arrow/src/compute/kernels/sort.rs

@@ -17,14 +17,12 @@
 
 //! Defines sort kernel for `ArrayRef`
 
-use std::cmp::Ordering;
-
 use crate::array::*;
 use crate::buffer::MutableBuffer;
 use crate::compute::take;
 use crate::datatypes::*;
 use crate::error::{ArrowError, Result};
-
+use std::cmp::Ordering;
 use TimeUnit::*;
 
 /// Sort the `ArrayRef` using `SortOptions`.
@@ -835,26 +833,55 @@ pub fn lexsort_to_indices(
         ));
     };
 
-    // map to data and DynComparator
-    let flat_columns = columns
-        .iter()
-        .map(
-            |column| -> Result<(&ArrayData, DynComparator, SortOptions)> {
-                // flatten and convert build comparators
-                // use ArrayData for is_valid checks later to avoid dynamic call
-                let values = column.values.as_ref();
-                let data = values.data_ref();
-                Ok((
-                    data,
-                    build_compare(values, values)?,
-                    column.options.unwrap_or_default(),
-                ))
-            },
-        )
-        .collect::<Result<Vec<(&ArrayData, DynComparator, SortOptions)>>>()?;
+    let mut value_indices = (0..row_count).collect::<Vec<usize>>();
+    let mut len = value_indices.len();
+
+    if let Some(limit) = limit {
+        len = limit.min(len);
+    }
+
+    let lexicographical_comparator = LexicographicalComparator::try_new(columns)?;
+    sort_by(&mut value_indices, len, |a, b| {
+        lexicographical_comparator.compare(a, b)
+    });
+
+    Ok(UInt32Array::from(
+        (&value_indices)[0..len]
+            .iter()
+            .map(|i| *i as u32)
+            .collect::<Vec<u32>>(),
+    ))
+}
+
+/// It's unstable_sort, may not preserve the order of equal elements
+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);
+}
+
+type LexicographicalCompareItem<'a> = (
+    &'a ArrayData,                              // data
+    Box<dyn Fn(usize, usize) -> Ordering + 'a>, // comparator
+    SortOptions,                                // sort_option
+);
+
+/// A lexicographical comparator that wraps given array data (columns) and can lexicographically compare data
+/// at given two indices. The lifetime is the same at the data wrapped.
+pub(super) struct LexicographicalComparator<'a> {
+    compare_items: Vec<LexicographicalCompareItem<'a>>,
+}
 
-    let lex_comparator = |a_idx: &usize, b_idx: &usize| -> Ordering {
-        for (data, comparator, sort_option) in flat_columns.iter() {
+impl LexicographicalComparator<'_> {
+    /// lexicographically compare values at the wrapped columns with given indices.
+    pub(super) fn compare<'a, 'b>(
+        &'a self,
+        a_idx: &'b usize,
+        b_idx: &'b usize,
+    ) -> Ordering {
+        for (data, comparator, sort_option) in &self.compare_items {
             match (data.is_valid(*a_idx), data.is_valid(*b_idx)) {
                 (true, true) => {
                     match (comparator)(*a_idx, *b_idx) {
@@ -889,31 +916,29 @@ pub fn lexsort_to_indices(
         }
 
         Ordering::Equal
-    };
-
-    let mut value_indices = (0..row_count).collect::<Vec<usize>>();
-    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)[0..len]
+    /// Create a new lex comparator that will wrap the given sort columns and give comparison
+    /// results with two indices.
+    pub(super) fn try_new(
+        columns: &[SortColumn],
+    ) -> Result<LexicographicalComparator<'_>> {
+        let compare_items = columns
             .iter()
-            .map(|i| *i as u32)
-            .collect::<Vec<u32>>(),
-    ))
-}
-
-/// It's unstable_sort, may not preserve the order of equal elements
-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);
+            .map(|column| {
+                // flatten and convert build comparators
+                // use ArrayData for is_valid checks later to avoid dynamic call
+                let values = column.values.as_ref();
+                let data = values.data_ref();
+                Ok((
+                    data,
+                    build_compare(values, values)?,
+                    column.options.unwrap_or_default(),
+                ))
+            })
+            .collect::<Result<Vec<_>>>()?;
+        Ok(LexicographicalComparator { compare_items })
+    }
 }
 
 #[cfg(test)]