ARROW-10609: [Rust] Optimize min/max of non null strings

Applies the same optimization as in ARROW-10595. Difference is smaller, but still there:

```
min string 512          time:   [3.4096 us 3.4378 us 3.4683 us]
                        change: [-13.563% -13.111% -12.628%] (p = 0.00 < 0.05)
                        Performance has improved.
```

Closes #8673 from Dandandan/min_max_string

Authored-by: Heres, Daniel <danielheres@gmail.com>
Signed-off-by: Andrew Lamb <andrew@nerdnetworks.org>

rust/arrow/benches/aggregate_kernels.rs

@@ -19,16 +19,15 @@
 extern crate criterion;
 use criterion::Criterion;
 
-use rand::Rng;
-use std::sync::Arc;
+use rand::{distributions::Alphanumeric, Rng};
 
 extern crate arrow;
 
 use arrow::array::*;
 use arrow::compute::kernels::aggregate::*;
 use arrow::util::test_util::seedable_rng;
 
-fn create_array(size: usize, with_nulls: bool) -> ArrayRef {
+fn create_array(size: usize, with_nulls: bool) -> Float32Array {
     // use random numbers to avoid spurious compiler optimizations wrt to branching
     let mut rng = seedable_rng();
     let mut builder = Float32Builder::new(size);
@@ -40,19 +39,40 @@ fn create_array(size: usize, with_nulls: bool) -> ArrayRef {
             builder.append_value(rng.gen()).unwrap();
         }
     }
-    Arc::new(builder.finish())
+    builder.finish()
 }
 
-fn bench_sum(arr_a: &ArrayRef) {
-    let arr_a = arr_a.as_any().downcast_ref::<Float32Array>().unwrap();
+fn create_string_array(size: usize, with_nulls: bool) -> StringArray {
+    // use random numbers to avoid spurious compiler optimizations wrt to branching
+    let mut rng = seedable_rng();
+    let mut builder = StringBuilder::new(size);
+
+    for _ in 0..size {
+        if with_nulls && rng.gen::<f32>() > 0.5 {
+            builder.append_null().unwrap();
+        } else {
+            let string = seedable_rng()
+                .sample_iter(&Alphanumeric)
+                .take(10)
+                .collect::<String>();
+            builder.append_value(&string).unwrap();
+        }
+    }
+    builder.finish()
+}
+
+fn bench_sum(arr_a: &Float32Array) {
     criterion::black_box(sum(&arr_a).unwrap());
 }
 
-fn bench_min(arr_a: &ArrayRef) {
-    let arr_a = arr_a.as_any().downcast_ref::<Float32Array>().unwrap();
+fn bench_min(arr_a: &Float32Array) {
     criterion::black_box(min(&arr_a).unwrap());
 }
 
+fn bench_min_string(arr_a: &StringArray) {
+    criterion::black_box(min_string(&arr_a).unwrap());
+}
+
 fn add_benchmark(c: &mut Criterion) {
     let arr_a = create_array(512, false);
 
@@ -63,6 +83,14 @@ fn add_benchmark(c: &mut Criterion) {
 
     c.bench_function("sum nulls 512", |b| b.iter(|| bench_sum(&arr_a)));
     c.bench_function("min nulls 512", |b| b.iter(|| bench_min(&arr_a)));
+
+    let arr_b = create_string_array(512, false);
+    c.bench_function("min string 512", |b| b.iter(|| bench_min_string(&arr_b)));
+
+    let arr_b = create_string_array(512, true);
+    c.bench_function("min nulls string 512", |b| {
+        b.iter(|| bench_min_string(&arr_b))
+    });
 }
 
 criterion_group!(benches, add_benchmark);

rust/arrow/src/compute/kernels/aggregate.rs

@@ -32,19 +32,20 @@ fn min_max_string<T: StringOffsetSizeTrait, F: Fn(&str, &str) -> bool>(
     if null_count == array.len() {
         return None;
     }
-    let mut n = "";
-    let mut has_value = false;
     let data = array.data();
-
+    let mut n;
     if null_count == 0 {
-        for i in 0..data.len() {
+        n = array.value(0);
+        for i in 1..data.len() {
             let item = array.value(i);
-            if !has_value || cmp(&n, item) {
-                has_value = true;
+            if cmp(&n, item) {
                 n = item;
             }
         }
     } else {
+        n = "";
+        let mut has_value = false;
+
         for i in 0..data.len() {
             let item = array.value(i);
             if data.is_valid(i) && (!has_value || cmp(&n, item)) {
@@ -105,13 +106,9 @@ where
 
     if null_count == 0 {
         // optimized path for arrays without null values
-        n = m[0];
-
-        for item in &m[1..] {
-            if cmp(&n, item) {
-                n = *item
-            }
-        }
+        n = m[1..]
+            .iter()
+            .fold(m[0], |max, item| if cmp(&max, item) { *item } else { max });
     } else {
         n = T::default_value();
         let mut has_value = false;