Improve fold bench data generation (#9210)

Author: mattsu2020

src/uu/fold/benches/fold_bench.rs

@@ -4,20 +4,19 @@
 // file that was distributed with this source code.
 
 use divan::{Bencher, black_box};
-use std::fmt::Write;
 use uu_fold::uumain;
 use uucore::benchmark::{create_test_file, run_util_function};
 
 /// Benchmark folding many short lines
 #[divan::bench(args = [100_000])]
 fn fold_many_lines(bencher: Bencher, num_lines: usize) {
     let temp_dir = tempfile::tempdir().unwrap();
-    // Create long lines that need folding
-    let data = (0..num_lines)
-        .fold(String::new(), |mut acc, i| {
-            writeln!(&mut acc, "This is a very long line number {i} that definitely needs to be folded at the default width of 80 columns").unwrap();
-            acc
-        });
+    let mut data = String::with_capacity(num_lines * 110);
+    for i in 0..num_lines {
+        data.push_str("This is a very long line number ");
+        append_usize(&mut data, i);
+        data.push_str(" that definitely needs to be folded at the default width of 80 columns\n");
+    }
     let file_path = create_test_file(data.as_bytes(), temp_dir.path());
     let file_path_str = file_path.to_str().unwrap();
 
@@ -30,14 +29,12 @@ fn fold_many_lines(bencher: Bencher, num_lines: usize) {
 #[divan::bench(args = [50_000])]
 fn fold_custom_width(bencher: Bencher, num_lines: usize) {
     let temp_dir = tempfile::tempdir().unwrap();
-    let data = (0..num_lines).fold(String::new(), |mut acc, i| {
-        writeln!(
-            &mut acc,
-            "Line {i} with enough text to exceed width 40 characters and require folding"
-        )
-        .unwrap();
-        acc
-    });
+    let mut data = String::with_capacity(num_lines * 80);
+    for i in 0..num_lines {
+        data.push_str("Line ");
+        append_usize(&mut data, i);
+        data.push_str(" with enough text to exceed width 40 characters and require folding\n");
+    }
     let file_path = create_test_file(data.as_bytes(), temp_dir.path());
     let file_path_str = file_path.to_str().unwrap();
 
@@ -49,3 +46,21 @@ fn fold_custom_width(bencher: Bencher, num_lines: usize) {
 fn main() {
     divan::main();
 }
+
+fn append_usize(buf: &mut String, mut value: usize) {
+    let mut digits = [0u8; 20];
+    let mut idx = digits.len();
+
+    if value == 0 {
+        buf.push('0');
+        return;
+    }
+
+    while value > 0 {
+        idx -= 1;
+        digits[idx] = b'0' + (value % 10) as u8;
+        value /= 10;
+    }
+
+    buf.push_str(std::str::from_utf8(&digits[idx..]).unwrap());
+}