refactor: add fast branch for ascii-only string truncate (#1330)

This is just a first attempt to speed up what looked like a hot spot in samply's profiling results.

Benchmark code and results here: https://gist.github.com/ClementTsang/e242f12f7e1d1902ed414dcc18c3b321

.gitignore

@@ -42,3 +42,6 @@ dhat/
 
 # cargo vet
 supply-chain/
+
+# samply profiling
+profile.json

src/utils/gen_util.rs

@@ -100,9 +100,23 @@ fn grapheme_width(g: &str) -> usize {
 #[inline]
 fn truncate_str<U: Into<usize>>(content: &str, width: U) -> String {
     let width = width.into();
-    let mut text = String::with_capacity(width);
 
     if width > 0 {
+        if content.is_ascii() {
+            // If the entire string is ASCII, we can use a much simpler approach.
+
+            if content.len() <= width {
+                content.to_owned()
+            } else {
+                let mut text = String::with_capacity(width);
+                let (keep, _throw) = content.split_at(width - 1);
+                text.push_str(keep);
+                text.push('…');
+
+                text
+            }
+        } else {
+            let mut text = String::with_capacity(width);
             let mut curr_width = 0;
             let mut early_break = false;
 
@@ -134,10 +148,12 @@ fn truncate_str<U: Into<usize>>(content: &str, width: U) -> String {
                 }
                 text.push('…');
             }
-    }
-
             text
         }
+    } else {
+        String::default()
+    }
+}
 
 #[inline]
 pub const fn sort_partial_fn<T: PartialOrd>(is_descending: bool) -> fn(T, T) -> Ordering {
@@ -252,7 +268,7 @@ mod test {
         assert_eq!(
             truncate_str(cpu_header, 8_usize),
             cpu_header,
-            "should match base string as there is enough room"
+            "should match base string as there is extra room"
         );
 
         assert_eq!(
@@ -269,13 +285,36 @@ mod test {
     }
 
     #[test]
-    fn truncate_cjk() {
+    fn test_truncate_ascii() {
+        let content = "0123456";
+
+        assert_eq!(
+            truncate_str(content, 8_usize),
+            content,
+            "should match base string as there is extra room"
+        );
+
+        assert_eq!(
+            truncate_str(content, 7_usize),
+            content,
+            "should match base string as there is enough room"
+        );
+
+        assert_eq!(truncate_str(content, 6_usize), "01234…");
+        assert_eq!(truncate_str(content, 5_usize), "0123…");
+        assert_eq!(truncate_str(content, 4_usize), "012…");
+        assert_eq!(truncate_str(content, 1_usize), "…");
+        assert_eq!(truncate_str(content, 0_usize), "");
+    }
+
+    #[test]
+    fn test_truncate_cjk() {
         let cjk = "施氏食獅史";
 
         assert_eq!(
             truncate_str(cjk, 11_usize),
             cjk,
-            "should match base string as there is enough room"
+            "should match base string as there is extra room"
         );
 
         assert_eq!(
@@ -292,13 +331,13 @@ mod test {
     }
 
     #[test]
-    fn truncate_mixed() {
+    fn test_truncate_mixed() {
         let test = "Test (施氏食獅史) Test";
 
         assert_eq!(
             truncate_str(test, 30_usize),
             test,
-            "should match base string as there is enough room"
+            "should match base string as there is extra room"
         );
 
         assert_eq!(
@@ -325,7 +364,7 @@ mod test {
     }
 
     #[test]
-    fn truncate_flags() {
+    fn test_truncate_flags() {
         let flag = "🇨🇦";
         assert_eq!(truncate_str(flag, 3_usize), flag);
         assert_eq!(truncate_str(flag, 2_usize), flag);
@@ -368,7 +407,7 @@ mod test {
 
     /// This might not be the best way to handle it, but this at least tests that it doesn't crash...
     #[test]
-    fn truncate_hindi() {
+    fn test_truncate_hindi() {
         // cSpell:disable
         let test = "हिन्दी";
         assert_eq!(truncate_str(test, 10_usize), test);