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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
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@ -42,3 +42,6 @@ dhat/
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# cargo vet
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supply-chain/
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# samply profiling
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profile.json
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@ -100,43 +100,59 @@ fn grapheme_width(g: &str) -> usize {
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#[inline]
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fn truncate_str<U: Into<usize>>(content: &str, width: U) -> String {
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let width = width.into();
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let mut text = String::with_capacity(width);
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if width > 0 {
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let mut curr_width = 0;
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let mut early_break = false;
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if content.is_ascii() {
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// If the entire string is ASCII, we can use a much simpler approach.
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// This tracks the length of the last added string - note this does NOT match the grapheme *width*.
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let mut last_added_str_len = 0;
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// Cases to handle:
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// - Completes adding the entire string.
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// - Adds a character up to the boundary, then fails.
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// - Adds a character not up to the boundary, then fails.
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// Inspired by https://tomdebruijn.com/posts/rust-string-length-width-calculations/
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for g in UnicodeSegmentation::graphemes(content, true) {
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let g_width = grapheme_width(g);
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if curr_width + g_width <= width {
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curr_width += g_width;
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last_added_str_len = g.len();
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text.push_str(g);
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if content.len() <= width {
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content.to_owned()
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} else {
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early_break = true;
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break;
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}
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}
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let mut text = String::with_capacity(width);
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let (keep, _throw) = content.split_at(width - 1);
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text.push_str(keep);
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text.push('…');
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if early_break {
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if curr_width == width {
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// Remove the last grapheme cluster added.
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text.truncate(text.len() - last_added_str_len);
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text
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}
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text.push('…');
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} else {
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let mut text = String::with_capacity(width);
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let mut curr_width = 0;
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let mut early_break = false;
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// This tracks the length of the last added string - note this does NOT match the grapheme *width*.
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let mut last_added_str_len = 0;
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// Cases to handle:
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// - Completes adding the entire string.
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// - Adds a character up to the boundary, then fails.
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// - Adds a character not up to the boundary, then fails.
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// Inspired by https://tomdebruijn.com/posts/rust-string-length-width-calculations/
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for g in UnicodeSegmentation::graphemes(content, true) {
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let g_width = grapheme_width(g);
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if curr_width + g_width <= width {
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curr_width += g_width;
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last_added_str_len = g.len();
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text.push_str(g);
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} else {
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early_break = true;
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break;
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}
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}
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if early_break {
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if curr_width == width {
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// Remove the last grapheme cluster added.
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text.truncate(text.len() - last_added_str_len);
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}
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text.push('…');
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}
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text
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}
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} else {
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String::default()
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}
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text
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}
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#[inline]
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@ -252,7 +268,7 @@ mod test {
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assert_eq!(
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truncate_str(cpu_header, 8_usize),
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cpu_header,
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"should match base string as there is enough room"
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"should match base string as there is extra room"
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);
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assert_eq!(
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@ -269,13 +285,36 @@ mod test {
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}
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#[test]
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fn truncate_cjk() {
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fn test_truncate_ascii() {
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let content = "0123456";
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assert_eq!(
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truncate_str(content, 8_usize),
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content,
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"should match base string as there is extra room"
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);
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assert_eq!(
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truncate_str(content, 7_usize),
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content,
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"should match base string as there is enough room"
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);
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assert_eq!(truncate_str(content, 6_usize), "01234…");
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assert_eq!(truncate_str(content, 5_usize), "0123…");
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assert_eq!(truncate_str(content, 4_usize), "012…");
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assert_eq!(truncate_str(content, 1_usize), "…");
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assert_eq!(truncate_str(content, 0_usize), "");
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}
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#[test]
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fn test_truncate_cjk() {
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let cjk = "施氏食獅史";
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assert_eq!(
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truncate_str(cjk, 11_usize),
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cjk,
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"should match base string as there is enough room"
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"should match base string as there is extra room"
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);
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assert_eq!(
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@ -292,13 +331,13 @@ mod test {
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}
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#[test]
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fn truncate_mixed() {
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fn test_truncate_mixed() {
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let test = "Test (施氏食獅史) Test";
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assert_eq!(
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truncate_str(test, 30_usize),
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test,
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"should match base string as there is enough room"
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"should match base string as there is extra room"
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);
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assert_eq!(
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@ -325,7 +364,7 @@ mod test {
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}
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#[test]
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fn truncate_flags() {
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fn test_truncate_flags() {
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let flag = "🇨🇦";
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assert_eq!(truncate_str(flag, 3_usize), flag);
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assert_eq!(truncate_str(flag, 2_usize), flag);
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@ -368,7 +407,7 @@ mod test {
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/// This might not be the best way to handle it, but this at least tests that it doesn't crash...
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#[test]
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fn truncate_hindi() {
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fn test_truncate_hindi() {
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// cSpell:disable
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let test = "हिन्दी";
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assert_eq!(truncate_str(test, 10_usize), test);
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