⚡ Bolt: [performance improvement] Optimize terminal scrollback search fast-path

.jules/bolt.md

@@ -0,0 +1,3 @@
+## 2024-03-12 - [Optimizing Terminal Scrollback Search]
+**Learning:** [In Rust, optimizing text search hot loops (like terminal scrollback searches) can be achieved by extracting the first-character comparison out of the iterator chain and utilizing an ASCII fast-path. This avoids the overhead of setting up slice iterators, zipping, closures, and invoking heavy `chars_eq_ignore_case` functions for the vast majority of non-matches.]
+**Action:** [When implementing case-insensitive character matching hot loops in Rust, hoist the target character's ASCII lower/uppercase conversions outside the loop, compare the iterated haystack characters against these bounds (`h != first_lower && h != first_upper`), but ONLY short-circuit reject if the haystack character is also ASCII (`h.is_ascii()`), since non-ASCII characters can map to ASCII when case-folded. Use full Unicode case-insensitive matching for the fallback.]

crates/forktty-ui-gtk/src/gtk_app/terminal_search.rs

@@ -75,11 +75,24 @@ fn for_each_char_match_start(
         return;
     }
     let first_needle = needle[0];
+    let first_is_ascii = first_needle.is_ascii();
+    let first_lower = first_needle.to_ascii_lowercase();
+    let first_upper = first_needle.to_ascii_uppercase();
+
     let mut index = 0;
     while index + needle.len() <= haystack.len() {
+        let h = haystack[index];
         // Fast-path: short-circuit the full substring check if the first character
-        // doesn't match, avoiding iterator overhead in the common case.
-        if !chars_eq_ignore_case(haystack[index], first_needle) {
+        // doesn't match, avoiding function call overhead in the common case.
+        // We only short-circuit if both characters are ASCII. Non-ASCII characters
+        // (like the Kelvin sign) can fold to ASCII, so they must fall back to
+        // full Unicode comparison.
+        if first_is_ascii && h.is_ascii() && h != first_lower && h != first_upper {
+            index += 1;
+            continue;
+        }
+
+        if !chars_eq_ignore_case(h, first_needle) {
             index += 1;
             continue;
         }