⚡ Bolt: Optimize HintExtractor regex parsing

.jules/bolt.md

@@ -0,0 +1,3 @@
+## 2024-06-25 - Replace regex with character iteration in hot path
+**Learning:** Replacing lookaround and formatting regexes with a single manual character iteration loop using a `StringBuilder` significantly reduces compilation overhead, object allocation, and backtracking delays in hot paths for string processing.
+**Action:** Always consider converting complex, heavily-used `Regex` replacements into manual string parsing functions when performance is a priority and the processing logic involves straightforward conditions like prefix checking, whitespace handling, and character cases.

halogen-engine/src/commonMain/kotlin/halogen/engine/HintExtractor.kt

@@ -10,42 +10,81 @@ package halogen.engine
  */
 internal object HintExtractor {
 
-    private val PREFIX_PATTERN = Regex("""^(?:/r/|/category/|/topic/|/|#)""")
-    private val CAMEL_SPLIT = Regex("""(?<=[a-z])(?=[A-Z])""")
-    private val ID_PATTERN = Regex("""^[0-9a-f]{8,}$""", RegexOption.IGNORE_CASE)
-    private val NUMERIC_ONLY = Regex("""^\d+$""")
-    private val WHITESPACE_PATTERN = Regex("""\s+""")
-
     fun extract(key: String): String? {
         if (key.isBlank()) return null
 
         // Strip common prefixes
-        var cleaned = PREFIX_PATTERN.replace(key.trim(), "")
+        var cleaned = key.trim()
+        if (cleaned.startsWith("/r/")) {
+            cleaned = cleaned.substring(3)
+        } else if (cleaned.startsWith("/category/")) {
+            cleaned = cleaned.substring(10)
+        } else if (cleaned.startsWith("/topic/")) {
+            cleaned = cleaned.substring(7)
+        } else if (cleaned.startsWith("/")) {
+            cleaned = cleaned.substring(1)
+        } else if (cleaned.startsWith("#")) {
+            cleaned = cleaned.substring(1)
+        }
 
         // Remove leading/trailing slashes
-        cleaned = cleaned.trim('/')
+        var start = 0
+        var end = cleaned.length - 1
+        while (start <= end && cleaned[start] == '/') start++
+        while (end >= start && cleaned[end] == '/') end--
+        if (start > end) return null
+        cleaned = cleaned.substring(start, end + 1)
 
         // Take the last meaningful segment if it looks like a path
-        if ('/' in cleaned) {
-            cleaned = cleaned.substringAfterLast('/')
+        val lastSlashIndex = cleaned.lastIndexOf('/')
+        if (lastSlashIndex >= 0) {
+            cleaned = cleaned.substring(lastSlashIndex + 1)
         }
 
-        // Split camelCase
-        cleaned = CAMEL_SPLIT.replace(cleaned, " ")
+        val sb = StringBuilder(cleaned.length + 4) // Some extra space for added spaces
+        var lastWasSpace = true
 
-        // Split snake_case and kebab-case
-        cleaned = cleaned.replace('_', ' ').replace('-', ' ')
+        for (i in cleaned.indices) {
+            val c = cleaned[i]
+            if (c == '_' || c == '-' || c.isWhitespace()) {
+                if (!lastWasSpace) {
+                    sb.append(' ')
+                    lastWasSpace = true
+                }
+            } else {
+                if (i > 0 && c.isUpperCase() && cleaned[i - 1].isLowerCase()) {
+                    if (!lastWasSpace) {
+                        sb.append(' ')
+                    }
+                }
+                sb.append(c.lowercaseChar())
+                lastWasSpace = false
+            }
+        }
 
-        // Normalize whitespace
-        cleaned = cleaned.trim().replace(WHITESPACE_PATTERN, " ")
+        var result = sb.toString()
+        if (result.endsWith(" ")) {
+            result = result.substring(0, result.length - 1)
+        }
 
-        if (cleaned.isBlank()) return null
+        if (result.isBlank()) return null
 
         // Reject things that look like IDs
-        val noSpaces = cleaned.replace(" ", "")
-        if (ID_PATTERN.matches(noSpaces)) return null
-        if (NUMERIC_ONLY.matches(noSpaces)) return null
+        var noSpacesLen = 0
+        var allNumeric = true
+        var allHex = true
+        for (i in result.indices) {
+            val c = result[i]
+            if (c != ' ') {
+                noSpacesLen++
+                if (c !in '0'..'9') allNumeric = false
+                if (c !in '0'..'9' && c !in 'a'..'f') allHex = false
+            }
+        }
+
+        if (noSpacesLen > 0 && allNumeric) return null
+        if (noSpacesLen >= 8 && allHex) return null
 
-        return cleaned.lowercase()
+        return result
     }
 }