⚡ Bolt: Optimize string hotpaths away from regular expressions in HintExtractor

.jules/bolt.md

@@ -0,0 +1,3 @@
+## 2024-11-20 - [Optimizing Regex in Kotlin Hot Paths]
+**Learning:** In Kotlin multiplatform hot paths, relying heavily on sequential `Regex` operations (e.g. prefix stripping followed by substring processing followed by case conversion) generates immense overhead due to state machine creation, tracking, and backtracking.
+**Action:** Always replace heavy regex parsing chains in hot paths with raw manual string traversal via bounded indices and `StringBuilder`. Precalculate maximum string capacities and reduce object allocations to measurably increase execution speed.

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

@@ -10,41 +10,84 @@ 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
 
+        val trimmed = key.trim()
+        var startIdx = 0
+
         // Strip common prefixes
-        var cleaned = PREFIX_PATTERN.replace(key.trim(), "")
+        if (trimmed.startsWith("/r/")) startIdx = 3
+        else if (trimmed.startsWith("/category/")) startIdx = 10
+        else if (trimmed.startsWith("/topic/")) startIdx = 7
+        else if (trimmed.startsWith("/")) startIdx = 1
+        else if (trimmed.startsWith("#")) startIdx = 1
+
+        // Trim leading slashes if any
+        while (startIdx < trimmed.length && trimmed[startIdx] == '/') {
+            startIdx++
+        }
+
+        // Trim trailing slashes
+        var endIdx = trimmed.length - 1
+        while (endIdx >= startIdx && trimmed[endIdx] == '/') {
+            endIdx--
+        }
 
-        // Remove leading/trailing slashes
-        cleaned = cleaned.trim('/')
+        if (startIdx > endIdx) return null
 
         // Take the last meaningful segment if it looks like a path
-        if ('/' in cleaned) {
-            cleaned = cleaned.substringAfterLast('/')
+        val lastSlash = trimmed.lastIndexOf('/', endIdx)
+        if (lastSlash >= startIdx) {
+            startIdx = lastSlash + 1
         }
 
-        // Split camelCase
-        cleaned = CAMEL_SPLIT.replace(cleaned, " ")
+        // Process characters for camelCase, snake_case, kebab-case, and normalize whitespace
+        val sb = StringBuilder(endIdx - startIdx + 10)
+        var prevChar = ' '
 
-        // Split snake_case and kebab-case
-        cleaned = cleaned.replace('_', ' ').replace('-', ' ')
+        for (i in startIdx..endIdx) {
+            val c = trimmed[i]
 
-        // Normalize whitespace
-        cleaned = cleaned.trim().replace(WHITESPACE_PATTERN, " ")
+            val isSeparator = c == '_' || c == '-' || c.isWhitespace()
 
-        if (cleaned.isBlank()) return null
+            if (isSeparator) {
+                if (prevChar != ' ') {
+                    sb.append(' ')
+                    prevChar = ' '
+                }
+            } else {
+                // Split camelCase
+                if (prevChar in 'a'..'z' && c in 'A'..'Z') {
+                    sb.append(' ')
+                }
+                sb.append(c)
+                prevChar = c
+            }
+        }
+
+        val cleaned = sb.toString().trim()
+        if (cleaned.isEmpty()) 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 hexChars = 0
+        var digitChars = 0
+        var totalChars = 0
+
+        for (i in 0 until cleaned.length) {
+            val c = cleaned[i]
+            if (c == ' ') continue
+            totalChars++
+            if (c in '0'..'9') {
+                digitChars++
+                hexChars++
+            } else if (c in 'a'..'f' || c in 'A'..'F') {
+                hexChars++
+            }
+        }
+
+        if (totalChars >= 8 && hexChars == totalChars) return null
+        if (totalChars > 0 && digitChars == totalChars) return null
 
         return cleaned.lowercase()
     }