⚡ Bolt: Optimize hex color parsing and formatting for performance

.jules/bolt.md

@@ -0,0 +1,3 @@
+## 2024-05-18 - Manual parsing avoids allocations
+**Learning:** In Kotlin hot paths, replacing standard library string manipulations (`substring`, `toLong(16)`, `toString(16)`, `padStart`, `uppercase`) with explicit manual character array manipulation avoids intermediate `String` object allocations and measurably improves performance, especially when parsing small fixed-format structures like hex color strings used frequently in theme generation.
+**Action:** Always prefer manual parsing logic that constructs from primitive character arrays or strings directly, instead of using fluent sequence of `String` conversion methods in hot paths like core algorithms.

halogen-core/src/commonMain/kotlin/halogen/ThemeExpander.kt

@@ -112,16 +112,33 @@ public object ThemeExpander {
         require(hex.startsWith("#") && hex.length == 7) {
             "Invalid hex color: \"$hex\". Expected format: #RRGGBB"
         }
-        val rgb = hex.substring(1).toLong(16).toInt()
+        var rgb = 0
+        for (i in 1..6) {
+            val c = hex[i]
+            val digit = when {
+                c in '0'..'9' -> c - '0'
+                c in 'A'..'F' -> c - 'A' + 10
+                c in 'a'..'f' -> c - 'a' + 10
+                else -> throw IllegalArgumentException("Invalid hex color: \"$hex\". Expected format: #RRGGBB")
+            }
+            rgb = (rgb shl 4) or digit
+        }
         return rgb or (0xFF shl 24).toInt()
     }
 
     /**
      * Convert an ARGB integer to a hex color string like "#1A73E8".
      */
     public fun argbToHex(argb: Int): String {
-        val rgb = argb and 0xFFFFFF
-        return "#" + rgb.toString(16).padStart(6, '0').uppercase()
+        val chars = CharArray(7)
+        chars[0] = '#'
+        var rgb = argb and 0xFFFFFF
+        for (i in 6 downTo 1) {
+            val nibble = rgb and 0xF
+            chars[i] = if (nibble < 10) (nibble + '0'.code).toChar() else (nibble - 10 + 'A'.code).toChar()
+            rgb = rgb shr 4
+        }
+        return chars.concatToString()
     }
 
     private fun buildScheme(palette: HalogenPalette, isDark: Boolean): HalogenColorScheme {