⚡ Bolt: Optimize JSON Decoding in WasmJs LocalStorageThemeCache

.jules/bolt.md

@@ -0,0 +1,3 @@
+## 2024-04-22 - WasmJs JSON Decoding Overhead
+**Learning:** JSON decoding in Kotlin Multiplatform for WasmJs targets carries significant overhead when accessing localStorage. Re-parsing the same JSON structure repeatedly (e.g., manifest keys) negatively impacts performance on Web.
+**Action:** Use an in-memory cache initialized when needed (or standard memory properties) to keep a parsed copy of frequently accessed data, thus minimizing redundant `localStorage` operations and `kotlinx.serialization` parsing penalties on WasmJs targets.

halogen-engine/src/wasmJsMain/kotlin/halogen/engine/LocalStorageThemeCache.kt

@@ -49,18 +49,23 @@ public class LocalStorageThemeCache(
     private val json = Json { ignoreUnknownKeys = true }
     private val manifestKey = "${prefix}__keys__"
 
+    // Memory cache for manifest to avoid JSON decoding on every operation
+    private var manifestCache: MutableSet<String>? = null
+
     // ── Manifest helpers ────────────────────────────────────────────────
 
     private fun readManifest(): MutableSet<String> {
-        val raw = jsGetItem(manifestKey.toJsString())?.toString() ?: return mutableSetOf()
+        manifestCache?.let { return it }
+        val raw = jsGetItem(manifestKey.toJsString())?.toString() ?: return mutableSetOf<String>().also { manifestCache = it }
         return try {
-            json.decodeFromString<Set<String>>(raw).toMutableSet()
+            json.decodeFromString<Set<String>>(raw).toMutableSet().also { manifestCache = it }
         } catch (_: Exception) {
-            mutableSetOf()
+            mutableSetOf<String>().also { manifestCache = it }
         }
     }
 
     private fun writeManifest(keys: Set<String>) {
+        manifestCache = keys.toMutableSet()
         val encoded = json.encodeToString(keys)
         jsSetItem(manifestKey.toJsString(), encoded.toJsString())
     }