⚡ Bolt: Cache WasmJs localStorage manifest in memory

.jules/bolt.md

@@ -0,0 +1,3 @@
+## 2024-05-24 - WasmJs localStorage JSON parsing overhead
+**Learning:** JSON decoding in Kotlin Multiplatform for WasmJs targets carries significant overhead. Parsing the same JSON manifest from localStorage repeatedly causes a measurable performance bottleneck.
+**Action:** Use an in-memory cache initialized via `by lazy` to avoid redundant localStorage parsing for frequently accessed metadata like manifests.

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

@@ -49,17 +49,21 @@ public class LocalStorageThemeCache(
     private val json = Json { ignoreUnknownKeys = true }
     private val manifestKey = "${prefix}__keys__"
 
-    // ── Manifest helpers ────────────────────────────────────────────────
-
-    private fun readManifest(): MutableSet<String> {
-        val raw = jsGetItem(manifestKey.toJsString())?.toString() ?: return mutableSetOf()
-        return try {
+    private val inMemoryManifest: MutableSet<String> by lazy {
+        val raw = jsGetItem(manifestKey.toJsString())?.toString() ?: return@lazy mutableSetOf()
+        try {
             json.decodeFromString<Set<String>>(raw).toMutableSet()
         } catch (_: Exception) {
             mutableSetOf()
         }
     }
 
+    // ── Manifest helpers ────────────────────────────────────────────────
+
+    private fun readManifest(): MutableSet<String> {
+        return inMemoryManifest
+    }
+
     private fun writeManifest(keys: Set<String>) {
         val encoded = json.encodeToString(keys)
         jsSetItem(manifestKey.toJsString(), encoded.toJsString())
@@ -154,6 +158,7 @@ public class LocalStorageThemeCache(
         for (key in manifest) {
             removeEntry("$prefix$key")
         }
+        manifest.clear()
         jsRemoveItem(manifestKey.toJsString())
         _changes.tryEmit(CacheEvent.Cleared)
     }