⚡ Bolt: Add in-memory manifest cache to LocalStorageThemeCache

.jules/bolt.md

@@ -0,0 +1,3 @@
+## 2024-05-18 - Avoid redundant JSON parsing in wasmJs LocalStorage caching
+**Learning:** In the `wasmJs` target, the `LocalStorageThemeCache` previously parsed the JSON manifest array from `localStorage` on every operation (e.g. `size()`, `keys()`, `put()`, `evict()`), which is very expensive when done repeatedly. Caching the manifest array in memory significantly reduces overhead while still maintaining correctness for concurrent UI operations within a single session.
+**Action:** Always consider the cost of JSON parsing in browser environments, particularly for frequently accessed metadata. Add a lightweight in-memory caching layer on top of `localStorage` to memoize the deserialized state when it is safe to do so within the bounds of a single user session.

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

@@ -48,21 +48,27 @@ public class LocalStorageThemeCache(
     private val _changes = MutableSharedFlow<CacheEvent>(extraBufferCapacity = 64)
     private val json = Json { ignoreUnknownKeys = true }
     private val manifestKey = "${prefix}__keys__"
+    private var manifestCache: MutableSet<String>? = null
 
     // ── Manifest helpers ────────────────────────────────────────────────
 
     private fun readManifest(): MutableSet<String> {
+        manifestCache?.let { return it.toMutableSet() }
+
         val raw = jsGetItem(manifestKey.toJsString())?.toString() ?: return mutableSetOf()
-        return try {
+        val parsed = try {
             json.decodeFromString<Set<String>>(raw).toMutableSet()
         } catch (_: Exception) {
             mutableSetOf()
         }
+        manifestCache = parsed.toMutableSet()
+        return parsed
     }
 
     private fun writeManifest(keys: Set<String>) {
         val encoded = json.encodeToString(keys)
         jsSetItem(manifestKey.toJsString(), encoded.toJsString())
+        manifestCache = keys.toMutableSet()
     }
 
     // ── Entry helpers ───────────────────────────────────────────────────
@@ -155,6 +161,7 @@ public class LocalStorageThemeCache(
             removeEntry("$prefix$key")
         }
         jsRemoveItem(manifestKey.toJsString())
+        manifestCache = null
         _changes.tryEmit(CacheEvent.Cleared)
     }