From 88bcb80f75fabc0a3c84bfc1f46de6c25e0a303d Mon Sep 17 00:00:00 2001
From: "google-labs-jules[bot]"
 <161369871+google-labs-jules[bot]@users.noreply.github.com>
Date: Sun, 19 Apr 2026 02:31:40 +0000
Subject: [PATCH] =?UTF-8?q?=E2=9A=A1=20Bolt:=20Add=20in-memory=20manifest?=
 =?UTF-8?q?=20cache=20to=20LocalStorageThemeCache?=
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

Introduced an in-memory `manifestCache` to avoid repeated and expensive
JSON parsing from localStorage during cache operations.

Co-authored-by: himattm <6266621+himattm@users.noreply.github.com>
---
 .jules/bolt.md                                           | 3 +++
 .../kotlin/halogen/engine/LocalStorageThemeCache.kt      | 9 ++++++++-
 2 files changed, 11 insertions(+), 1 deletion(-)
 create mode 100644 .jules/bolt.md

diff --git a/.jules/bolt.md b/.jules/bolt.md
new file mode 100644
index 0000000..69ba52b
--- /dev/null
+++ b/.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.
diff --git a/halogen-engine/src/wasmJsMain/kotlin/halogen/engine/LocalStorageThemeCache.kt b/halogen-engine/src/wasmJsMain/kotlin/halogen/engine/LocalStorageThemeCache.kt
index aba1f32..0928c74 100644
--- a/halogen-engine/src/wasmJsMain/kotlin/halogen/engine/LocalStorageThemeCache.kt
+++ b/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)
     }