Prefetch a smaller feed and bound its local cache

1 files changed, 53 insertions, 0 deletions

diff --git a/src/feed-cache.js b/src/feed-cache.js
new file mode 100644
index 0000000..ea562f7
--- /dev/null
+++ b/src/feed-cache.js
@@ -0,0 +1,53 @@
+// Disposable public feed cache. Budget is conservative UTF-16 storage bytes.
+export const FEED_CACHE_POSTS = 30;
+const MAX_FEEDS = 4, MAX_UNITS = 256_000, MAX_AGE = 7 * 86400_000;
+export function createFeedCache(storage, prefix, now = Date.now) {
+  const keyOf = (id) => id === 'following' ? prefix : prefix + ':' + id;
+  const valid = (e) => e?.kind === 1 && typeof e.id === 'string' && typeof e.pubkey === 'string'
+    && typeof e.content === 'string' && Number.isFinite(e.created_at) && Array.isArray(e.tags);
+  function compact(notes) {
+    const kept = []; let size = 0;
+    for (const e of (Array.isArray(notes) ? notes : []).filter(valid).sort((a, b) => b.created_at - a.created_at)) {
+      const cost = JSON.stringify(e).length;
+      if (size + cost > MAX_UNITS - 1000) continue;
+      kept.push(e); size += cost;
+      if (kept.length === FEED_CACHE_POSTS) break;
+    }
+    return kept;
+  }
+  function sweep(preferred) {
+    const entries = [];
+    for (let i = 0; i < storage.length; i++) {
+      const key = storage.key(i);
+      if (key !== prefix && !key?.startsWith(prefix + ':')) continue;
+      try {
+        const text = storage.getItem(key);
+        const raw = JSON.parse(text);
+        const entry = Array.isArray(raw) ? { at: now(), notes: compact(raw) } : raw;
+        if (!entry || !Number.isFinite(entry.at) || now() - entry.at > MAX_AGE) entries.push({ key });
+        else entries.push({ key, text, value: { at: entry.at, notes: compact(entry.notes) } });
+      } catch { entries.push({ key }); }
+    }
+    entries.sort((a, b) => (b.key === preferred) - (a.key === preferred) || (b.value?.at || 0) - (a.value?.at || 0));
+    let units = 0, count = 0;
+    for (const e of entries) {
+      const json = e.value && JSON.stringify(e.value);
+      if (!json || count >= MAX_FEEDS || units + json.length > MAX_UNITS) storage.removeItem(e.key);
+      else { if (e.text !== json) storage.setItem(e.key, json); units += json.length; count++; }
+    }
+  }
+  return {
+    read(id) {
+      try {
+        sweep(keyOf(id));
+        const value = JSON.parse(storage.getItem(keyOf(id)) || 'null');
+        if (!value) return [];
+        value.at = now(); storage.setItem(keyOf(id), JSON.stringify(value));
+        return value.notes;
+      } catch { return []; }
+    },
+    save(id, notes) {
+      try { storage.setItem(keyOf(id), JSON.stringify({ at: now(), notes: compact(notes) })); sweep(keyOf(id)); } catch {}
+    },
+  };
+}