From f567acb58fdefd489052eb3fb721c9d5e58f07a3 Mon Sep 17 00:00:00 2001
From: Alex Gleason <alex@alexgleason.me>
Date: Sat, 6 May 2023 20:30:11 -0500
Subject: [PATCH] Cache rich media cards for 12 hours

---
 src/deps.ts      |  1 +
 src/transmute.ts | 19 +++++++++++++++++--
 2 files changed, 18 insertions(+), 2 deletions(-)

diff --git a/src/deps.ts b/src/deps.ts
index 75fe3d8..4cb15bf 100644
--- a/src/deps.ts
+++ b/src/deps.ts
@@ -30,3 +30,4 @@ import 'npm:linkify-plugin-hashtag@^4.1.0';
 // @deno-types="npm:@types/mime@3.0.0"
 export { default as mime } from 'npm:mime@^3.0.0';
 export { unfurl } from 'npm:unfurl.js@^6.3.1';
+export { default as TTLCache } from 'npm:@isaacs/ttlcache@^1.4.0';
diff --git a/src/transmute.ts b/src/transmute.ts
index 08f6569..5da97b3 100644
--- a/src/transmute.ts
+++ b/src/transmute.ts
@@ -1,4 +1,4 @@
-import { findReplyTag, lodash, nip19, unfurl, z } from '@/deps.ts';
+import { findReplyTag, lodash, nip19, TTLCache, unfurl, z } from '@/deps.ts';
 import { type Event } from '@/event.ts';
 import { type MetaContent, parseMetaContent } from '@/schema.ts';
 
@@ -107,7 +107,7 @@ async function toStatus(event: Event<1>) {
   return {
     id: event.id,
     account,
-    card: firstUrl ? await unfurlCard(firstUrl) : null,
+    card: firstUrl ? await unfurlCardCached(firstUrl) : null,
     content: html,
     created_at: new Date(event.created_at * 1000).toISOString(),
     in_reply_to_id: replyTag ? replyTag[1] : null,
@@ -196,4 +196,19 @@ async function unfurlCard(url: string): Promise<PreviewCard | null> {
   }
 }
 
+const TWELVE_HOURS = 12 * 60 * 60 * 1000;
+
+const previewCardCache = new TTLCache({ ttl: TWELVE_HOURS, max: 500 });
+
+/** Unfurl card from cache if available, otherwise fetch it. */
+async function unfurlCardCached(url: string): Promise<PreviewCard | null> {
+  const cached = previewCardCache.get<PreviewCard | null>(url);
+  if (cached !== undefined) return cached;
+
+  const card = await unfurlCard(url);
+  previewCardCache.set(url, card);
+
+  return card;
+}
+
 export { toAccount, toStatus };