Add a hotness thread sort (#6649)

* Add a hotness thread sort

* Bump @atproto/api

src/state/queries/post-thread.ts

@@ -237,7 +237,11 @@ export function sortThread(
         }
       }
 
-      if (opts.sort === 'oldest') {
+      if (opts.sort === 'hotness') {
+        const aHotness = getHotness(a.post)
+        const bHotness = getHotness(b.post)
+        return bHotness - aHotness
+      } else if (opts.sort === 'oldest') {
         return a.post.indexedAt.localeCompare(b.post.indexedAt)
       } else if (opts.sort === 'newest') {
         return b.post.indexedAt.localeCompare(a.post.indexedAt)
@@ -269,6 +273,21 @@ export function sortThread(
 // internal methods
 // =
 
+// Inspired by https://join-lemmy.org/docs/contributors/07-ranking-algo.html
+// We want to give recent comments a real chance (and not bury them deep below the fold)
+// while also surfacing well-liked comments from the past. In the future, we can explore
+// something more sophisticated, but we don't have much data on the client right now.
+function getHotness(post: AppBskyFeedDefs.PostView) {
+  const hoursAgo =
+    (new Date().getTime() - new Date(post.indexedAt).getTime()) /
+    (1000 * 60 * 60)
+  const likeCount = post.likeCount ?? 0
+  const likeOrder = Math.log(3 + likeCount)
+  const timePenaltyExponent = 1.5 + 1.5 / (1 + Math.log(1 + likeCount))
+  const timePenalty = Math.pow(hoursAgo + 2, timePenaltyExponent)
+  return likeOrder / timePenalty
+}
+
 function responseToThreadNodes(
   node: ThreadViewNode,
   depth = 0,