[D1X] Use user action and viewing history to inform suggested follows (#4727)

* Use user action and viewing history to inform suggested follows

* Remove dynamic spreads

* Track more info about seen posts

* Add ranking

---------

Co-authored-by: Dan Abramov <dan.abramov@gmail.com>

src/state/userActionHistory.ts

@@ -0,0 +1,71 @@
+import React from 'react'
+
+const LIKE_WINDOW = 100
+const FOLLOW_WINDOW = 100
+const SEEN_WINDOW = 100
+
+export type SeenPost = {
+  uri: string
+  likeCount: number
+  repostCount: number
+  replyCount: number
+  isFollowedBy: boolean
+  feedContext: string | undefined
+}
+
+export type UserActionHistory = {
+  /**
+   * The last 100 post URIs the user has liked
+   */
+  likes: string[]
+  /**
+   * The last 100 DIDs the user has followed
+   */
+  follows: string[]
+  /**
+   * The last 100 post URIs the user has seen from the Discover feed only
+   */
+  seen: SeenPost[]
+}
+
+const userActionHistory: UserActionHistory = {
+  likes: [],
+  follows: [],
+  seen: [],
+}
+
+export function getActionHistory() {
+  return userActionHistory
+}
+
+export function useActionHistorySnapshot() {
+  return React.useState(() => getActionHistory())[0]
+}
+
+export function like(postUris: string[]) {
+  userActionHistory.likes = userActionHistory.likes
+    .concat(postUris)
+    .slice(-LIKE_WINDOW)
+}
+export function unlike(postUris: string[]) {
+  userActionHistory.likes = userActionHistory.likes.filter(
+    uri => !postUris.includes(uri),
+  )
+}
+
+export function follow(dids: string[]) {
+  userActionHistory.follows = userActionHistory.follows
+    .concat(dids)
+    .slice(-FOLLOW_WINDOW)
+}
+export function unfollow(dids: string[]) {
+  userActionHistory.follows = userActionHistory.follows.filter(
+    uri => !dids.includes(uri),
+  )
+}
+
+export function seen(posts: SeenPost[]) {
+  userActionHistory.seen = userActionHistory.seen
+    .concat(posts)
+    .slice(-SEEN_WINDOW)
+}