1 files changed, 236 insertions, 0 deletions
diff --git a/src/lib/api/feed/merge.ts b/src/lib/api/feed/merge.ts
new file mode 100644
index 000000000..51a619589
--- /dev/null
+++ b/src/lib/api/feed/merge.ts
@@ -0,0 +1,236 @@
+import {AppBskyFeedDefs, AppBskyFeedGetTimeline} from '@atproto/api'
+import shuffle from 'lodash.shuffle'
+import {RootStoreModel} from 'state/index'
+import {timeout} from 'lib/async/timeout'
+import {bundleAsync} from 'lib/async/bundle'
+import {feedUriToHref} from 'lib/strings/url-helpers'
+import {FeedAPI, FeedAPIResponse, FeedSourceInfo} from './types'
+
+const REQUEST_WAIT_MS = 500 // 500ms
+const POST_AGE_CUTOFF = 60e3 * 60 * 24 // 24hours
+
+export class MergeFeedAPI implements FeedAPI {
+  following: MergeFeedSource_Following
+  customFeeds: MergeFeedSource_Custom[] = []
+  feedCursor = 0
+  itemCursor = 0
+  sampleCursor = 0
+
+  constructor(public rootStore: RootStoreModel) {
+    this.following = new MergeFeedSource_Following(this.rootStore)
+  }
+
+  reset() {
+    this.following = new MergeFeedSource_Following(this.rootStore)
+    this.customFeeds = [] // just empty the array, they will be captured in _fetchNext()
+    this.feedCursor = 0
+    this.itemCursor = 0
+    this.sampleCursor = 0
+  }
+
+  async peekLatest(): Promise<AppBskyFeedDefs.FeedViewPost> {
+    const res = await this.rootStore.agent.getTimeline({
+      limit: 1,
+    })
+    return res.data.feed[0]
+  }
+
+  async fetchNext({limit}: {limit: number}): Promise<FeedAPIResponse> {
+    // we capture here to ensure the data has loaded
+    this._captureFeedsIfNeeded()
+
+    const promises = []
+
+    // always keep following topped up
+    if (this.following.numReady < limit) {
+      promises.push(this.following.fetchNext(30))
+    }
+
+    // pick the next feeds to sample from
+    const feeds = this.customFeeds.slice(this.feedCursor, this.feedCursor + 3)
+    this.feedCursor += 3
+    if (this.feedCursor > this.customFeeds.length) {
+      this.feedCursor = 0
+    }
+
+    // top up the feeds
+    for (const feed of feeds) {
+      if (feed.numReady < 5) {
+        promises.push(feed.fetchNext(10))
+      }
+    }
+
+    // wait for requests (all capped at a fixed timeout)
+    await Promise.all(promises)
+
+    // assemble a response by sampling from feeds with content
+    const posts: AppBskyFeedDefs.FeedViewPost[] = []
+    while (posts.length < limit) {
+      let slice = this.sampleItem()
+      if (slice[0]) {
+        posts.push(slice[0])
+      } else {
+        break
+      }
+    }
+
+    return {
+      cursor: posts.length ? 'fake' : undefined,
+      feed: posts,
+    }
+  }
+
+  sampleItem() {
+    const i = this.itemCursor++
+    const candidateFeeds = this.customFeeds.filter(f => f.numReady > 0)
+    const canSample = candidateFeeds.length > 0
+    const hasFollows = this.following.numReady > 0
+
+    // this condition establishes the frequency that custom feeds are woven into follows
+    const shouldSample =
+      i >= 15 && candidateFeeds.length >= 2 && (i % 4 === 0 || i % 5 === 0)
+
+    if (!canSample && !hasFollows) {
+      // no data available
+      return []
+    }
+    if (shouldSample || !hasFollows) {
+      // time to sample, or the user isnt following anybody
+      return candidateFeeds[this.sampleCursor++ % candidateFeeds.length].take(1)
+    }
+    // not time to sample
+    return this.following.take(1)
+  }
+
+  _captureFeedsIfNeeded() {
+    if (!this.rootStore.preferences.homeFeedMergeFeedEnabled) {
+      return
+    }
+    if (this.customFeeds.length === 0) {
+      this.customFeeds = shuffle(
+        this.rootStore.me.savedFeeds.all.map(
+          feed =>
+            new MergeFeedSource_Custom(
+              this.rootStore,
+              feed.uri,
+              feed.displayName,
+            ),
+        ),
+      )
+    }
+  }
+}
+
+class MergeFeedSource {
+  sourceInfo: FeedSourceInfo | undefined
+  cursor: string | undefined = undefined
+  queue: AppBskyFeedDefs.FeedViewPost[] = []
+  hasMore = true
+
+  constructor(public rootStore: RootStoreModel) {}
+
+  get numReady() {
+    return this.queue.length
+  }
+
+  get needsFetch() {
+    return this.hasMore && this.queue.length === 0
+  }
+
+  reset() {
+    this.cursor = undefined
+    this.queue = []
+    this.hasMore = true
+  }
+
+  take(n: number): AppBskyFeedDefs.FeedViewPost[] {
+    return this.queue.splice(0, n)
+  }
+
+  async fetchNext(n: number) {
+    await Promise.race([this._fetchNextInner(n), timeout(REQUEST_WAIT_MS)])
+  }
+
+  _fetchNextInner = bundleAsync(async (n: number) => {
+    const res = await this._getFeed(this.cursor, n)
+    if (res.success) {
+      this.cursor = res.data.cursor
+      if (res.data.feed.length) {
+        this.queue = this.queue.concat(res.data.feed)
+      } else {
+        this.hasMore = false
+      }
+    } else {
+      this.hasMore = false
+    }
+  })
+
+  protected _getFeed(
+    _cursor: string | undefined,
+    _limit: number,
+  ): Promise<AppBskyFeedGetTimeline.Response> {
+    throw new Error('Must be overridden')
+  }
+}
+
+class MergeFeedSource_Following extends MergeFeedSource {
+  async fetchNext(n: number) {
+    return this._fetchNextInner(n)
+  }
+
+  protected async _getFeed(
+    cursor: string | undefined,
+    limit: number,
+  ): Promise<AppBskyFeedGetTimeline.Response> {
+    const res = await this.rootStore.agent.getTimeline({cursor, limit})
+    // filter out mutes pre-emptively to ensure better mixing
+    res.data.feed = res.data.feed.filter(
+      post => !post.post.author.viewer?.muted,
+    )
+    return res
+  }
+}
+
+class MergeFeedSource_Custom extends MergeFeedSource {
+  minDate: Date
+
+  constructor(
+    public rootStore: RootStoreModel,
+    public feedUri: string,
+    public feedDisplayName: string,
+  ) {
+    super(rootStore)
+    this.sourceInfo = {
+      displayName: feedDisplayName,
+      uri: feedUriToHref(feedUri),
+    }
+    this.minDate = new Date(Date.now() - POST_AGE_CUTOFF)
+  }
+
+  protected async _getFeed(
+    cursor: string | undefined,
+    limit: number,
+  ): Promise<AppBskyFeedGetTimeline.Response> {
+    const res = await this.rootStore.agent.app.bsky.feed.getFeed({
+      cursor,
+      limit,
+      feed: this.feedUri,
+    })
+    // NOTE
+    // some custom feeds fail to enforce the pagination limit
+    // so we manually truncate here
+    // -prf
+    if (limit && res.data.feed.length > limit) {
+      res.data.feed = res.data.feed.slice(0, limit)
+    }
+    // filter out older posts
+    res.data.feed = res.data.feed.filter(
+      post => new Date(post.post.indexedAt) > this.minDate,
+    )
+    // attach source info
+    for (const post of res.data.feed) {
+      post.__source = this.sourceInfo
+    }
+    return res
+  }
+}