Re-add follow recommendations API (#7918)

* Re-add follow recommendations API

    GET /api/v1/suggestions

Removed in 8efa081f210d72ed450c39ac4cde0fd84fb3d3fb due to Neo4J
dependency. The algorithm uses triadic closures, takes into account
suspensions, blocks, mutes, domain blocks, excludes locked and moved
accounts, and prefers more recently updated accounts.

* Track interactions with people you don't follow

Replying to, favouriting and reblogging someone you're not following
will make them show up in follow recommendations. The interactions
have different weights:

- Replying is 1
- Favouriting is 10 (decidedly positive interaction, but private)
- Reblogging is 20

Following them, muting or blocking will remove them from the list,
obviously.

* Remove triadic closures, ensure potential friendships are trimmed

1 files changed, 39 insertions, 0 deletions

diff --git a/app/lib/potential_friendship_tracker.rb b/app/lib/potential_friendship_tracker.rb
new file mode 100644
index 000000000..362482669
--- /dev/null
+++ b/app/lib/potential_friendship_tracker.rb
@@ -0,0 +1,39 @@
+# frozen_string_literal: true
+
+class PotentialFriendshipTracker
+  EXPIRE_AFTER = 90.days.seconds
+  MAX_ITEMS    = 80
+
+  WEIGHTS = {
+    reply: 1,
+    favourite: 10,
+    reblog: 20,
+  }.freeze
+
+  class << self
+    def record(account_id, target_account_id, action)
+      key    = "interactions:#{account_id}"
+      weight = WEIGHTS[action]
+
+      redis.zincrby(key, weight, target_account_id)
+      redis.zremrangebyrank(key, 0, -MAX_ITEMS)
+      redis.expire(key, EXPIRE_AFTER)
+    end
+
+    def remove(account_id, target_account_id)
+      redis.zrem("interactions:#{account_id}", target_account_id)
+    end
+
+    def get(account_id, limit: 20, offset: 0)
+      account_ids = redis.zrevrange("interactions:#{account_id}", offset, limit)
+      return [] if account_ids.empty?
+      Account.searchable.where(id: account_ids)
+    end
+
+    private
+
+    def redis
+      Redis.current
+    end
+  end
+end