Track trending tags (#7638)

* Track trending tags

- Half-life of 1 day
- Historical usage in daily buckets (last 7 days stored)
- GET /api/v1/trends

Fix #271

* Add trends to web UI

* Don't render compose form on search route, adjust search results header

* Disqualify tag from trends if it's in disallowed hashtags setting

* Count distinct accounts using tag, ignore silenced accounts

1 files changed, 61 insertions, 0 deletions

diff --git a/app/models/trending_tags.rb b/app/models/trending_tags.rb
new file mode 100644
index 000000000..eedd92644
--- /dev/null
+++ b/app/models/trending_tags.rb
@@ -0,0 +1,61 @@
+# frozen_string_literal: true
+
+class TrendingTags
+  KEY                  = 'trending_tags'
+  HALF_LIFE            = 1.day.to_i
+  MAX_ITEMS            = 500
+  EXPIRE_HISTORY_AFTER = 7.days.seconds
+
+  class << self
+    def record_use!(tag, account, at_time = Time.now.utc)
+      return if disallowed_hashtags.include?(tag.name) || account.silenced?
+
+      increment_vote!(tag.id, at_time)
+      increment_historical_use!(tag.id, at_time)
+      increment_unique_use!(tag.id, account.id, at_time)
+    end
+
+    def get(limit)
+      tag_ids = redis.zrevrange(KEY, 0, limit).map(&:to_i)
+      tags    = Tag.where(id: tag_ids).to_a.map { |tag| [tag.id, tag] }.to_h
+      tag_ids.map { |tag_id| tags[tag_id] }.compact
+    end
+
+    private
+
+    def increment_vote!(tag_id, at_time)
+      redis.zincrby(KEY, (2**((at_time.to_i - epoch) / HALF_LIFE)).to_f, tag_id.to_s)
+      redis.zremrangebyrank(KEY, 0, -MAX_ITEMS) if rand < (2.to_f / MAX_ITEMS)
+    end
+
+    def increment_historical_use!(tag_id, at_time)
+      key = "activity:tags:#{tag_id}:#{at_time.beginning_of_day.to_i}"
+      redis.incrby(key, 1)
+      redis.expire(key, EXPIRE_HISTORY_AFTER)
+    end
+
+    def increment_unique_use!(tag_id, account_id, at_time)
+      key = "activity:tags:#{tag_id}:#{at_time.beginning_of_day.to_i}:accounts"
+      redis.pfadd(key, account_id)
+      redis.expire(key, EXPIRE_HISTORY_AFTER)
+    end
+
+    # The epoch needs to be 2.5 years in the future if the half-life is one day
+    # While dynamic, it will always be the same within one year
+    def epoch
+      @epoch ||= Date.new(Date.current.year + 2.5, 10, 1).to_datetime.to_i
+    end
+
+    def disallowed_hashtags
+      return @disallowed_hashtags if defined?(@disallowed_hashtags)
+
+      @disallowed_hashtags = Setting.disallowed_hashtags.nil? ? [] : Setting.disallowed_hashtags
+      @disallowed_hashtags = @disallowed_hashtags.split(' ') if @disallowed_hashtags.is_a? String
+      @disallowed_hashtags = @disallowed_hashtags.map(&:downcase)
+    end
+
+    def redis
+      Redis.current
+    end
+  end
+end