Add trending links (#16917)

* Add trending links

* Add overriding specific links trendability

* Add link type to preview cards and only trend articles

Change trends review notifications from being sent every 5 minutes to being sent every 2 hours

Change threshold from 5 unique accounts to 15 unique accounts

* Fix tests

1 files changed, 111 insertions, 0 deletions

diff --git a/app/models/trends/tags.rb b/app/models/trends/tags.rb
new file mode 100644
index 000000000..13e0ab56b
--- /dev/null
+++ b/app/models/trends/tags.rb
@@ -0,0 +1,111 @@
+# frozen_string_literal: true
+
+class Trends::Tags < Trends::Base
+  PREFIX = 'trending_tags'
+
+  self.default_options = {
+    threshold: 15,
+    review_threshold: 10,
+    max_score_cooldown: 2.days.freeze,
+    max_score_halflife: 4.hours.freeze,
+  }
+
+  def register(status, at_time = Time.now.utc)
+    original_status = status.reblog? ? status.reblog : status
+
+    return unless original_status.public_visibility? && status.public_visibility? &&
+                  !original_status.account.silenced? && !status.account.silenced?
+
+    original_status.tags.each do |tag|
+      add(tag, status.account_id, at_time) if tag.usable?
+    end
+  end
+
+  def add(tag, account_id, at_time = Time.now.utc)
+    tag.history.add(account_id, at_time)
+    record_used_id(tag.id, at_time)
+  end
+
+  def refresh(at_time = Time.now.utc)
+    tags = Tag.where(id: (recently_used_ids(at_time) + currently_trending_ids(false, -1)).uniq)
+    calculate_scores(tags, at_time)
+    trim_older_items
+  end
+
+  def get(allowed, limit)
+    tag_ids = currently_trending_ids(allowed, limit)
+    tags = Tag.where(id: tag_ids).index_by(&:id)
+    tag_ids.map { |id| tags[id] }.compact
+  end
+
+  def request_review
+    tags = Tag.where(id: currently_trending_ids(false, -1))
+
+    tags_requiring_review = tags.filter_map do |tag|
+      next unless would_be_trending?(tag.id) && !tag.trendable? && tag.requires_review_notification?
+
+      tag.touch(:requested_review_at)
+      tag
+    end
+
+    return if tags_requiring_review.empty?
+
+    User.staff.includes(:account).find_each do |user|
+      AdminMailer.new_trending_tags(user.account, tags_requiring_review).deliver_later! if user.allows_trending_tag_emails?
+    end
+  end
+
+  protected
+
+  def key_prefix
+    PREFIX
+  end
+
+  private
+
+  def calculate_scores(tags, at_time)
+    tags.each do |tag|
+      expected  = tag.history.get(at_time - 1.day).accounts.to_f
+      expected  = 1.0 if expected.zero?
+      observed  = tag.history.get(at_time).accounts.to_f
+      max_time  = tag.max_score_at
+      max_score = tag.max_score
+      max_score = 0 if max_time.nil? || max_time < (at_time - options[:max_score_cooldown])
+
+      score = begin
+        if expected > observed || observed < options[:threshold]
+          0
+        else
+          ((observed - expected)**2) / expected
+        end
+      end
+
+      if score > max_score
+        max_score = score
+        max_time  = at_time
+
+        # Not interested in triggering any callbacks for this
+        tag.update_columns(max_score: max_score, max_score_at: max_time)
+      end
+
+      decaying_score = max_score * (0.5**((at_time.to_f - max_time.to_f) / options[:max_score_halflife].to_f))
+
+      if decaying_score.zero?
+        redis.zrem("#{PREFIX}:all", tag.id)
+        redis.zrem("#{PREFIX}:allowed", tag.id)
+      else
+        redis.zadd("#{PREFIX}:all", decaying_score, tag.id)
+
+        if tag.trendable?
+          redis.zadd("#{PREFIX}:allowed", decaying_score, tag.id)
+        else
+          redis.zrem("#{PREFIX}:allowed", tag.id)
+        end
+      end
+    end
+  end
+
+  def would_be_trending?(id)
+    score(id) > score_at_rank(options[:review_threshold] - 1)
+  end
+end