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

app/models/trends/links.rb

@@ -0,0 +1,117 @@
+# frozen_string_literal: true
+
+class Trends::Links < Trends::Base
+  PREFIX = 'trending_links'
+
+  self.default_options = {
+    threshold: 15,
+    review_threshold: 10,
+    max_score_cooldown: 2.days.freeze,
+    max_score_halflife: 8.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.spoiler_text?
+
+    original_status.preview_cards.each do |preview_card|
+      add(preview_card, status.account_id, at_time) if preview_card.appropriate_for_trends?
+    end
+  end
+
+  def add(preview_card, account_id, at_time = Time.now.utc)
+    preview_card.history.add(account_id, at_time)
+    record_used_id(preview_card.id, at_time)
+  end
+
+  def get(allowed, limit)
+    preview_card_ids = currently_trending_ids(allowed, limit)
+    preview_cards = PreviewCard.where(id: preview_card_ids).index_by(&:id)
+    preview_card_ids.map { |id| preview_cards[id] }.compact
+  end
+
+  def refresh(at_time = Time.now.utc)
+    preview_cards = PreviewCard.where(id: (recently_used_ids(at_time) + currently_trending_ids(false, -1)).uniq)
+    calculate_scores(preview_cards, at_time)
+    trim_older_items
+  end
+
+  def request_review
+    preview_cards = PreviewCard.where(id: currently_trending_ids(false, -1))
+
+    preview_cards_requiring_review = preview_cards.filter_map do |preview_card|
+      next unless would_be_trending?(preview_card.id) && !preview_card.trendable? && preview_card.requires_review_notification?
+
+      if preview_card.provider.nil?
+        preview_card.provider = PreviewCardProvider.create(domain: preview_card.domain, requested_review_at: Time.now.utc)
+      else
+        preview_card.provider.touch(:requested_review_at)
+      end
+
+      preview_card
+    end
+
+    return if preview_cards_requiring_review.empty?
+
+    User.staff.includes(:account).find_each do |user|
+      AdminMailer.new_trending_links(user.account, preview_cards_requiring_review).deliver_later! if user.allows_trending_tag_emails?
+    end
+  end
+
+  protected
+
+  def key_prefix
+    PREFIX
+  end
+
+  private
+
+  def calculate_scores(preview_cards, at_time)
+    preview_cards.each do |preview_card|
+      expected  = preview_card.history.get(at_time - 1.day).accounts.to_f
+      expected  = 1.0 if expected.zero?
+      observed  = preview_card.history.get(at_time).accounts.to_f
+      max_time  = preview_card.max_score_at
+      max_score = preview_card.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
+        preview_card.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", preview_card.id)
+        redis.zrem("#{PREFIX}:allowed", preview_card.id)
+      else
+        redis.zadd("#{PREFIX}:all", decaying_score, preview_card.id)
+
+        if preview_card.trendable?
+          redis.zadd("#{PREFIX}:allowed", decaying_score, preview_card.id)
+        else
+          redis.zrem("#{PREFIX}:allowed", preview_card.id)
+        end
+      end
+    end
+  end
+
+  def would_be_trending?(id)
+    score(id) > score_at_rank(options[:review_threshold] - 1)
+  end
+end