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/base.rb

@@ -0,0 +1,80 @@
+# frozen_string_literal: true
+
+class Trends::Base
+  include Redisable
+
+  class_attribute :default_options
+
+  attr_reader :options
+
+  # @param [Hash] options
+  # @option options [Integer] :threshold Minimum amount of uses by unique accounts to begin calculating the score
+  # @option options [Integer] :review_threshold Minimum rank (lower = better) before requesting a review
+  # @option options [ActiveSupport::Duration] :max_score_cooldown For this amount of time, the peak score (if bigger than current score) is decayed-from
+  # @option options [ActiveSupport::Duration] :max_score_halflife How quickly a peak score decays
+  def initialize(options = {})
+    @options = self.class.default_options.merge(options)
+  end
+
+  def register(_status)
+    raise NotImplementedError
+  end
+
+  def add(*)
+    raise NotImplementedError
+  end
+
+  def refresh(*)
+    raise NotImplementedError
+  end
+
+  def request_review
+    raise NotImplementedError
+  end
+
+  def get(*)
+    raise NotImplementedError
+  end
+
+  def score(id)
+    redis.zscore("#{key_prefix}:all", id) || 0
+  end
+
+  def rank(id)
+    redis.zrevrank("#{key_prefix}:allowed", id)
+  end
+
+  def currently_trending_ids(allowed, limit)
+    redis.zrevrange(allowed ? "#{key_prefix}:allowed" : "#{key_prefix}:all", 0, limit.positive? ? limit - 1 : limit).map(&:to_i)
+  end
+
+  protected
+
+  def key_prefix
+    raise NotImplementedError
+  end
+
+  def recently_used_ids(at_time = Time.now.utc)
+    redis.smembers(used_key(at_time)).map(&:to_i)
+  end
+
+  def record_used_id(id, at_time = Time.now.utc)
+    redis.sadd(used_key(at_time), id)
+    redis.expire(used_key(at_time), 1.day.seconds)
+  end
+
+  def trim_older_items
+    redis.zremrangebyscore("#{key_prefix}:all", '-inf', '(1')
+    redis.zremrangebyscore("#{key_prefix}:allowed", '-inf', '(1')
+  end
+
+  def score_at_rank(rank)
+    redis.zrevrange("#{key_prefix}:allowed", 0, rank, with_scores: true).last&.last || 0
+  end
+
+  private
+
+  def used_key(at_time)
+    "#{key_prefix}:used:#{at_time.beginning_of_day.to_i}"
+  end
+end