class EmailDigestArticleCollector
  def initialize(user)
    @user = user
  end

  def articles_to_send
    return [] unless should_receive_email?

    articles = if user_has_followings?
                 experience_level_rating = (@user.experience_level || 5)
                 experience_level_rating_min = experience_level_rating - 3.6
                 experience_level_rating_max = experience_level_rating + 3.6

                 @user.followed_articles
                   .select(:title, :description, :path)
                   .published
                   .where("published_at > ?", cutoff_date)
                   .where(email_digest_eligible: true)
                   .where.not(user_id: @user.id)
                   .where("score > ?", 12)
                   .where("experience_level_rating > ? AND experience_level_rating < ?",
                          experience_level_rating_min, experience_level_rating_max)
                   .order(score: :desc)
                   .limit(6)
               else
                 Article.select(:title, :description, :path)
                   .published
                   .where("published_at > ?", cutoff_date)
                   .where(featured: true, email_digest_eligible: true)
                   .where.not(user_id: @user.id)
                   .where("score > ?", 25)
                   .order(score: :desc)
                   .limit(6)
               end

    articles.length < 3 ? [] : articles
  end

  private

  def should_receive_email?
    return true unless last_email_sent_at

    # Has it been at least x days since @user received an email?
    Time.current - last_email_sent_at >= days_until_next_email
  end

  def days_until_next_email
    # Relies on hyperbolic tangent function to model the frequency of the digest email
    max_day = SiteConfig.periodic_email_digest_max
    min_day = SiteConfig.periodic_email_digest_min
    result = max_day * (1 - Math.tanh(2 * open_rate))
    result = result.round

    [result, min_day].max
  end

  def open_rate
    email_count = last_user_emails.count

    # Will stick with 50% open rate if @user has no/not-enough email digest history
    return 0.5 if email_count < 10

    past_opened_emails_count = last_user_emails.count { |msg| msg.opened_at.present? }
    past_opened_emails_count / email_count
  end

  def last_email_sent_at
    last_user_emails.last&.sent_at
  end

  def cutoff_date
    a_few_days_ago = 4.days.ago.utc
    return a_few_days_ago unless last_email_sent_at

    [a_few_days_ago, last_email_sent_at].max
  end

  def user_has_followings?
    @user.following_users_count.positive? || @user.cached_followed_tag_names.any?
  end

  def last_user_emails
    @last_user_emails ||= @user.email_messages.select(:sent_at,
                                                      :opened_at).where(mailer: "DigestMailer#digest_email").limit(10)
  end
end