class Notification::RemoveOldNotificationJob < ApplicationJob
  queue_as :purgable

  NOTIFICATION_LIMIT = 300
  OLD_NOTIFICATION_THRESHOLD = 1.month

  def perform
    remove_old_notifications
    trim_user_notifications
  end

  private

  def remove_old_notifications
    Notification.where('created_at < ?', OLD_NOTIFICATION_THRESHOLD.ago)
                .delete_all
  end

  def trim_user_notifications
    # Find users with more than NOTIFICATION_LIMIT notifications
    user_ids_exceeding_limit.each do |user_id|
      trim_notifications_for_user(user_id)
    end
  end

  def user_ids_exceeding_limit
    Notification.group(:user_id)
                .having('COUNT(*) > ?', NOTIFICATION_LIMIT)
                .pluck(:user_id)
  end

  def trim_notifications_for_user(user_id)
    # Find the cutoff notification (the 301st when we want to keep top 300)
    # Order by created_at DESC, then id DESC for deterministic ordering
    cutoff = Notification.where(user_id: user_id)
                         .order(created_at: :desc, id: :desc)
                         .offset(NOTIFICATION_LIMIT)
                         .limit(1)
                         .pick(:created_at, :id)

    return unless cutoff

    cutoff_time, cutoff_id = cutoff

    # Delete notifications older than cutoff, or same timestamp but lower/equal ID
    # Since we order by id DESC, higher IDs are kept (come first), lower IDs deleted
    # This avoids race conditions: notifications created after finding the cutoff
    # will have timestamps > cutoff_time and won't be incorrectly deleted
    Notification.where(user_id: user_id)
                .where('created_at < ? OR (created_at = ? AND id <= ?)',
                       cutoff_time, cutoff_time, cutoff_id)
                .delete_all
  end
end