discourse/app/services/post_alerter.rb

class PostAlerter

  def self.post_created(post)
    alerter = PostAlerter.new
    alerter.after_create_post(post)
    alerter.after_save_post(post)
    post
  end

  def after_create_post(post)
    if post.topic.private_message?
      # If it's a private message, notify the topic_allowed_users
      post.topic.all_allowed_users.reject{ |user| user.id == post.user_id }.each do |user|
        next if user.blank?

        if TopicUser.get(post.topic, user).try(:notification_level) == TopicUser.notification_levels[:tracking]
          next unless post.reply_to_post_number
          next unless post.reply_to_post.user_id == user.id
        end

        create_notification(user, Notification.types[:private_message], post)
      end
    elsif post.post_type != Post.types[:moderator_action]
      # If it's not a private message and it's not an automatic post caused by a moderator action, notify the users
      notify_post_users(post)
    end
  end

  def after_save_post(post)
    return if post.topic.private_message?

    mentioned_users = extract_mentioned_users(post)
    quoted_users = extract_quoted_users(post)
    linked_users = extract_linked_users(post)

    reply_to_user = post.reply_notification_target

    notified = [reply_to_user]

    notify_users(mentioned_users - notified, :mentioned, post)

    notified += mentioned_users

    notify_users(quoted_users - notified, :quoted, post)

    notified += quoted_users

    notify_users(linked_users - notified, :linked, post)
  end

  def unread_posts(user, topic)
    Post.where('post_number > COALESCE((
               SELECT last_read_post_number FROM topic_users tu
               WHERE tu.user_id = ? AND tu.topic_id = ? ),0)',
                user.id, topic.id)
        .where('reply_to_user_id = ? OR exists(
            SELECT 1 from topic_users tu
            WHERE tu.user_id = ? AND
              tu.topic_id = ? AND
              notification_level = ?
            )', user.id, user.id, topic.id, TopicUser.notification_levels[:watching])
        .where(topic_id: topic.id)
  end

  def first_unread_post(user, topic)
    unread_posts(user, topic).order('post_number').first
  end

  def unread_count(user, topic)
    unread_posts(user, topic).count
  end

  def destroy_notifications(user, type, topic)
    return if user.blank?
    return unless Guardian.new(user).can_see?(topic)

    user.notifications.where(notification_type: type,
                             topic_id: topic.id).destroy_all
  end

  def create_notification(user, type, post, opts={})
    return if user.blank?

    # Make sure the user can see the post
    return unless Guardian.new(user).can_see?(post)

    # skip if muted on the topic
    return if TopicUser.get(post.topic, user).try(:notification_level) == TopicUser.notification_levels[:muted]

    # Don't notify the same user about the same notification on the same post
    return if user.notifications.exists?(notification_type: type, topic_id: post.topic_id, post_number: post.post_number)

    collapsed = false

    if  type == Notification.types[:replied] ||
        type == Notification.types[:posted]

      destroy_notifications(user, Notification.types[:replied] , post.topic)
      destroy_notifications(user, Notification.types[:posted] , post.topic)
      collapsed = true
    end

    if  type == Notification.types[:private_message]
      destroy_notifications(user, type, post.topic)
      collapsed = true
    end

    original_post = post
    original_username = opts[:display_username] || post.username

    if collapsed
      post = first_unread_post(user,post.topic) || post
      count = unread_count(user, post.topic)
      opts[:display_username] = I18n.t('embed.replies', count: count) if count > 1
    end

    UserActionObserver.log_notification(original_post, user, type)

    # Create the notification
    user.notifications.create(notification_type: type,
                              topic_id: post.topic_id,
                              post_number: post.post_number,
                              post_action_id: opts[:post_action_id],
                              data: { topic_title: post.topic.title,
                                      original_post_id: original_post.id,
                                      original_username: original_username,
                                      display_username: opts[:display_username] || post.user.username }.to_json)
  end

  # TODO: Move to post-analyzer?
  # Returns a list users who have been mentioned
  def extract_mentioned_users(post)
    User.where(username_lower: post.raw_mentions).where("id <> ?", post.user_id)
  end

  # TODO: Move to post-analyzer?
  # Returns a list of users who were quoted in the post
  def extract_quoted_users(post)
    post.raw.scan(/\[quote=\"([^,]+),.+\"\]/).uniq.map do |m|
      User.find_by("username_lower = :username and id != :id", username: m.first.strip.downcase, id: post.user_id)
    end.compact
  end

  def extract_linked_users(post)
    post.topic_links.map do |link|
      linked_post = link.link_post
      if !linked_post && topic = link.link_topic
        linked_post = topic.posts(post_number: 1).first
      end
      linked_post && post.user_id != linked_post.user_id && linked_post.user
    end.compact
  end

  # Notify a bunch of users
  def notify_users(users, type, post)
    users = [users] unless users.is_a?(Array)
    users.each do |u|
      create_notification(u, Notification.types[type], post)
    end
  end

  # TODO: This should use javascript for parsing rather than re-doing it this way.
  def notify_post_users(post)
    # Is this post a reply to a user?
    reply_to_user = post.reply_notification_target
    notify_users(reply_to_user, :replied, post)

    exclude_user_ids = [] <<
        post.user_id <<
        extract_mentioned_users(post).map(&:id) <<
        extract_quoted_users(post).map(&:id)

    exclude_user_ids << reply_to_user.id if reply_to_user.present?
    exclude_user_ids.flatten!
    TopicUser
      .where(topic_id: post.topic_id, notification_level: TopicUser.notification_levels[:watching])
      .includes(:user).each do |tu|
        create_notification(tu.user, Notification.types[:posted], post) unless exclude_user_ids.include?(tu.user_id)
      end
  end
end