DEV: Move discourse-chat to the core repo. (#18776)

As part of this move, we are also renaming `discourse-chat` to `chat`.

plugins/chat/lib/post_notification_handler.rb

@@ -0,0 +1,40 @@
+# frozen_string_literal: true
+
+##
+# Handles :post_alerter_after_save_post events from
+# core. Used for notifying users that their chat message
+# has been quoted in a post.
+class Chat::PostNotificationHandler
+  attr_reader :post
+
+  def initialize(post, notified_users)
+    @post = post
+    @notified_users = notified_users
+  end
+
+  def handle
+    return false if post.post_type == Post.types[:whisper]
+    return false if post.topic.blank?
+    return false if post.topic.private_message?
+
+    quoted_users = extract_quoted_users(post)
+    if @notified_users.present?
+      quoted_users = quoted_users.where("users.id NOT IN (?)", @notified_users)
+    end
+
+    opts = { user_id: post.user.id, display_username: post.user.username }
+    quoted_users.each do |user|
+      # PostAlerter.create_notification handles many edge cases, such as
+      # muting, ignoring, double notifications etc.
+      PostAlerter.new.create_notification(user, Notification.types[:chat_quoted], post, opts)
+    end
+  end
+
+  private
+
+  def extract_quoted_users(post)
+    usernames =
+      post.raw.scan(/\[chat quote=\"([^;]+);.+\"\]/).uniq.map { |q| q.first.strip.downcase }
+    User.where.not(id: post.user_id).where(username_lower: usernames)
+  end
+end