# == Mentionable concern
#
# Contains functionality related to objects that can mention Users, Issues, MergeRequests, or Commits by
# GFM references.
#
# Used by Issue, Note, MergeRequest, and Commit.
#
module Mentionable
  extend ActiveSupport::Concern

  module ClassMethods
    # Indicate which attributes of the Mentionable to search for GFM references.
    def attr_mentionable(*attrs)
      mentionable_attrs.concat(attrs.map(&:to_s))
    end

    # Accessor for attributes marked mentionable.
    def mentionable_attrs
      @mentionable_attrs ||= []
    end
  end

  # Returns the text used as the body of a Note when this object is referenced
  #
  # By default this will be the class name and the result of calling
  # `to_reference` on the object.
  def gfm_reference(from_project = nil)
    # "MergeRequest" > "merge_request" > "Merge request" > "merge request"
    friendly_name = self.class.to_s.underscore.humanize.downcase

    "#{friendly_name} #{to_reference(from_project)}"
  end

  # Construct a String that contains possible GFM references.
  def mentionable_text
    self.class.mentionable_attrs.map { |attr| send(attr) }.compact.join("\n\n")
  end

  # The GFM reference to this Mentionable, which shouldn't be included in its #references.
  def local_reference
    self
  end

  # Determine whether or not a cross-reference Note has already been created between this Mentionable and
  # the specified target.
  def has_mentioned?(target)
    SystemNoteService.cross_reference_exists?(target, local_reference)
  end

  def mentioned_users(current_user = nil)
    return [] if mentionable_text.blank?

    ext = Gitlab::ReferenceExtractor.new(self.project, current_user)
    ext.analyze(mentionable_text)
    ext.users.uniq
  end

  # Extract GFM references to other Mentionables from this Mentionable. Always excludes its #local_reference.
  def references(p = project, current_user = self.author, text = mentionable_text)
    return [] if text.blank?

    ext = Gitlab::ReferenceExtractor.new(p, current_user)
    ext.analyze(text)

    (ext.issues + ext.merge_requests + ext.commits).uniq - [local_reference]
  end

  # Create a cross-reference Note for each GFM reference to another Mentionable found in +mentionable_text+.
  def create_cross_references!(p = project, a = author, without = [])
    refs = references(p)

    # We're using this method instead of Array diffing because that requires
    # both of the object's `hash` values to be the same, which may not be the
    # case for otherwise identical Commit objects.
    refs.reject! { |ref| without.include?(ref) }

    refs.each do |ref|
      SystemNoteService.cross_reference(ref, local_reference, a)
    end
  end

  # If the mentionable_text field is about to change, locate any *added* references and create cross references for
  # them. Invoke from an observer's #before_save implementation.
  def notice_added_references(p = project, a = author)
    changes = detect_mentionable_changes

    return if changes.empty?

    original_text = changes.collect { |_, vals| vals.first }.join(' ')

    preexisting = references(p, self.author, original_text)
    create_cross_references!(p, a, preexisting)
  end

  private

  # Returns a Hash of changed mentionable fields
  #
  # Preference is given to the `changes` Hash, but falls back to
  # `previous_changes` if it's empty (i.e., the changes have already been
  # persisted).
  #
  # See ActiveModel::Dirty.
  #
  # Returns a Hash.
  def detect_mentionable_changes
    source = (changes.present? ? changes : previous_changes).dup

    mentionable = self.class.mentionable_attrs

    # Only include changed fields that are mentionable
    source.select { |key, val| mentionable.include?(key) }
  end
end