module Gitlab
  # Custom parser for GitLab-flavored Markdown
  #
  # It replaces references in the text with links to the appropriate items in
  # GitLab.
  #
  # Supported reference formats are:
  #   * @foo for team members
  #   * #123 for issues
  #   * !123 for merge requests
  #   * $123 for snippets
  #   * 123456 for commits
  #
  # It also parses Emoji codes to insert images. See
  # http://www.emoji-cheat-sheet.com/ for a list of the supported icons.
  #
  # Examples
  #
  #   >> gfm("Hey @david, can you fix this?")
  #   => "Hey <a href="/gitlab/team_members/1">@david</a>, can you fix this?"
  #
  #   >> gfm("Commit 35d5f7c closes #1234")
  #   => "Commit <a href="/gitlab/commits/35d5f7c">35d5f7c</a> closes <a href="/gitlab/issues/1234">#1234</a>"
  #
  #   >> gfm(":trollface:")
  #   => "<img alt=\":trollface:\" class=\"emoji\" src=\"/images/trollface.png" title=\":trollface:\" />
  module Markdown
    REFERENCE_PATTERN = %r{
      (\W)?           # Prefix (1)
      (               # Reference (2)
        @([\w\._]+)   # User name (3)
        |[#!$](\d+)   # Issue/MR/Snippet ID (4)
        |([\h]{6,40}) # Commit ID (5)
      )
      (\W)?           # Suffix (6)
    }x.freeze

    EMOJI_PATTERN = %r{(:(\S+):)}.freeze

    attr_reader :html_options

    # Public: Parse the provided text with GitLab-Flavored Markdown
    #
    # text         - the source text
    # html_options - extra options for the reference links as given to link_to
    #
    # Note: reference links will only be generated if @project is set
    def gfm(text, html_options = {})
      return text if text.nil?

      # Duplicate the string so we don't alter the original, then call to_str
      # to cast it back to a String instead of a SafeBuffer. This is required
      # for gsub calls to work as we need them to.
      text = text.dup.to_str

      @html_options = html_options

      # Extract pre blocks so they are not altered
      # from http://github.github.com/github-flavored-markdown/
      extractions = {}
      text.gsub!(%r{<pre>.*?</pre>|<code>.*?</code>}m) do |match|
        md5 = Digest::MD5.hexdigest(match)
        extractions[md5] = match
        "{gfm-extraction-#{md5}}"
      end

      # TODO: add popups with additional information

      text = parse(text)

      # Insert pre block extractions
      text.gsub!(/\{gfm-extraction-(\h{32})\}/) do
        extractions[$1]
      end

      sanitize text.html_safe, attributes: ActionView::Base.sanitized_allowed_attributes + %w(id class)
    end

    private

    # Private: Parses text for references and emoji
    #
    # text - Text to parse
    #
    # Note: reference links will only be generated if @project is set
    #
    # Returns parsed text
    def parse(text)
      parse_references(text) if @project
      parse_emoji(text)

      text
    end

    def parse_references(text)
      # parse reference links
      text.gsub!(REFERENCE_PATTERN) do |match|
        prefix     = $1 || ''
        reference  = $2
        identifier = $3 || $4 || $5
        suffix     = $6 || ''

        # Avoid HTML entities
        if prefix.ends_with?('&') || suffix.starts_with?(';')
          match
        elsif ref_link = reference_link(reference, identifier)
          prefix + ref_link + suffix
        else
          match
        end
      end
    end

    def parse_emoji(text)
      # parse emoji
      text.gsub!(EMOJI_PATTERN) do |match|
        if valid_emoji?($2)
          image_tag("emoji/#{$2}.png", size: "20x20", class: 'emoji', title: $1, alt: $1)
        else
          match
        end
      end
    end

    # Private: Checks if an emoji icon exists in the image asset directory
    #
    # emoji - Identifier of the emoji as a string (e.g., "+1", "heart")
    #
    # Returns boolean
    def valid_emoji?(emoji)
      File.exists?(Rails.root.join('app', 'assets', 'images', 'emoji', "#{emoji}.png"))
    end

    # Private: Dispatches to a dedicated processing method based on reference
    #
    # reference  - Object reference ("@1234", "!567", etc.)
    # identifier - Object identifier (Issue ID, SHA hash, etc.)
    #
    # Returns string rendered by the processing method
    def reference_link(reference, identifier)
      case reference
      when /^@/  then reference_user(identifier)
      when /^#/  then reference_issue(identifier)
      when /^!/  then reference_merge_request(identifier)
      when /^\$/ then reference_snippet(identifier)
      when /^\h/ then reference_commit(identifier)
      end
    end

    def reference_user(identifier)
      if user = @project.users.where(name: identifier).first
        member = @project.users_projects.where(user_id: user).first
        link_to("@#{identifier}", project_team_member_path(@project, member), html_options.merge(class: "gfm gfm-team_member #{html_options[:class]}")) if member
      end
    end

    def reference_issue(identifier)
      if issue = @project.issues.where(id: identifier).first
        link_to("##{identifier}", project_issue_path(@project, issue), html_options.merge(title: "Issue: #{issue.title}", class: "gfm gfm-issue #{html_options[:class]}"))
      end
    end

    def reference_merge_request(identifier)
      if merge_request = @project.merge_requests.where(id: identifier).first
        link_to("!#{identifier}", project_merge_request_path(@project, merge_request), html_options.merge(title: "Merge Request: #{merge_request.title}", class: "gfm gfm-merge_request #{html_options[:class]}"))
      end
    end

    def reference_snippet(identifier)
      if snippet = @project.snippets.where(id: identifier).first
        link_to("$#{identifier}", project_snippet_path(@project, snippet), html_options.merge(title: "Snippet: #{snippet.title}", class: "gfm gfm-snippet #{html_options[:class]}"))
      end
    end

    def reference_commit(identifier)
      if commit = @project.commit(identifier)
        link_to(identifier, project_commit_path(@project, id: commit.id), html_options.merge(title: CommitDecorator.new(commit).link_title, class: "gfm gfm-commit #{html_options[:class]}"))
      end
    end
  end
end
