Add HTMLGitlab formatter

This custom formatter for rouge is needed to generate HTML output
specifically for gitlab. Since its usecase is mostly suitable for
gitlab it had been rejected upstream:
https://github.com/jneen/rouge/pull/268

Thanks a lot to @stanhu, @jneen and @tsigo for review!

app/helpers/blob_helper.rb

@@ -8,7 +8,7 @@ module BlobHelper
     )
     begin
-      @lexer ||= Rouge::Lexer.guess(filename: blob_name, source: blob_content)
+      @lexer ||= Rouge::Lexer.guess(filename: blob_name, source: blob_content).new
       result = @formatter.format(@lexer.lex(blob_content, continue: continue)).html_safe
     rescue
       @lexer = Rouge::Lexers::PlainText

lib/rouge/formatters/html_gitlab.rb

+require 'cgi'
+
+module Rouge
+  module Formatters
+    class HTMLGitlab < Rouge::Formatter
+      tag 'html_gitlab'
+
+      # Creates a new <tt>Rouge::Formatter::HTMLGitlab</tt> instance.
+      #
+      # [+nowrap+]          If set to True, don't wrap the output at all, not
+      #                     even inside a <tt><pre></tt> tag (default: false).
+      # [+cssclass+]        CSS class for the wrapping <tt><div></tt> tag
+      #                     (default: 'highlight').
+      # [+linenos+]         If set to 'table', output line numbers as a table
+      #                     with two cells, one containing the line numbers,
+      #                     the other the whole code. This is copy paste friendly,
+      #                     but may cause alignment problems with some browsers
+      #                     or fonts. If set to 'inline', the line numbers will
+      #                     be integrated in the <tt><pre></tt> tag that contains
+      #                     the code (default: nil).
+      # [+linenostart+]     The line number for the first line (default: 1).
+      # [+lineanchors+]     If set to true the formatter will wrap each output
+      #                     line in an anchor tag with a name of L-linenumber.
+      #                     This allows easy linking to certain lines
+      #                     (default: false).
+      # [+lineanchorsid+]   If lineanchors is true the name of the anchors can
+      #                     be changed with lineanchorsid to e.g. foo-linenumber
+      #                     (default: 'L').
+      # [+anchorlinenos+]   If set to true, will wrap line numbers in <tt><a></tt>
+      #                     tags. Used in combination with linenos and lineanchors
+      #                     (default: false).
+      # [+inline_theme+]    Inline CSS styles for the <pre> tag (default: false).
+      def initialize(
+          nowrap: false,
+          cssclass: 'highlight',
+          linenos: nil,
+          linenostart: 1,
+          lineanchors: false,
+          lineanchorsid: 'L',
+          anchorlinenos: false,
+          inline_theme: nil
+        )
+        @nowrap = nowrap
+        @cssclass = cssclass
+        @linenos = linenos
+        @linenostart = linenostart
+        @lineanchors = lineanchors
+        @lineanchorsid = lineanchorsid
+        @anchorlinenos = anchorlinenos
+        @inline_theme = Theme.find(@inline_theme).new if @inline_theme.is_a?(String)
+      end
+
+      def render(tokens)
+        case @linenos
+        when 'table'
+          render_tableized(tokens)
+        when 'inline'
+          render_untableized(tokens)
+        else
+          render_untableized(tokens)
+        end
+      end
+
+      alias_method :format, :render
+
+      private
+
+      def render_untableized(tokens)
+        data = process_tokens(tokens)
+
+        html = ''
+        html << "<pre class=\"#{@cssclass}\"><code>" unless @nowrap
+        html << wrap_lines(data[:code])
+        html << "</code></pre>\n" unless @nowrap
+        html
+      end
+
+      def render_tableized(tokens)
+        data = process_tokens(tokens)
+
+        html = ''
+        html << "<div class=\"#{@cssclass}\">" unless @nowrap
+        html << '<table><tbody>'
+        html << "<td class=\"linenos\"><pre>"
+        html << wrap_linenos(data[:numbers])
+        html << '</pre></td>'
+        html << "<td class=\"lines\"><pre><code>"
+        html << wrap_lines(data[:code])
+        html << '</code></pre></td>'
+        html << '</tbody></table>'
+        html << '</div>' unless @nowrap
+        html
+      end
+
+      def process_tokens(tokens)
+        num_lines = 0
+        last_val = ''
+        rendered = ''
+
+        tokens.each do |tok, val|
+          last_val = val
+          num_lines += val.scan(/\n/).size
+          rendered << span(tok, val)
+        end
+
+        numbers = (@linenostart..num_lines + @linenostart - 1).to_a
+
+        { numbers: numbers, code: rendered }
+      end
+
+      def wrap_linenos(numbers)
+        if @anchorlinenos
+          numbers.map! do |number|
+            "<a href=\"##{@lineanchorsid}#{number}\">#{number}</a>"
+          end
+        end
+        numbers.join("\n")
+      end
+
+      def wrap_lines(rendered)
+        if @lineanchors
+          lines = rendered.split("\n")
+          lines = lines.each_with_index.map do |line, index|
+            number = index + @linenostart
+
+            if @linenos == 'inline'
+              "<a name=\"L#{number}\"></a>" \
+              "<span class=\"linenos\">#{number}</span>" \
+              "<span id=\"#{@lineanchorsid}#{number}\" class=\"line\">#{line}" \
+              '</span>'
+            else
+              "<span id=\"#{@lineanchorsid}#{number}\" class=\"line\">#{line}" \
+              '</span>'
+            end
+          end
+          lines.join("\n")
+        else
+          if @linenos == 'inline'
+            lines = rendered.split("\n")
+            lines = lines.each_with_index.map do |line, index|
+              number = index + @linenostart
+              "<span class=\"linenos\">#{number}</span>#{line}"
+            end
+            lines.join("\n")
+          else
+            rendered
+          end
+        end
+      end
+
+      def span(tok, val)
+        # http://stackoverflow.com/a/1600584/2587286
+        val = CGI.escapeHTML(val)
+
+        if tok.shortname.empty?
+          val
+        else
+          if @inline_theme
+            rules = @inline_theme.style_for(tok).rendered_rules
+            "<span style=\"#{rules.to_a.join(';')}\">#{val}</span>"
+          else
+            "<span class=\"#{tok.shortname}\">#{val}</span>"
+          end
+        end
+      end
+    end
+  end
+end