resolves #264 add line highlighting option

README.md

@@ -79,6 +79,11 @@ The built-in formatters are:
   your code into lines, each contained in its own div. The `class` option will
   be used to add a class name to the div, given the line number.
 
+* `Rouge::Formatters::HTMLLineHighlighter.new(formatter, highlight_lines: [3, 5])`
+  will split your code into lines and wrap the lines specified by the
+  `highlight_lines` option in a span with a class name specified by the
+  `highlight_line_class` option (default: `hll`).
+
 * `Rouge::Formatters::HTMLLineTable.new(formatter, opts={})` will output an HTML
   table containing numbered lines, each contained in its own table-row. Options
   are:

lib/rouge.rb

@@ -83,6 +83,7 @@ def load_lexers
 Rouge.load_file 'formatters/html_pygments'
 Rouge.load_file 'formatters/html_legacy'
 Rouge.load_file 'formatters/html_linewise'
+Rouge.load_file 'formatters/html_line_highlighter'
 Rouge.load_file 'formatters/html_line_table'
 Rouge.load_file 'formatters/html_inline'
 Rouge.load_file 'formatters/terminal256'

lib/rouge/formatters/html_line_highlighter.rb

@@ -0,0 +1,26 @@
+# -*- coding: utf-8 -*- #
+# frozen_string_literal: true
+
+module Rouge
+  module Formatters
+    class HTMLLineHighlighter < Formatter
+      tag 'html_line_highlighter'
+
+      def initialize(delegate, opts = {})
+        @delegate = delegate
+        @highlight_line_class = opts.fetch(:highlight_line_class, 'hll')
+        @highlight_lines = opts[:highlight_lines] || []
+      end
+
+      def stream(tokens)
+        lineno = 0
+        token_lines(tokens) do |tokens_in_line|
+          lineno += 1
+          line = %(#{@delegate.format(tokens_in_line)}\n)
+          line = %(<span class="#{@highlight_line_class}">#{line}</span>) if @highlight_lines.include? lineno
+          yield line
+        end
+      end
+    end
+  end
+end

spec/formatters/html_line_highlighter_spec.rb

@@ -0,0 +1,28 @@
+# -*- coding: utf-8 -*- #
+# frozen_string_literal: true
+
+describe Rouge::Formatters::HTMLLineHighlighter do
+  let(:subject) { Rouge::Formatters::HTMLLineHighlighter.new(formatter, options) }
+  let(:formatter) { Rouge::Formatters::HTML.new }
+
+  let(:options) { {} }
+  let(:output) { subject.format(input_stream) }
+
+  describe 'highlight lines' do
+    let(:input_stream) { [[Token['Text'], "foo\n"], [Token['Name'], "bar\n"]] }
+    let(:options) { { highlight_lines: [2] } }
+
+    it 'should add highlight line class to lines specified by :highlight_lines option' do
+      assert { output == %(foo\n<span class="hll"><span class="n">bar</span>\n</span>) }
+    end
+  end
+
+  describe 'configure highlight line class' do
+    let(:input_stream) { [[Token['Text'], "foo\n"], [Token['Name'], "bar\n"]] }
+    let(:options) { { highlight_lines: [1, 2], highlight_line_class: 'hiline' } }
+
+    it 'should add highlight line class to lines specified by :highlight_lines option' do
+      assert { output == %(<span class="hiline">foo\n</span><span class="hiline"><span class="n">bar</span>\n</span>) }
+    end
+  end
+end