Generate meta tags based on page's content (#1091)

lib/rdoc/generator/darkfish.rb

@@ -780,4 +780,19 @@ def template_for file, page = true, klass = ERB
     template
   end
 
+  # Returns an excerpt of the content for usage in meta description tags
+  def excerpt(content)
+    text = content.is_a?(RDoc::Comment) ? content.text : content
+
+    # Match from a capital letter to the first period, discarding any links, so
+    # that we don't end up matching badges in the README
+    first_paragraph_match = text.match(/[A-Z][^\.:\/]+\./)
+    return text[0...150].gsub(/\n/, " ").squeeze(" ") unless first_paragraph_match
+
+    extracted_text = first_paragraph_match[0]
+    second_paragraph = first_paragraph_match.post_match.match(/[A-Z][^\.:\/]+\./)
+    extracted_text << " " << second_paragraph[0] if second_paragraph
+
+    extracted_text[0...150].gsub(/\n/, " ").squeeze(" ")
+  end
 end

lib/rdoc/generator/template/darkfish/_head.rhtml

@@ -3,6 +3,28 @@
 
 <title><%= h @title %></title>
 
+<%- if defined?(klass) -%>
+  <meta name="keywords" content="ruby,<%= h "#{klass.type},#{klass.full_name}" %>">
+
+  <%- if klass.comment.empty? -%>
+    <meta name="description" content="Documentation for the <%= h "#{klass.full_name} #{klass.type}" %>">
+  <%- else -%>
+    <meta name="description" content="<%= h "#{klass.type} #{klass.full_name}: #{excerpt(klass.comment)}" %>">
+  <%- end -%>
+<%- elsif defined?(file) -%>
+  <meta name="keywords" content="ruby,documentation,<%= h file.page_name %>">
+  <meta name="description" content="<%= h "#{file.page_name}: #{excerpt(file.comment)}" %>">
+<%- elsif @title -%>
+  <meta name="keywords" content="ruby,documentation,<%= h @title %>">
+
+  <%- if @options.main_page and
+      main_page = @files.find { |f| f.full_name == @options.main_page } then %>
+    <meta name="description" content="<%= h "#{@title}: #{excerpt(main_page.comment)}" %>">
+  <%- else -%>
+    <meta name="description" content="Documentation for <%= h @title %>">
+  <%- end -%>
+<%- end -%>
+
 <script type="text/javascript">
   var rdoc_rel_prefix = "<%= h asset_rel_prefix %>/";
   var index_rel_prefix = "<%= h rel_prefix %>/";

test/rdoc/test_rdoc_generator_darkfish.rb

@@ -322,6 +322,51 @@ def test_title_escape
     assert_main_title(File.binread('index.html'), title)
   end
 
+  def test_meta_tags_for_index
+    @options.title = "My awesome Ruby project"
+    @g.generate
+
+    content = File.binread("index.html")
+
+    assert_include(content, '<meta name="keywords" content="ruby,documentation,My awesome Ruby project">')
+    assert_include(content, '<meta name="description" content="Documentation for My awesome Ruby project">')
+  end
+
+  def test_meta_tags_for_classes
+    top_level = @store.add_file("file.rb")
+    top_level.add_class(@klass.class, @klass.name)
+    inner = @klass.add_class(RDoc::NormalClass, "Inner")
+    inner.add_comment("This is a normal class. It is fully documented.", top_level)
+
+    @g.generate
+
+    content = File.binread("Klass/Inner.html")
+    assert_include(content, '<meta name="keywords" content="ruby,class,Klass::Inner">')
+    assert_include(
+      content,
+      '<meta name="description" content="class Klass::Inner: This is a normal class. It is fully documented.">',
+    )
+  end
+
+  def test_meta_tags_for_pages
+    top_level = @store.add_file("CONTRIBUTING.rdoc", parser: RDoc::Parser::Simple)
+    top_level.comment = <<~RDOC
+      = Contributing
+
+      Here are the instructions for contributing. Begin by installing Ruby.
+    RDOC
+
+    @g.generate
+
+    content = File.binread("CONTRIBUTING_rdoc.html")
+    assert_include(content, '<meta name="keywords" content="ruby,documentation,CONTRIBUTING">')
+    assert_include(
+      content,
+      "<meta name=\"description\" content=\"CONTRIBUTING: Contributing Here are the instructions for contributing." \
+      " Begin by installing Ruby.\">",
+    )
+  end
+
   ##
   # Asserts that +filename+ has a link count greater than 1 if hard links to
   # @tmpdir are supported.