Add front matter stripping option to renderer

lib/govuk_markdown.rb

@@ -7,7 +7,7 @@
 
 module GovukMarkdown
   def self.render(markdown, govuk_options = {})
-    renderer = GovukMarkdown::Renderer.new(govuk_options, { with_toc_data: true, link_attributes: { class: "govuk-link" } })
+    renderer = GovukMarkdown::Renderer.new(govuk_options, { with_toc_data: true, strip_front_matter: true, link_attributes: { class: "govuk-link" } })
     Redcarpet::Markdown.new(renderer, tables: true, no_intra_emphasis: true).render(markdown).strip
   end
 end

lib/govuk_markdown/preprocessor.rb

@@ -37,6 +37,14 @@ def inject_details
       self
     end
 
+    def strip_front_matter(enabled)
+      return self unless enabled
+
+      @output = output.gsub(%r{^---\n.*\n---}m, "")
+
+      self
+    end
+
   private
 
     # parse as markdown if there are multiple lines of content

lib/govuk_markdown/renderer.rb

@@ -2,6 +2,8 @@ module GovukMarkdown
   class Renderer < ::Redcarpet::Render::HTML
     def initialize(govuk_options, options = {})
       @headings_start_with = govuk_options[:headings_start_with]
+      @strip_front_matter = govuk_options[:strip_front_matter]
+
       super options
     end
 
@@ -93,6 +95,7 @@ def preprocess(document)
         .new(document)
         .inject_inset_text
         .inject_details
+        .strip_front_matter(@strip_front_matter)
         .output
     end
   end

spec/preprocessor_spec.rb

@@ -320,6 +320,63 @@
     end
   end
 
+  describe "strip front matter" do
+    let(:input) do
+      <<~MD
+        ---
+        title: Hi
+        tags: hello, world
+        ---
+
+        Waffle waffle waffle waffle.
+
+        Waffle waffle waffle waffle.
+      MD
+    end
+
+    let(:actual_output) { render(input, { strip_front_matter: }) }
+
+    context "when front matter is stripped" do
+      let(:strip_front_matter) { true }
+
+      let(:expected_output) do
+        <<~EXPECTED.strip
+          <p class="govuk-body-m">Waffle waffle waffle waffle.</p>
+          <p class="govuk-body-m">Waffle waffle waffle waffle.</p>
+        EXPECTED
+      end
+
+      it "renders no front matter" do
+        expect(actual_output).not_to include("title")
+      end
+
+      it "renders without front matter" do
+        expect(actual_output).to eql(expected_output)
+      end
+    end
+
+    context "when front matter is not stripped" do
+      let(:strip_front_matter) { false }
+
+      let(:expected_output) do
+        <<~EXPECTED.strip
+          <p class="govuk-body-m">Waffle waffle waffle waffle.</p>
+          <p class="govuk-body-m">Waffle waffle waffle waffle.</p>
+        EXPECTED
+      end
+
+      it "renders the front matter" do
+        expect(actual_output).to include(%(<hr class="govuk-section-break govuk-section-break--xl govuk-section-break--visible">))
+        expect(actual_output).to include(%(<p class="govuk-body-m">title: Hi</p>))
+        expect(actual_output).to include(%(<h2 id="tags-hello-world" class="govuk-heading-l">tags: hello, world</h2>))
+      end
+
+      it "renders the content too" do
+        expect(actual_output).to include(expected_output)
+      end
+    end
+  end
+
   describe "multiple preprocessing steps" do
     let(:expected_output) do
       <<~HTML

spec/spec_helper.rb

@@ -1,8 +1,8 @@
 require_relative "./../lib/govuk_markdown"
 require "pry"
 
-def render(content)
-  GovukMarkdown.render(content)
+def render(content, govuk_options = {})
+  GovukMarkdown.render(content, govuk_options)
 end
 
 def expect_equal_ignoring_ws(first, second)