Use :http_prefix when calculating the path to the site root, when using absolute links.

This configuration setting defaults to '/' so we can rely on it to always have a value.

Author: eddgrant

lib/govuk_tech_docs/path_helpers.rb

@@ -21,7 +21,8 @@ def path_to_site_root(config, page_path)
         ascents = number_of_ascents_to_site_root.zero? ? ["."] : number_of_ascents_to_site_root.times.collect { ".." }
         path_to_site_root = ascents.join("/").concat("/")
       else
-        path_to_site_root = "/"
+        middleman_http_prefix = config[:http_prefix]
+        path_to_site_root = middleman_http_prefix.end_with?("/") ? middleman_http_prefix : "#{middleman_http_prefix}/"
       end
       path_to_site_root
     end

spec/govuk_tech_docs/path_helpers_spec.rb

@@ -31,12 +31,25 @@
     it "calculates the path from a page to the site root when using absolute links" do
       page_path = "/documentation/introduction/index.html"
 
-      config = {}
+      config = {
+        http_prefix: "/", # This is Middleman's default setting.
+      }
 
       path_to_site_root = path_to_site_root(config, page_path)
       expect(path_to_site_root).to eql("/")
     end
 
+    it "calculates the path from a page to the site root when a middleman http prefix" do
+      page_path = "/bananas/documentation/introduction/index.html"
+
+      config = {
+        http_prefix: "/bananas",
+      }
+
+      path_to_site_root = path_to_site_root(config, page_path)
+      expect(path_to_site_root).to eql("/bananas/")
+    end
+
     it "calculates the path from a page to the site root when using relative links" do
       page_path = "/documentation/introduction/index.html"