AB test - replace some content on UTR page

app/controllers/content_items_controller.rb

@@ -24,6 +24,35 @@ class ContentItemsController < ApplicationController
   def show
     load_content_item
 
+    # TEMPORARY (author: richard.towers, expected end date: November 30 2023)
+    # Content specific AB test for the Find your UTR number page
+    if @content_item.base_path == "/find-utr-number"
+      ab_test = GovukAbTesting::AbTest.new(
+        "find_utr_number_video_links",
+        dimension: 300, # TODO: which dimension should we use?
+        allowed_variants: %w[HelpText VideoLink],
+        control_variant: "HelpText",
+      )
+      @requested_variant = ab_test.requested_variant(request.headers)
+      @requested_variant.configure_response(response)
+
+      if @requested_variant.variant? "VideoLink"
+        # NOTE: this is a fragile way of doing an AB test on content.
+        #
+        # Any change to the base content, or to the way the content is rendered
+        # could potentially break the B variant of the test, and result in both
+        # variants being the same.
+        # We're aware of this risk, and we're going to be careful in this one off
+        # situation. This is not a sustainable way of doing AB tests in the
+        # future.
+        @content_item.body.sub!(
+          /<li>\s*in\ the\s+<a\ href="[^"]*"><abbr\ title="[^"]+">HMRC<\/abbr>\s+app<\/a>\s*<\/li>/,
+          '<li>in the <a href="https://www.gov.uk/guidance/download-the-hmrc-app"><abbr title="HM Revenue and Customs">HMRC</abbr> app</a> - watch a <a href="https://www.youtube.com/watch?v=LXw9ily9rTo">video about finding your UTR number in the app</a>',
+        )
+      end
+    end
+    # /TEMPORARY
+
     set_expiry
 
     if is_service_manual?

test/controllers/content_items_controller_test.rb

@@ -364,6 +364,21 @@ class ContentItemsControllerTest < ActionController::TestCase
     assert_equal "true", @response.headers[Slimmer::Headers::REMOVE_SEARCH_HEADER]
   end
 
+  test "AB test replaces content on the find-utr-number page" do
+    content_item = content_store_has_schema_example("answer", "answer")
+    content_item["base_path"] = "/find-utr-number"
+    content_item["details"]["body"] = "<li>in the <a href=\"\"><abbr title=\"HM Revenue and Customs\">HMRC</abbr> app</a>\n</li>"
+    content_item["locale"] = "en"
+
+    stub_content_store_has_item(content_item["base_path"], content_item)
+
+    request.headers["HTTP_GOVUK_ABTEST_FIND_UTR_NUMBER_VIDEO_LINKS"] = "VideoLink"
+
+    get :show, params: { path: path_for(content_item) }
+    assert_response :success
+    assert_match 'watch a <a href="https://www.youtube.com/watch?v=LXw9ily9rTo">video about finding your UTR number in the app</a>', response.body
+  end
+
   def path_for(content_item, locale = nil)
     base_path = content_item["base_path"].sub(/^\//, "")
     base_path.gsub!(/\.#{locale}$/, "") if locale