AB test - replace some content on UTR page

HMRC would like to see whether a link to a video performs better than
some descriptive help text. We've agreed to run an AB test to compare
these variants.

There's no elegant way to AB test bits of content in GOV.UK, so we've
had to resort to the fragile and ugly approach of find / replacing the
content.

As per the comments in the code, this will break if the content in the
content item changes, and we'll have to update the hardcoded replacement
if the content in the B variant needs to change.

Nevertheless, we think this is a pragmatic thing to do as a one off, as
it keeps an immportant stakeholder happy, helps us learn about video
content (which is a strategic priority), and helps us learn about the
use case for content AB tests in mainstream content.

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>/,
+          '<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>'
+    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