navigator redirects user if they veer off journey

app/controllers/claims_controller.rb

@@ -4,6 +4,7 @@ class ClaimsController < BasePublicController
   skip_before_action :send_unstarted_claimants_to_the_start, only: [:new, :create]
   before_action :initialize_session_slug_history
   before_action :check_page_is_in_sequence, only: [:show, :update]
+  before_action :check_page_is_permissible, only: [:show]
   before_action :set_backlink_path, only: [:show, :update]
   before_action :check_claim_not_in_progress, only: [:new]
   before_action :clear_claim_session, only: [:new], unless: -> { journey.start_with_magic_link? }
@@ -120,6 +121,14 @@ def check_page_is_in_sequence
     redirect_to claim_path(current_journey_routing_name, next_required_slug) unless page_sequence.has_completed_journey_until?(params[:slug])
   end
 
+  def check_page_is_permissible
+    return unless journey.use_navigator?
+
+    unless navigator.permissible_slug?
+      redirect_to claim_path(current_journey_routing_name, navigator.furthest_permissible_slug)
+    end
+  end
+
   def initialize_session_slug_history
     session[:slugs] ||= []
   end

app/models/journeys/further_education_payments/slug_sequence.rb

@@ -139,8 +139,22 @@ def slugs
           if answers.skip_postcode_search == true
             sequence.delete("select-home-address")
           end
+
+          if !eligibility_checker.ineligible?
+            sequence.delete("ineligible")
+          end
         end
       end
+
+      private
+
+      def eligibility_checker
+        @eligibility_checker ||= journey::EligibilityChecker.new(journey_session:)
+      end
+
+      def journey
+        Journeys.for_routing_name(journey_session.journey)
+      end
     end
   end
 end

app/models/journeys/navigator.rb

@@ -9,6 +9,8 @@ def initialize(current_slug:, slug_sequence:, params:, session:)
       @session = session
     end
 
+    # when showing a form
+    # work out where the backlink should take the user
     def previous_slug
       last_valid_slug = nil
 
@@ -35,6 +37,9 @@ def previous_slug
       end
     end
 
+    # given the current slug
+    # returns the next slug in the sequence
+    # used when a user hits continue on a form
     def next_slug
       return "ineligible" if eligibility_checker.ineligible?
 
@@ -78,8 +83,74 @@ def next_slug
       end
     end
 
+    # is a user allowed to visit the current_slug?
+    # use this to guard against a user jumping ahead in a journey
+    def permissible_slug?
+      if current_slug == "ineligible" && eligibility_checker.ineligible?
+        return true
+      end
+
+      forms.each do |form|
+        slug = journey.slug_for_form(form:)
+
+        if current_slug == slug
+          return true
+        end
+
+        if form.respond_to?(:completed?)
+          if !form.completed?
+            return false
+          end
+        elsif form.invalid?
+          return false
+        end
+      end
+    end
+
+    # ignoring the current_slug
+    # return the last slug the user is allowed to visit
+    # this is the furthest slug available in the journey given their state
+    # use this when user requests a non permissible slug
+    def furthest_permissible_slug
+      last_slug = nil
+
+      forms.each_with_index do |form, index|
+        slug = journey.slug_for_form(form:)
+
+        if form.respond_to?(:completed?)
+          if form.completed?
+            last_slug = slug
+          else
+            return slug
+          end
+        elsif form.valid?
+          last_slug = slug
+        else
+          return slug
+        end
+      end
+
+      last_slug
+    end
+
     private
 
+    def forms
+      @forms ||= slug_sequence.slugs.map do |slug|
+        form_class = journey.form_class_for_slug(slug:)
+
+        raise "Form not found for journey: #{journey} slug: #{slug}" if form_class.nil?
+
+        form = form_class.new(
+          journey:,
+          journey_session:,
+          params:,
+          session:
+        )
+        form
+      end
+    end
+
     def journey
       Journeys.for_routing_name(slug_sequence.journey_session.journey)
     end

spec/features/further_education_payments/jump_around_spec.rb

@@ -0,0 +1,17 @@
+require "rails_helper"
+
+RSpec.feature "Further education payments" do
+  scenario "visiting impermissible slug redirects back to last permissible slug" do
+    when_further_education_payments_journey_configuration_exists
+
+    visit landing_page_path(Journeys::FurtherEducationPayments::ROUTING_NAME)
+    expect(page).to have_link("Start now")
+    click_link "Start now"
+
+    expect(page).to have_content("Are you a member of staff with teaching responsibilities?")
+
+    visit claim_path(Journeys::FurtherEducationPayments::ROUTING_NAME, slug: "address")
+
+    expect(page).to have_content("Are you a member of staff with teaching responsibilities?")
+  end
+end

spec/models/journeys/navigator_spec.rb

@@ -74,4 +74,83 @@
       end
     end
   end
+
+  describe "#permissible_slug?" do
+    context "when permissible" do
+      let(:answers) do
+        build(
+          :further_education_payments_answers
+        )
+      end
+
+      it "returns truthy" do
+        expect(subject.permissible_slug?).to be_truthy
+      end
+    end
+
+    context "when not permissible" do
+      let(:current_slug) { "address" }
+      let(:answers) do
+        build(
+          :further_education_payments_answers
+        )
+      end
+
+      it "returns falsey" do
+        expect(subject.permissible_slug?).to be_falsey
+      end
+    end
+  end
+
+  describe "#furthest_permissible_slug" do
+    context "when new journey" do
+      let(:answers) do
+        build(
+          :further_education_payments_answers
+        )
+      end
+
+      it "returns first slug" do
+        expect(subject.furthest_permissible_slug).to eql("teaching-responsibilities")
+      end
+    end
+
+    context "when mid-journey" do
+      let(:current_slug) { "foo" }
+
+      let(:answers) do
+        build(
+          :further_education_payments_answers,
+          teaching_responsibilities: true,
+          provision_search: "ply"
+        )
+      end
+
+      before do
+        create(:school, name: "Plymouth")
+      end
+
+      it "returns relevant slug" do
+        expect(subject.furthest_permissible_slug).to eql("select-provision")
+      end
+    end
+
+    context "when about to submit" do
+      let(:current_slug) { "foo" }
+      let(:school) { create(:school) }
+
+      let(:answers) do
+        build(
+          :further_education_payments_answers,
+          :submittable,
+          provision_search: school.name,
+          skip_postcode_search: true
+        )
+      end
+
+      it "returns check-answers slug" do
+        expect(subject.furthest_permissible_slug).to eql("check-your-answers")
+      end
+    end
+  end
 end