Make RandomExample/RandomItemGenerator deterministic (use seed)

We're now building govuk-developer-docs hourly, but our heavy use
of GovukSchemas::RandomExample is causing ~50,000 line changes
for every commit to gh-pages, despite the underlying schemas
remaining unchanged.

We want the example to be random and schema-validated, but not
necessarily a new random on every build. Hence we would like to
set the 'seed' so that we can predictably execute the same random
generation every time. In theory, this means our gh-pages output
will be identical between builds until an underlying schema changes.

govuk_schemas did not support this out of the box because of its
reliance on SecureRandom, which, as the name implies, does not
allow outside interference/configuration by way of seed values
(it gives a truly random result every time). We don't _need_ a
'secure' random output - just something we can be reasonably
confident will give a different result every time if the seed is
omitted. I've reproduced its 'uuid' and 'hex' methods using arrays
of chars/numbers and the native 'rand' method, which _does_
respect the global seed value.

lib/govuk_schemas/random.rb

@@ -1,5 +1,3 @@
-require "securerandom"
-
 module GovukSchemas
   # @private
   module Random
@@ -27,7 +25,7 @@ def uri
       end
 
       def base_path
-        "/" + rand(1..5).times.map { SecureRandom.uuid }.join("/")
+        "/" + rand(1..5).times.map { uuid }.join("/")
       end
 
       def govuk_subdomain_url
@@ -48,19 +46,29 @@ def bool
       end
 
       def anchor
-        "##{SecureRandom.hex}"
+        "##{hex}"
       end
 
       def random_identifier(separator:)
         Utils.parameterize(WORDS.sample(rand(1..10)).join("-")).gsub("-", separator)
       end
 
+      def uuid
+        # matches uuid regex e.g. e058aad7-ce86-5181-8801-4ddcb3c8f27c
+        # /^[a-f0-9]{8}-[a-f0-9]{4}-[1-5][a-f0-9]{3}-[89ab][a-f0-9]{3}-[a-f0-9]{12}$/
+        "#{hex(8)}-#{hex(4)}-1#{hex(3)}-a#{hex(3)}-#{hex(12)}"
+      end
+
+      def hex(length = 10)
+        length.times.map { bool ? random_letter : random_number }.join("")
+      end
+
       def string_for_regex(pattern)
         case pattern.to_s
         when "^(placeholder|placeholder_.+)$"
           ["placeholder", "placeholder_#{WORDS.sample}"].sample
         when "^[a-f0-9]{8}-[a-f0-9]{4}-[1-5][a-f0-9]{3}-[89ab][a-f0-9]{3}-[a-f0-9]{12}$"
-          SecureRandom.uuid
+          uuid
         when "^/(([a-zA-Z0-9._~!$&'()*+,;=:@-]|%[0-9a-fA-F]{2})+(/([a-zA-Z0-9._~!$&'()*+,;=:@-]|%[0-9a-fA-F]{2})*)*)?$"
           base_path
         when "^[1-9][0-9]{3}[-/](0[1-9]|1[0-2])[-/](0[1-9]|[12][0-9]|3[0-1])$"
@@ -80,7 +88,7 @@ def string_for_regex(pattern)
         when "^https://([a-zA-Z0-9]([a-zA-Z0-9-]{0,61}[A-Za-z0-9])?\\.)*gov\\.uk(/(([a-zA-Z0-9._~!$&'()*+,;=:@-]|%[0-9a-fA-F]{2})+(/([a-zA-Z0-9._~!$&'()*+,;=:@-]|%[0-9a-fA-F]{2})*)*)?(\\?([a-zA-Z0-9._~!$&'()*+,;=:@-]|%[0-9a-fA-F]{2})*)?(#([a-zA-Z0-9._~!$&'()*+,;=:@-]|%[0-9a-fA-F]{2})*)?)?$"
           govuk_subdomain_url
         when '[a-z0-9\-_]'
-          "#{SecureRandom.hex}-#{SecureRandom.hex}"
+          "#{hex}-#{hex}"
         else
           raise <<-DOC
             Don't know how to generate random string for pattern #{pattern.inspect}
@@ -96,6 +104,18 @@ def string_for_regex(pattern)
           DOC
         end
       end
+
+    private
+
+      def random_letter
+        letters = ("a".."f").to_a
+        letters[rand(0..letters.count - 1)]
+      end
+
+      def random_number
+        numbers = ("0".."9").to_a
+        numbers[rand(0..numbers.count - 1)]
+      end
     end
   end
 end

lib/govuk_schemas/random_example.rb

@@ -24,6 +24,11 @@ class RandomExample
     #     schema = GovukSchemas::Schema.find(frontend_schema: "detailed_guide")
     #     GovukSchemas::RandomExample.new(schema: schema).payload
     #
+    # If you need a 'consistent' random response, you can set the seed using
+    # (for example) `srand(777)` and know that the payload will always be the
+    # same. Note that timestamps in the payload will continue to reflect the
+    # current time.
+    #
     # @param [Hash] schema A JSON schema.
     # @return [GovukSchemas::RandomExample]
     def initialize(schema:)

spec/lib/random_example_spec.rb

@@ -25,6 +25,16 @@
       end
     end
 
+    it "returns the same output if a seed is detected" do
+      schema = GovukSchemas::Schema.random_schema(schema_type: "frontend")
+      srand(777) # these srand calls would be in the upstream application
+      first_payload = GovukSchemas::RandomExample.new(schema: schema).payload
+      srand(777)
+      second_payload = GovukSchemas::RandomExample.new(schema: schema).payload
+
+      expect(first_payload).to eql(second_payload)
+    end
+
     it "can customise the payload" do
       schema = GovukSchemas::Schema.random_schema(schema_type: "frontend")