Generate unique items for arrays with the "uniqueItems" property

This re-calculates the values of an array if the "uniqueItems" property
is set and a non-unique item has been generated. This has been to help
resolve flaky tests that expect a random array and occasionally fail due
to the generation of an array with collisions.

As there is a risk that this can produce an eternal loop I've put in a
check that if there are very number of attempts to generate the unique
value then it should abort. This does produce a risk of failure if there
is a low amount of variance in the allowed variables.

This is to resolve issues such as:

```
GovukSchemas::InvalidContentGenerated: An invalid content item was generated.

This probably means there's a bug in the generator that causes it to output
invalid values. Below you'll find the generated payload, the validation errors
and the schema that was used.

...

Validation errors:
--------------------------

[
  {
    "schema": "869a7d30-f94e-5044-84e7-dc3c01c8f1ba#",
    "fragment": "#/details/featured_attachments",
    "message": "The property '#/details/featured_attachments' contained duplicated array values in schema 869a7d30-f94e-5044-84e7-dc3c01c8f1ba#",
    "failed_attribute": "UniqueItems"
  },
  {
    "schema": "869a7d30-f94e-5044-84e7-dc3c01c8f1ba",
    "fragment": "#/details/featured_attachments",
    "message": "The property '#/details/featured_attachments' contained duplicated array values in schema 869a7d30-f94e-5044-84e7-dc3c01c8f1ba",
    "failed_attribute": "UniqueItems"
  }
]
```

lib/govuk_schemas/random_schema_generator.rb

@@ -116,12 +116,27 @@ def generate_random_object(subschema)
     def generate_random_array(props)
       min = props["minItems"] || 0
       max = props["maxItems"] || 10
+      unique = props["uniqueItems"] == true
       num_items = @random.rand(min..max)
+      items = []
+      attempts = 0
+      max_attempts = num_items * 100
 
-      num_items.times.map do
+      until items.length == num_items
         # sometimes arrays don't have `items` specified, not sure if this is a bug
-        generate_value(props["items"] || {})
+        new_value = generate_value(props["items"] || {})
+
+        if unique && items.include?(new_value)
+          attempts += 1
+          raise "Failed to create a unique array item after #{max_attempts} attempts" if attempts >= max_attempts
+          next
+        end
+
+        attempts = 0
+        items << new_value
       end
+
+      items
     end
 
     def generate_random_string(props)

spec/lib/random_schema_generator_spec.rb

@@ -110,5 +110,44 @@
       expect(generator.payload["my_enum"]).to eq("a")
       expect(generator.payload.keys).to include("my_field")
     end
+
+    describe "unique arrays" do
+      it "handles arrays that require unique items" do
+        schema = {
+          "type" => "array",
+          "items" => { "type" => "string" },
+          "uniqueItems" => true,
+          "minItems" => 5,
+          "maxItems" => 5,
+        }
+
+        generator = GovukSchemas::RandomSchemaGenerator.new(schema: schema)
+
+        # These stubs are to ensure determinism in the random array value
+        # generation.
+        allow(generator).to receive(:generate_value).and_call_original
+        allow(generator)
+          .to receive(:generate_value)
+          .with({ "type" => "string" })
+          .and_return(*%w[a a b b c c d d e e])
+
+        expect(generator.payload).to match_array(%w[a b c d e])
+      end
+
+      it "raises an error for a situation where it can't generate a random array" do
+        schema = {
+          "type" => "array",
+          "items" => { "enum" => %w[a b] },
+          "uniqueItems" => true,
+          "minItems" => 3,
+          "maxItems" => 3,
+        }
+
+        generator = GovukSchemas::RandomSchemaGenerator.new(schema: schema)
+
+        expect { generator.payload }
+          .to raise_error "Failed to create a unique array item after 300 attempts"
+      end
+    end
   end
 end