NHS: fix occasional bad numbers (faker-ruby#1425)

* Add additional tests around NHS number edge cases

* Add code to handle check digit 10 and 11 edge cases

lib/faker/nhs.rb

@@ -4,7 +4,10 @@ module Faker
   class NationalHealthService < Base
     class << self
       def british_number
-        base_number = rand(400_000_000...499_999_999)
+        base_number = rand(400_000_001...499_999_999)
+        # If the check digit is equivalent to 10, the number is invalid.
+        # See https://en.wikipedia.org/wiki/NHS_number
+        base_number -= 1 if check_digit(base_number) == 10
         "#{base_number}#{check_digit(base_number)}".to_s
                                                    .chars
                                                    .insert(3, ' ')
@@ -18,7 +21,13 @@ def check_digit(number = 0)
           position = idx + 1
           sum += (digit.to_i * (11 - position))
         end
-        11 - (sum % 11)
+        result = 11 - (sum % 11)
+
+        # A value of 11 is considered the same as 0
+        # See https://en.wikipedia.org/wiki/NHS_number
+        return 0 if result == 11
+
+        result
       end
     end
   end

test/test_faker_national_health_service.rb

@@ -8,12 +8,20 @@ def setup
   end
 
   def test_nhs_british_number
-    omit 'fix me'
-
     assert_match(/\A\d{3}\s\d{3}\s\d{4}\z/, @tester.british_number)
   end
 
+  def test_nhs_check_digit_equals_10
+    Faker::NationalHealthService.stub(:rand, 458_617_434) do
+      assert_match('458 617 4331', @tester.british_number)
+    end
+  end
+
   def test_nhs_check_digit
     assert_equal 6, @tester.check_digit(400_012_114)
   end
+
+  def test_nhs_check_digit_11
+    assert_equal 0, @tester.check_digit(418_513_625)
+  end
 end