Test and improve #valid_ulid? method

Co-authored-by: Benjamin Quorning <[email protected]>

lib/ulid/rails/type.rb

@@ -44,15 +44,19 @@ def cast_string_to_ulid(value, data_class: Data)
       end
 
       class Data < ActiveModel::Type::Binary::Data
+        INVALID_CHARACTERS_REGEX = /[ilo]/i.freeze
+
         def self.from_serialized(data)
           deserialized = Base32::Crockford.encode(data.hex).rjust(26, "0")
           new(deserialized)
         end
 
         def self.valid_ulid?(str)
           return true if str.nil?
+          return false unless str.length == 26
+          return false if INVALID_CHARACTERS_REGEX.match?(str)
 
-          str.length == 26 && Base32::Crockford.valid?(str)
+          Base32::Crockford.valid?(str)
         end
 
         def initialize(value)

test/ulid/rails/type_test.rb

@@ -0,0 +1,34 @@
+require "test_helper"
+
+class ULID::Rails::Type::DataTest < Minitest::Test
+  def test_has_26_valid_characters
+    id = "01BX5ZZKBKACTAV9WEVGEMMVRZ"
+
+    assert ULID::Rails::Type::Data.valid_ulid?(id)
+  end
+
+  def test_length_less_than_26
+    id = "A" * 25
+
+    refute ULID::Rails::Type::Data.valid_ulid?(id)
+  end
+
+  def test_length_greater_than_26
+    id = "A" * 27
+
+    refute ULID::Rails::Type::Data.valid_ulid?(id)
+  end
+
+  # Crockford's Base32 is used as shown. This alphabet excludes
+  # the letters I, L, O, and U to avoid confusion and abuse.
+  # https://github.com/ulid/spec#encoding
+  def test_has_invalid_characters
+    invalid_characters = %w(I L O U)
+
+    invalid_characters.each do |char|
+      id = char * 26
+
+      refute ULID::Rails::Type::Data.valid_ulid?(id)
+    end
+  end
+end