Add ActiveModel’s Luhn Algorithm validator

lib/luhn_algorithm.rb

@@ -1,4 +1,5 @@
 require 'luhn_algorithm/version'
+require 'luhn_algorithm/validator'
 
 module LuhnAlgorithm
   NUMBER_ONLY = /^\d+$/

lib/luhn_algorithm/validator.rb

@@ -0,0 +1,19 @@
+require 'active_model'
+
+module ActiveModel
+  module Validations
+    # active model validator for Luhn Algorithm
+    class LuhnAlgorithmValidator < ActiveModel::EachValidator
+      def validate_each(record, attribute, value)
+        return if LuhnAlgorithm.valid?(value)
+        record.errors.add(attribute, options[:message] || :invalid)
+      end
+    end
+
+    module HelperMethods
+      def validates_luhn_algorithm_of(*attr_names)
+        validates_with LuhnAlgorithmValidator, _merge_attributes(attr_names)
+      end
+    end
+  end
+end

luhn_algorithm.gemspec

@@ -36,9 +36,10 @@ Gem::Specification.new do |spec|
   spec.executables   = spec.files.grep(%r{^exe/}) { |f| File.basename(f) }
   spec.require_paths = ["lib"]
 
-  spec.add_dependency "activemodel", ">= 3", "<= 6"
+  spec.add_runtime_dependency "activemodel", ">= 3", "<= 6"
 
   spec.add_development_dependency "bundler", "~> 1.17"
   spec.add_development_dependency "rake", "~> 10.0"
   spec.add_development_dependency "rspec", "~> 3.0"
+  spec.add_development_dependency "byebug"
 end

spec/luhn_algorithm/validator_spec.rb

@@ -0,0 +1,20 @@
+class SampleModel
+  include ::ActiveModel::Validations
+
+  attr_accessor :number1, :number2
+
+  validates_luhn_algorithm_of :number1
+  validates :number2, luhn_algorithm: true
+end
+
+RSpec.describe ActiveModel::Validations::LuhnAlgorithmValidator do
+  let(:model) { SampleModel.new }
+
+  context 'validates attributes based on luhn_algorithm' do
+    it 'calls LuhnAlgorithm.valid?' do
+      model.number1 = 6_011_111_111_111_118
+      model.number2 = '4111111111111111'
+      expect(LuhnAlgorithm).to receive(:valid?).and_call_original.twice
+    end
+  end
+end