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I would look to implement a structure with separate implementations for the three cases:

1. IBAN13
2. IBAN10
3. Neither of the above

You can provide that with something along these lines, in which IBAN#new is overridden to detect the number of integers provided, and to initialise the correct object to process it.

The real trick here is to provide the Invalid module, which responds to everything that you expect the Thirteen and Ten objects to respond to, in an implementation similar to a Nil Object pattern – in this case an Invalid Object.

I've omitted the implementation of the #valid? etc classes, but would point you to the Lisbn gem in which we have implemented an extremely fast algorithm https://github.com/ragalie/lisbn/blob/master/lib/lisbn/lisbn.rb#L111. Benchmarked against Enumerable-based methods, which often look quite elegant, this algorithm is incredibly fast – 40x faster than the same logic implemented in some ISBN gems.

Here's a code outline:

module IBAN
 def self.new(string)
 numeric = string.gsub(/[^0-9]/,"")
 case numeric.size
 when 13
 then Thirteen.new(string)
 when 10
 then Ten.new(string)
 else
 Invalid
 end
 end
 class Thirteen
 def initialize(number)
 @number = number
 end
 def checksum
 # TODO
 end
 def valid?
 # TODO
 end
 attr_reader :number
 end
 class Ten
 def initialize(number)
 @number = number
 end
 def checksum
 # TODO
 end
 def valid?
 # TODO
 end
 attr_reader :number
 end
 module Invalid
 def self.valid?
 false
 end
 def self.checksum
 nil
 end
 end
end
 

ps. You might prefer to implement Invalid as a singleton instead of as a module.

I would look to implement a structure with separate implementations for the three cases:

1. IBAN13
2. IBAN10
3. Neither of the above

You can provide that with something along these lines, in which IBAN#new is overridden to detect the number of integers provided, and to initialise the correct object to process it.

The real trick here is to provide the Invalid module, which responds to everything that you expect the Thirteen and Ten objects to respond to, in an implementation similar to a Nil Object pattern – in this case an Invalid Object.

I've omitted the implementation of the #valid? etc classes, but would point you to the Lisbn gem in which we have implemented an extremely fast algorithm https://github.com/ragalie/lisbn/blob/master/lib/lisbn/lisbn.rb#L111. Benchmarked against Enumerable-based methods, which often look quite elegant, this algorithm is incredibly fast – 40x faster than the same logic implemented in some ISBN gems.

Here's a code outline:

module IBAN
 def self.new(string)
 numeric = string.gsub(/[^0-9]/,"")
 case numeric.size
 when 13
 then Thirteen.new(string)
 when 10
 then Ten.new(string)
 else
 Invalid
 end
 end
 class Thirteen
 def initialize(number)
 @number = number
 end
 def checksum
 # TODO
 end
 def valid?
 # TODO
 end
 attr_reader :number
 end
 class Ten
 def initialize(number)
 @number = number
 end
 def checksum
 # TODO
 end
 def valid?
 # TODO
 end
 attr_reader :number
 end
 module Invalid
 def self.valid?
 false
 end
 def self.checksum
 nil
 end
 end
end
 

ps. You might prefer to implement Invalid as a singleton instead of as a module.

 ...
 else
 Invalid.instance
 end
 ...
 class Invalid
 include Singleton
 def valid?
 false
 end
 def checksum
 nil
 end
 end
end