Partita IVA (Italian VAT identification number) checksum calculation and validation in Ruby

vat_number.rb

# https://it.wikipedia.org/wiki/Partita_IVA#Struttura_della_partita_IVA
module ChecksumCalculation 
  def calculate(ary)
    (10 - t(ary)) % 10
  end
 
  private

  def t(ary)
    odd, even = separate_odd_even(ary)
    (x(odd) + y(even)) % 10
  end

  def x(odd)
    odd.inject(0){ |sum, x| sum += x }
  end

  def y(even)
    even.inject(0){ |sum, x| sum += process_even(x)}
  end

  def separate_odd_even(ary)
    hash = Hash[*ary]
    return hash.keys, hash.values
  end

  def process_even(n)
    n*2 > 9 ? n*2 - 9 : n*2
  end
end

module ChecksumValidation
  def validate(ary)
    (10 - t(ary)) % 10
  end

  private

  def t(ary)
    odd, even = separate_odd_even(ary)
    (x(odd) + y(even)+ z(even)) % 10
  end

  def x(odd)
    odd.inject(0){ |sum, x| sum += x }
  end

  def y(even)
    even.inject(0){ |sum, x| sum += x*2} 
  end

  def z(even)
    even.count{|x| x >= 5}
  end

  def separate_odd_even(ary)
    hash = Hash[*ary]
    return hash.keys, hash.values
  end
end

class VatNumber
  extend ChecksumValidation

  class << self
    def is_valid?(vat_number)
      return false if !check_length(vat_number)
      validate_checksum(vat_number)
    end

    def check_length(vat_number)
      vat_number.size == 11
    end

    def validate_checksum(vat_number)
      ary = vat_number.split('').map(&:to_i)
      check_value = ary.pop
      validate(ary) == check_value
    end
  end
end