Calculates the digits of a CPF, given the first 9 digits of it.

calculate_cpf_digits.rb

# Usage: `ruby calculate_cpf_digits.rb "123456789"`

cpf = ARGV[0]
cpf_array = cpf.split("").map(&:to_i)

def calculate_digit(base_array:)
  # Constants
  divisor = 11 # Constant, fixed number
  weighted_array = [10, 9, 8, 7, 6, 5, 4, 3, 2] # Digits, starts with 10 and decrements by 1
  # Algorithm
  weighted_sum = 0
  (0...9).each do |index|
    weighted_sum += base_array[index] * weighted_array[index]
  end
  remainder = weighted_sum % divisor
  case remainder
  when 0, 1
    0
  else
    divisor - remainder
  end
end

def calculate_first_digit(cpf_array:)
  calculate_digit(base_array: cpf_array)
end

def calculate_second_digit(cpf_array:, first_digit:)
  second_digit_array = cpf_array.drop(1) + [first_digit]
  calculate_digit(base_array: second_digit_array)
end

first_digit = calculate_first_digit(cpf_array: cpf_array)
second_digit = calculate_second_digit(cpf_array: cpf_array, first_digit: first_digit)
puts("CPF: #{cpf}-#{first_digit}#{second_digit}")