module FFaker::PhoneNumber

def imei(serial_number = nil)
  # IMEI Format:
  # AA-BBBBBB-CCCCCC-D
  characters = Array.new(15, 0)
  rbi_codes  = %w[01 10 30 33 35 44 45 49 50 51 52 53 54 86 91 98 99]
  serial_number ||= rand(0...1_000_000)
  serial_number = format('%06d', serial_number).chars.map(&:to_i)
  first_two_chars = fetch_sample(rbi_codes)
  characters[0, 2] = first_two_chars.chars.map(&:to_i)
  # serial number part
  2.upto(7) do |current_position|
    characters[current_position] = fetch_sample((0..9).to_a)
  end
  8.upto(13) do |current_position|
    characters[current_position] = serial_number[current_position - 8]
  end
  current_checksum = characters.reverse.each_with_index.inject(0) do |sum, (digit, i)|
    digit *= 2 if i.odd?
    digit -= 9 if digit > 9
    sum + digit
  end
  final_digit = (10 - (current_checksum % 10)) % 10
  characters[14] = final_digit
  characters.join
end