class SiretFormatValidator < ActiveModel::EachValidator def validate_each(record, attribute, value) if !format_is_valid(value) record.errors.add(attribute, :length) elsif !luhn_passed(value) record.errors.add(attribute, :checksum) end end private LA_POSTE_SIREN = '356000000' def format_is_valid(value) value&.match?(/^\d{14}$/) end def luhn_passed(value) # Do not enforce Luhn for La Poste SIRET numbers, the only exception to this rule siret_is_attached_to_la_poste(value) || (luhn_checksum(value) % 10 == 0) end def siret_is_attached_to_la_poste(value) value[0..8] == LA_POSTE_SIREN end def luhn_checksum(value) value.reverse.each_char.map(&:to_i).map.with_index do |digit, index| t = index.even? ? digit : digit * 2 t < 10 ? t : t - 9 end.sum end end