Add checks for valid GTINs

[ferrisoxide]

GTINs contain basic checksums we can use to verify that the data in the database is actually valid.

We could implement this at either the app or db level, run a script to mark any product with an invalid GTIN.

def valid_gtin?(gtin)
  return false unless gtin.match?(/^\d+$/) && [8, 12, 13, 14].include?(gtin.length)

  digits = gtin.chars.map(&:to_i)
  check_digit = digits.pop

  sum = digits.reverse.each_with_index.sum do |digit, index|
    index.odd? ? digit * 3 : digit
  end

  calculated_check_digit = (10 - sum % 10) % 10
  calculated_check_digit == check_digit
end

puts valid_gtin?("012345678905") # Example usage

or

create or replace function is_valid_gtin(gtin text) returns boolean as $$
declare
    gtin_length int;
    check_digit int;
    gtin_without_check_digit text;
    gtin_reverse text;
    gtin_digits int[];
    i int;
    sum int := 0;
    computed_check_digit int;
begin
    -- remove non-numeric characters
    gtin := regexp_replace(gtin, '\\d', '', 'g');
    gtin_length := length(gtin);

    -- ensure gtin length is valid (8, 12, 13, or 14 digits)
    if gtin_length not in (8, 12, 13, 14) then
      return false;
    end if;

    gtin_without_check_digit := left(gtin, gtin_length - 1);
    check_digit := cast(right(gtin, 1) as int);

    -- reverse the gtin (excluding check digit) and convert to an array of integers
    gtin_reverse := reverse(gtin_without_check_digit);
    gtin_digits := string_to_array(gtin_reverse, null)::int[];

    -- compute the check digit using the luhn algorithm
    for i in 1..array_upper(gtin_digits, 1) loop
      sum := sum + gtin_digits[i] * case i % 2 when 0 then 3 else 1 end;
    end loop;

    computed_check_digit := (10 - sum % 10) % 10;

    -- compare computed check digit with the actual check digit
    return computed_check_digit = check_digit;
end;
$$ language plpgsql;