passes method

@override FutureOr<bool> passes(ValidationContext context)

Validates the value using the given context.

Returns true if valid, otherwise false.

Implementation

@override
FutureOr<bool> passes(ValidationContext context) {
  final value = context.value;
  if (value == null || value is! String) return false;

  // Remove all non-digits
  final clean = value.replaceAll(RegExp(r'\D'), '');
  if (clean.isEmpty) return false;

  // Luhn algorithm
  int sum = 0;
  bool alternate = false;
  for (int i = clean.length - 1; i >= 0; i--) {
    int n = int.parse(clean[i]);
    if (alternate) {
      n *= 2;
      if (n > 9) {
        n = (n % 10) + 1;
      }
    }
    sum += n;
    alternate = !alternate;
  }

  return (sum % 10 == 0);
}