exercise: Luhn
version: 2
plan: 16
subs: is_luhn_valid
tests: |-
  is $subs{is_luhn_valid}->($_->{input}), $_->{expected}, $_->{description} foreach @{$C_DATA->{cases}};

example: |-
  sub is_luhn_valid {
    my ($input) = @_;
    $input =~ s/\s+//g;
    if (length $input < 2 || $input =~ /\D+/) {return 0}

    my @num = split(//, $input);
    unshift @num, 0 if scalar @num % 2;

    my $count = 1;
    my $sum = 0;

    foreach (@num) {
      if ($count % 2) {
        $sum += $_ * 2;
        $sum -= 9 if $_ * 2 > 9;
      }
      else {
        $sum += $_;
      }
      $count += 1;
    }

    if ($sum % 10) {return 0};
    return 1;
  }

stub: |-
  sub is_luhn_valid {
    my ($input) = @_;
  }