package LargestSeriesProduct;
use strict;
use warnings;
use List::Util qw( product );
use Exporter qw<import>;
our @EXPORT_OK = qw<largest_product>;

sub largest_product {
  my ($input) = @_;
  die "digits input must only contain digits"
    if ($input->{digits} =~ m/\D+/);
  die "span must be greater than zero" if ($input->{span} < 0);
  my @digits = split //, $input->{digits};
  my $length = scalar @digits;
  die "span must be smaller than string length"
    if ($input->{span} > $length);
  my $max = 0;
  foreach my $i (0 .. $length - $input->{span}) {
    my @substring = @digits[$i .. ($i + $input->{span} - 1)];
    my $product = product @substring;
    $max = $product if ($product > $max);
  }
  return $max;
}

1;