#!/usr/bin/perl
use strict;
use warnings;

# Seed can be longer than 3 digits but always 0,1,2,3
my $seed = $ARGV[0] || "000";
if ($seed =~ /[^0-3]/) {
  die "Invalid seed.  Seed may only contain digits from 0 through 3.\n";
}

my $max_differences = 2;

frobnicate($seed);

sub frobnicate {
  my ($prefix, $suffix, $difference_count) = @_;
  unless (defined $suffix) {
    $suffix = $prefix;
    $prefix = '';
  }

  $difference_count++;

  while (length $suffix) {
    my $initial = substr($suffix, 0, 1);
    substr($suffix, 0, 1) = '';

    for my $substitute (0..3) {
      next if $substitute == $initial;

      print "\t" unless $difference_count == 1;
      print $prefix, $substitute, $suffix;

      if ($difference_count < $max_differences && length $suffix) {
        frobnicate($prefix . $substitute, $suffix, $difference_count);
      }

      print "\n" if $difference_count == 1;
    }
    $prefix .= $initial;
  }
}