#!/usr/bin/perl -l

# http://perlmonks.org/?node_id=1188292

use strict;
use warnings;

my $n = shift // 3;  # alphabet size
my $t = shift // $n; # size of tuples

my @alphabet = ('A'..'Z', '0'..'9', 'a'..'z');
my %next;
@next{'', @alphabet} = @alphabet;

$_ = $alphabet[0] x $t; # start of string
my $over = $alphabet[$n]; # outside of alphabet
my $wantedlength = $n ** $t + $t - 1;

while( length $_ >= $t )
  {
  if( /^(?=(.{$t})).+\1/ ) { s/^./$next{$&}/; }
  elsif( s/$over(.)/$next{$1}/ ) { }
  elsif( $wantedlength == length $_ )
    {
    print;
    # prove it is correct
    my %all;
    $all{$1}++ while /(?=(.{$t}))/g;
    my $count = keys %all;
    print "want @{[ $n ** $t ]} unique strings, have $count";
    die "duplicate string" if grep $_ > 1, values %all;
    exit;
    }
  else { $_ = $alphabet[0] . $_; }
  }