#! perl
use strict;
use warnings;
use autodie;

die "To use me, give me the word list file as argument\n"
    unless defined $ARGV[0];

my %letter_lines;
my $evt;
my $niter = 0;

for (my $cycled = 0;
       !$cycled;
        warn "going back to the beginning\n" unless $cycled)
{
    open my $fh, '<', $ARGV[0];

    FILE_ITER:
    while (my $word = <$fh>)
    {
        my @letters = split '', $word;
        ++$niter;

        if (defined $evt)
        {
            WORD_ITER:
            for my $i (0 .. $#letters)
            {
                if ($evt eq $letters[$i])
                {
                    my $j = $i + 1;
                    $evt  = $i < $#letters ? $letters[$j] : undef;

                    if (defined $evt)
                    {
                        if (defined $letter_lines{$.}{$j})
                        {
                            warn "we have cycled: line $. letter $j\n";
                            $cycled = 1;
                            last FILE_ITER;
                        }

                        warn "adding to hash line $. letter $j\n";
                        $letter_lines{$.}{$j} = 1;
                        print $evt;
                    }

                    last WORD_ITER;
                }
            }
        }
        elsif (defined $letter_lines{$.}{0})
        {
            warn "we have cycled: doing line $. letter 0 again\n";
            $cycled = 1;
            last FILE_ITER;
        }
        else
        {
            $letter_lines{$.}{0} = 1;
            warn "adding to hash line $. letter 0\n";
            $evt = shift @letters;
            print $evt;
        }
    }

    close $fh;
}

warn "cycled in $niter iterations\n";