#!/usr/bin/perl
use strict;
use warnings;
use Algorithm::Loops 'NestedLoops';

# This solution says take all possible pairs of indices
# Then count in base 4, replacing the value at that index

my $input = $ARGV[0] || '000';
my @seed = split //, $input;

# Don't replace two values with exact same two values 
my %seen;

my $idx_iter = combo(2, 0 .. $#seed);
while (my @idx = $idx_iter->()) {
    my $base4_iter = NestedLoops([([0..3]) x 2]);
    while (my @base4 = $base4_iter->()) {
        my @neighbor = @seed;
        @neighbor[@idx] = @base4;
        print "@neighbor\n" if ! $seen{"@neighbor"}++;  
    }
}

# Taken from [id://393064]
sub combo {
    my $by = shift;
    return sub { () } if ! $by || $by =~ /\D/ || @_ < $by;
    my @list = @_;

    my @position = (0 .. $by - 2, $by - 2);
    my @stop     = @list - $by .. $#list;
    my $end_pos  = $#position;
    my $done     = undef;

    return sub {
        return () if $done;
        my $cur = $end_pos;
        {
            if ( ++$position[ $cur ] > $stop[ $cur ] ) {
                $position[ --$cur ]++;
                redo if $position[ $cur ] > $stop[ $cur ];
                my $new_pos = $position[ $cur ];
                @position[ $cur .. $end_pos ] = $new_pos .. $new_pos +
+ $by;
            }
        }
        $done = 1 if $position[0] == $stop[0];
        return @list[ @position ];
    }
}
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