use strict;
use warnings;

# Four digit mechanical lock: no repeated digits, order doesn't matter.
#
#https://math.stackexchange.com/questions/156928/number-of-4-digit-numbers-with-no-repeated-digit
# Start by finding the permutations: For the first choice, you have 10 possible digits to choose from. 
#For the second choice, you have 9 digits because you used one for the first choice. 
#The third choice comes from 8 possibilities and the fourth from 7 possibilities. 
#Now we multiply these together: 10 x 9 x 8 x 7 = 90 x 56 = 5040. That's the number of permutations. 
#No digits repeat, but 0123 is different from 0321.

# Now to find the number of combinations, I have to know how many different ways there are of arranging four digits. 
#That's the same kind of problem: the first position could be from 4 possibilities, the second from 3 possiblities, 
#the third from 2 choices and the last has to be the 1 left. So there are 4 x 3 x 2 x 1 = 24 possible ways of 
#arranging 4 items.

# Therefore I divide 5040 / 24 = 210. So there are 210 different combinations of four digits chosen 
#from 0-9 where the digits don't repeat.

my %output;
foreach(123..9876){
    my $num = sprintf "%04d", $_;
    next if $num =~ /(\d).*\1/;
    my @digits = sort split //, $num;
    my $num_sorted = join '', @digits;
    #print "$num: @digits - $num_sorted\n";
    if (not exists $output{$num_sorted} ) {
        #print "adding $num_sorted\n";
        $output{$num_sorted}=1;
    }
}

print "found ", scalar keys %output, " combinations.\n";
print "$_\n" foreach sort keys %output;