kaprekar.pl

#!/usr/bin/env perl

# PRAGMA
use warnings;
use strict;


# DECLARE
sub kaprekar_distance(_);
sub fmt(_);


# CONFIGURE
$, = " ";
$\ = "\n";
our $DEBUG = 0;


# MAIN
if (@ARGV) {
    print @$_ for map {[fmt, kaprekar_distance]} @ARGV;
} else {
    print map kaprekar_distance, 0 .. 9999;
}


# DEFINE
sub fmt(_) { sprintf "%04d", $_[0] }

sub kaprekar_distance(_) {
    my $n = int shift;
    return "X" if $n > 9999;
    return "Z" if $n < 0;
    return  0  if $n == 6174; # Kaprekar's constant

    my $n = fmt $n;
    return "*" if $n =~ /(\d)\1{3}/; # quads make 0 instead of converging to K 

    my $sorted = join "", sort split //, $n;
    my $reversed = reverse $sorted;
    my $new = $reversed - $sorted;
    print STDERR "$reversed - $sorted = $new" if $DEBUG;

    return 1 + kaprekar_distance $new;
}