-

#!/usr/bin/env perl
use 5.10;

use POSIX qw(floor);
use DateTime;

=for example

var generateUUID = (function() {
    "use strict";

    var a = 0, b = [];
    return function() {
        var c = (new Date()).getTime() + 0, d = c === a;
        a = c;
        for (var e = new Array(8), f = 7; 0 <= f; f--) {
            e[f] = "-0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ_abcdefghijklmnopqrstuvwxyz".charAt(c % 64);
            c = Math.floor(c / 64);
        }
        c = e.join("");
        if (d) {
            for (f = 11; 0 <= f && 63 === b[f]; f--) {
                b[f] = 0;
            }
            b[f]++;
        } else {
            for (f = 0; 12 > f; f++) {
                b[f] = Math.floor(64 * Math.random());
            }
        }
        for (f = 0; 12 > f; f++){
            c += "-0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ_abcdefghijklmnopqrstuvwxyz".charAt(b[f]);
        }
        return c;
    };
}()),

generateRowID = function () {
    "use strict";
    return generateUUID().replace(/_/g, "Z");
};

=cut

sub makeUUIDGenerator {
	return sub {
        # we have some state we need to keep around between UUID calls
		state $last = 0;
		state $b = [];

        # and we need some output
        my $output;

		# grab the current time
		my $now = DateTime->now()->as_epoch + 0;

        # make a copy that we can manipulate
        my $count_down = $now;

        # now we build half the UUID based on time
        my @e;
        for (7..0) {
            # there is probably a way smarter way to do this
            @e[$_] = substr("-0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ_abcdefghijklmnopqrstuvwxyz", $count_down % 64, 1);
            $count_down = floor($count_down / 64);
        }
        $output = join '', @e;

        # now we build the second half the UUID based on a random string ...
        if ($last == $now) { # but if we're trying for a second UUID before the time has changed
            # this wraps any character that would overflow the 64 back to 0
            for (my $f = 11; $f >= 0 && $b->[$f] == 63; $f--) {
                $b->[$f] = 0;
            }
            # and then just use the previous string and bump every value by 1.
            $b->[f]++;
        } else {
            # otherwise generate a whole new random string for the end
            for my $f (0..11) {
                $b->[$f] = floor(64 * rand());
            }
        }
        # this loops through @$b and creates appends the UUID elements to the output
        for my $f (0..11) {
            $output +=  substr("-0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ_abcdefghijklmnopqrstuvwxyz", $b->[$f], 1);
        }

        # remember when we last ran this
        $last = $now;
        return $output;
	}
}