That's very nice. ++

Here is a less elegant, and slower, solution that uses some of the POSIX functions:

    #!/usr/bin/perl -wl

    use strict;
    use POSIX qw(frexp ldexp);


    sub prev_power2 {

        return 0 if $_[0] == 0;
        my $num          = $_[0];
        my ($mant, $exp) = frexp($num);
        my $sign         = $mant < 0 ? -1 : 1;

        return ldexp(0.5 *$sign, $exp);
    }


    sub next_power2 {

        my $num  = $_[0];
        my $prev = prev_power2($num);
        my $next = $prev;

        $next = $prev *2 if $prev != $num;
        return $next;
    }
[download]

The following seems to be faster than both of the above. It doesn't handle numbers less than 1 but that could be fixed.

    sub prev_power2 {
        return 2 ** int log($_[0]) / log 2;
    }
[download]

Finally, how not to do it: Power Twool

--
John.