Re: Re (tilly) 1: Power of two round up.


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Re: Re (tilly) 1: Power of two round up.

by japhy (Canon)

on Dec 15, 2000 at 23:31 UTC ( [id://46896]=note: print w/replies, xml )

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in reply to Re (tilly) 1: Power of two round up.
in thread Power of two round up.

And an even faster WTDI:

sub next_pwr {
  my ($x,$p) = (@_,2);  # default to next_pwr(X,2)
  my $log = log($x)/log($p);
  $log = int($log+1) if $log != int($log);
  return $p**$log;
}
[download]

You can even add a log() memoization in there:

{
  my %LOG = (2 => log(2));  # most common
  sub next_pwr {
    my ($x,$p) = (@_,2);  # default to next_pwr(X,2)

    # assertions, etc.
    die "illegal base: $p" if $p < 2;
    return 1 if $x == 1;
    $p = int $p;

    my $log =
      ($LOG{$x} ||= log($x)) / ($LOG{$p} ||= log($p));
    $log = int($log+1) if $log != int($log);

    return $p ** $log;
  }
}
[download]

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