Re: How can I tell if a number is a power of 2?

[larryk]

After a session in the CB:

#!perl
use strict;
use warnings;

use Benchmark qw/cmpthese/;

sub one {
    my $number = shift;
    sprintf('%b',$number) =~ /^10*$/ ? 1 : 0;
}

sub two {
    my $number = shift;
    1 == reverse sprintf('%b',$number) ? 1 : 0;
}

sub three {
    my $number = shift;
    1 == substr(sprintf('%b',$number),0,1) ? 1 : 0;
}

my %h = map { $_ ** 2 => 1 } (0..63);

sub jeffa {
    my $number = shift;
    exists $h{$number} ? 1 : 0;
}

sub tye {
    my $x = shift;
    0 == ( $x & ($x-1) ) ? 1 : 0;
}

my %h2 = ();
@h2{map $_**2,0..63}; # word of mouth: by tilly

sub tilly { # really just another impl. of jeffa's lookup
    my $number = shift;
    exists $h2{$number} ? 1 : 0;
}

cmpthese( -5,
    {
        one => q!one(rand 1000000)!,
        two => q!two(rand 1000000)!,
        three => q!three(rand 1000000)!,
        tye => q!tye(rand 1000000)!,
        jeffa => q!jeffa(rand 1000000)!,
        tilly => q!tilly(rand 1000000)!,
    }
);

__END__
Benchmark: running jeffa, one, three, tilly, two, tye, each for at lea
+st 5 CPU seconds...
     jeffa:  6 wallclock secs ( 5.01 usr +  0.00 sys =  5.01 CPU) @ 62
+422.92/s (n=312614)
       one:  5 wallclock secs ( 5.14 usr +  0.00 sys =  5.14 CPU) @ 11
+9667.70/s (n=614733)
     three:  5 wallclock secs ( 5.25 usr +  0.00 sys =  5.25 CPU) @ 90
+412.81/s (n=474396)
     tilly:  6 wallclock secs ( 5.01 usr +  0.00 sys =  5.01 CPU) @ 63
+202.16/s (n=316390)
       two:  6 wallclock secs ( 5.14 usr +  0.00 sys =  5.14 CPU) @ 11
+9035.80/s (n=611725)
       tye:  6 wallclock secs ( 5.25 usr +  0.00 sys =  5.25 CPU) @ 21
+0491.04/s (n=1104657)
          Rate jeffa tilly three   two   one   tye
jeffa  62423/s    --   -1%  -31%  -48%  -48%  -70%
tilly  63202/s    1%    --  -30%  -47%  -47%  -70%
three  90413/s   45%   43%    --  -24%  -24%  -57%
two   119036/s   91%   88%   32%    --   -1%  -43%
one   119668/s   92%   89%   32%    1%    --  -43%
tye   210491/s  237%  233%  133%   77%   76%    --