Nice! But the pedant in me feels compelled to point out that for a 35% increase in length, this obfu can be made significantly more time-efficient:
$m=250;print 2;for($b=3;$b<=$m;$b+=2){if(!$d[$a]){for($a=$b*$b;$a<=$m;
+$a+=$b){$c[$a]=1}}print",$b"}
The speed increase is negligible for low values of $m, but for large value it becomes significant (e.g. an increase of nearly 8 times in speed for $m equal to ten million):
use strict;
use warnings;
use Benchmark qw(:all) ;
my $m = 1e7;
cmpthese
(
5,
{
Op => sub { op(); },
Ath => sub { ath(); },
}
);
sub op
{
my @c;
my $last = 2;
for (2 .. $m)
{
for (my $a = $_ * 2; $a <= $m; $a += $_)
{
$c[$a]++
}
$last = $_ if !$c[$_];
}
print "$last\n";
}
sub ath
{
my @d;
my $last = 2;
for (my $a = 3; $a <= $m; $a += 2)
{
next if $d[$a];
for (my $b = $a * $a; $b <= $m; $b += $a)
{
$d[$b]++
}
$last = $a;
}
print "$last\n";
}
Output:
23:05 >perl 1205_Obfu.pl
9999991
9999991
9999991
9999991
9999991
9999991
9999991
9999991
9999991
9999991
s/iter Op Ath
Op 62.2 -- -87%
Ath 8.02 676% --
23:11 >
But note that this version is still inefficient. See johngg’s recent post: Re^3: Number functions I have lying around, which saves on (1) cpu time by iterating only up to sqrt($m), and (2) memory by storing the sieve in a bit vector.
Hope this is of interest,
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