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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,

Athanasius <°(((><contra mundum Iustus alius egestas vitae, eros Piratica,


In reply to Re: This One's a Sieve by Athanasius
in thread This One's a Sieve by Schmunzie

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