I was just playing with this for funsies after seeing a youtube video from Vi Hart about Ulam's Spiral. http://www.youtube.com/embed/Yhlv5Aeuo_k

So, I wrote the following spiral generator that writes the output to a SVG file. It takes about 10 seconds on my laptop to generate a graph of all primes between 1 and 10,000,000.

I see now that lot's of other people have taken a crack at this, but TMTOWTDI :)

#!/usr/bin/perl

my $x = 1;
my $y = 1;

use Math::Prime::XS qw(is_prime);

my $maxcount = 1000000;

open(WRT,">primegrid.svg");

print WRT q(<?xml version="1.0" encoding="UTF-8" standalone="no"?>
<!-- Created with Inkscape (http://www.inkscape.org/) -->

<svg
   xmlns:dc="http://purl.org/dc/elements/1.1/"
   xmlns:cc="http://creativecommons.org/ns#"
   xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
   xmlns:svg="http://www.w3.org/2000/svg"
   xmlns="http://www.w3.org/2000/svg"
   xmlns:sodipodi="http://sodipodi.sourceforge.net/DTD/sodipodi-0.dtd"
   xmlns:inkscape="http://www.inkscape.org/namespaces/inkscape"
   width="1"
   height="1"
   id="svg2"
   version="1.1"
   inkscape:version="0.48.3.1 r9886"
   sodipodi:docname="New document 1">
  <defs
     id="defs4" />
  <sodipodi:namedview
     id="base"
     pagecolor="#ffffff"
     bordercolor="#666666"
     borderopacity="1.0"
     inkscape:pageopacity="1"
     inkscape:pageshadow="2"
     inkscape:zoom="42.250643"
     inkscape:cx="8.8460039"
     inkscape:cy="2.865365"
     inkscape:document-units="px"
     inkscape:current-layer="layer1"
     showgrid="false"
     inkscape:window-width="1073"
     inkscape:window-height="740"
     inkscape:window-x="0"
     inkscape:window-y="0"
     inkscape:window-maximized="0" />
  <metadata
     id="metadata7">
    <rdf:RDF>
      <cc:Work
         rdf:about="">
        <dc:format>image/svg+xml</dc:format>
        <dc:type
           rdf:resource="http://purl.org/dc/dcmitype/StillImage" />
        <dc:title></dc:title>
      </cc:Work>
    </rdf:RDF>
  </metadata>);

my $gridholder;
my $direction;

foreach my $number ( 1 ... $maxcount ){
    print "$number\r";
    if($direction eq ""){
        $gridholder->{$x}->{$y} = $number;
        markprime($x,$y,$number);
        $direction = 'r';
        next;
    }
    if($direction eq 'r'){
        $x++;
        $gridholder->{$x}->{$y} = $number;
        markprime($x,$y,$number);
        my $yp = $y + 1;
        if($gridholder->{$x}->{$yp} eq ""){
            $direction = 'd';
        }
        next; 
    }
    if($direction eq 'd'){
                $y++;
                $gridholder->{$x}->{$y} = $number;
                markprime($x,$y,$number);
                my $xp = $x - 1;
                if($gridholder->{$xp}->{$y} eq ""){
                        $direction = 'l'; 
                }
                next;
        }
    if($direction eq 'l'){
        $x--;
        $gridholder->{$x}->{$y} = $number;
        markprime($x,$y,$number);
        my $yp = $y - 1;
        if($gridholder->{$x}->{$yp} eq ""){
            $direction = 'u';
        }
        next; 
    }
    if($direction eq 'u'){
                $y--;
                $gridholder->{$x}->{$y} = $number;
                markprime($x,$y,$number);
                my $xp = $x + 1;
                if($gridholder->{$xp}->{$y} eq ""){
                        $direction = 'r'; 
                }
                next;
        }

}


sub markprime {
    my ($x, $y, $number) = @_;

    if(is_prime($number)){
print WRT qq(    <rect
       style="fill:#000000;fill-opacity:1;stroke:none"
       id="rect2985"
       width="1"
       height="1"
       x="$x"
       y="$y"
       ry="0" />
);
    }
    return();
}

print WRT q(
</svg>);

print "done\n";
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