An alternative name for web spider - a program which automatically fetches WWW pages, parses them for some information and links and then proceeds to these links.

Below is code to one such crawler, written in everybody's favourite chainsaw. It will start from arbitrary address, or if none provided, from first hit in google search of random dictionary word. There it will proceed to pick a random link it hasn't visited yet, always choosing an off-site link in preference to in-site link, move there, repeat, crawl back if it runs out of links and summon up a new dictionary search if it runs out of link history.

The code is pretty crufty, but it'll give you the idea. Besides, this code in itself is useful for amusing things; I made it this way for the reason of collecting almost-random site addresses, for the "today's link" part of my dynamic plan generator for batmud. Also available in www.iki.fi/kaatunut/webcrawl.pl where applicable.

#!/usr/bin/perl -w -s
# webcrawler.pl - crawls the web in single thread, collecting URLs and nothing more

# Parameters:

# -start=address        set the starting point for crawl
# -output=file          print all found valid addresses to file instead of stdout
# -report=file          print progress report to file instead of stderr
# -quiet                don't print progress reports

use HTML::TreeBuilder;
use LWP;
use IO::Socket;

@dictionary=`cat /usr/dict/words`;
$linkstackmax=10;
%approved_ext=(
    ''          => '',
    html        => '',
    htm         => '',
    shtml       => '',
    php         => '',
    php3        => '',
    asp         => '',
);


use vars qw($start $output $quiet $report);
select REPORT;
$|++;
select STDOUT;
$ua=new LWP::UserAgent;
$ua->agent("Gol " . $ua->agent);

if (defined $output) {
    open(OUTPUT,">$output") or die "can't open $output for writing!";
} else {
    open(OUTPUT,">&1") or die "can't redirect stdout to output!";
}
if (!$quiet) {
    if ($report) {
        open(REPORT,">$report") or die "can't open $report for writing!";
    } else {
        open(REPORT,">&2") or die "can't redirect stderr to report!";
    }
}

while (1) {
    $h=$start || getroot();
    print REPORT "**$h\n" unless $quiet;
    while (1) {
        if (@lt=get_links($h)) {
            push @link_history,[ @lt ];
            shift @link_history if @link_history>$linkstackmax;
            undef $visited{$h};
            print REPORT "$h\n";
        }
        last unless @link_history;
        $h=splice @{$link_history[$#link_history]},
         int(rand(@{$link_history[$#link_history]})),1;
        unless (@{$link_history[$#link_history]}) {
            print REPORT "<--\n" unless $quiet;
            pop @link_history;
        }
    }
}

sub getroot {
    my $word;
    chomp($word=$dictionary[int rand @dictionary]);
    print REPORT "word: $word\n" unless $quiet;

    my $sock=new IO::Socket::INET(PeerAddr=>"www.google.com",PeerPort=>80,Proto=>"tcp");
    die "sock error: $!" unless $sock;
    
    print $sock "GET /search?q=$word&btnI= HTTP/1.0\r\n\r\n";
    while (<$sock>) {
        if (/^Location: (.*?)\r?\n/) {
            return $1;
        }
    }
}

sub get_links {
    my $link=shift;
    my($proto,$host,$dir)=$link =~ m!^(\w+):(?://)?([^/]*)((?:/.*?)?)[^/]*$!;
    $dir.="/" unless $dir =~ /\/$/;
    my @links;
    my @links2;
    my $i;
    my $tree=new HTML::TreeBuilder;
    my $req=new HTTP::Request GET => $link;
    print REPORT "$link ->" unless $quiet;
    
    my $res= $ua->request($req);
    print REPORT "\r".(" " x length "$link ->")."\r" unless $quiet;
    return () unless $res->is_success;

    $tree->parse($res->content);
    
    @links=
# sort same-host links first
           sort { (($b =~ m!^\w+://([^/]*)!)[0] eq $host &&
                   ($a =~ m!^\w+://([^/]*)!)[0] ne $host) ?  1 : 
                  (($b =~ m!^\w+://([^/]*)!)[0] ne $host &&
                   ($a =~ m!^\w+://([^/]*)!)[0] eq $host) ? -1 : ($a cmp $b) }
           grep { $_ ne $link and !exists $visited{$_} }
# kill ".."
           map { while (s![^/]*/\.\./!!) { } $_ }
# handle local links
           map { (/^\w+:(?:\/\/)?/ ? "" : "$proto://$host".(/^\// ? "" : "$dir")).$_ }
# no CGI queries, only "approved" types
           grep { !/\?/ && exists $approved_ext{(m!.([^/.]*)$!)[0] || ""} }
           grep { ((/^(\w+):(?:\/\/)?/)[0] || "http") eq "http" }
# eliminate #, convert from href
           map { (($_->attr('_tag') eq "a"
                   ? $_->attr('href')
                   : $_->attr('src')) =~ /^([^#]*)/)[0] }
# need link, only http protocol (<frame src=...> considered a link)
           grep { $_->attr('href') or $_->attr('src') }
           $tree->look_down( sub {
               $_[0]->attr('_tag') eq "a"
            or $_[0]->attr('_tag') eq "frame" } );
    for ($i=1;$i<@links;++$i) {
        splice @links,$i--,1 if $links[$i] eq $links[$i-1];
    }
    my $split;
    for ($split=0; $split<@links && $links[$split] =~ m!http://$host!; ++$split) { }
    @links2=splice @links,$split;

    return @links2;
}

Web crawler is also a euphamism used often in the comic books The Amazing Spider Man and Peter Parker to describe the hero Spider Man. Used often by J. Jonah Jameson (his boss) and myriads of foes, sometimes shortened to Webby.

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