I was recently reading an article in DDJ(April 2001) about suffix arrays. As described in the article, the procedure in C is to 1) fill an array with pointers to every position in a string, 2) sort the array, and 3) search the array for long phrases. This is an extremely fast way to find the longest duplicated substrings. The crux of the technique is to create pointers into a string, which in C is really an array, and avoids the problem of copying or moving lots of substrings in memory.

My question is, how could this be duplicated in Perl? Is there a technique for creating references to parts inside a scalar? I know there is a module for this, but it is simply a frontend to a c program. Can this be done strictly in Perl without using a pile of memory?

I took a shot at doing this, based on the DDJ code, but it is slow as bejeezus. Please Perl Monks, grant me your wisdom!! 

#!/usr/bin/perl -w

use strict;
my $input;

{ local $/=undef; 
    $input=<>; 
}

my @arr=(0 .. (length($input)-2));

    my @arr2=sort {
    makestr($a)
        cmp 
        makestr($b)
    } @arr;

my $maxlen=0;    
my $maxindex=0;
for(my $i=0; $i<(length($input)-2); $i++) {
    my $temp=checklen($arr2[$i], $arr2[$i+1], \$input);
    if ($temp > $maxlen) {
    $maxlen=$temp;
    $maxindex=$i;
    }
}

print "maxlen:$maxlen index:$maxindex string:",substr($input,0,$maxlen
+),"\n";

sub makestr{
    return substr($input, $_[0], (length($input)-$_[0])); 
}

sub checklen{
    my $aa= $_[0];
    my $bb=$_[1];
    my $input=$_[2];
    my $counter=0;
    my $len=length($$input)-1;
    while(($aa < $len)&& ($bb < $len)) {
    last unless (substr($$input, $aa++,1) eq substr($$input, $bb++,1))
+;            
    $counter++;
    }
    return $counter;
}

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In reply to suffix arrays by Anonymous Monk

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