Glen Solsberry Glen Solsberry - 7 months ago 9
Perl Question

How do I determine the longest similar portion of several strings?

As per the title, I'm trying to find a way to programmatically determine the longest portion of similarity between several strings.

Example:


  • file:///home/gms8994/Music/t.A.T.u./

  • file:///home/gms8994/Music/nina%20sky/

  • file:///home/gms8994/Music/A%20Perfect%20Circle/



Ideally, I'd get back
file:///home/gms8994/Music/
, because that's the longest portion that's common for all 3 strings.

Specifically, I'm looking for a Perl solution, but a solution in any language (or even pseudo-language) would suffice.

From the comments: yes, only at the beginning; but there is the possibility of having some other entry in the list, which would be ignored for this question.

Answer

Edit: I'm sorry for mistake. My pity that I overseen that using my variable inside countit(x, q{}) is big mistake. This string is evaluated inside Benchmark module and @str was empty there. This solution is not as fast as I presented. See correction below. I'm sorry again.

Perl can be fast:

use strict;
use warnings;

package LCP;

sub LCP {
    return '' unless @_;
    return $_[0] if @_ == 1;
    my $i          = 0;
    my $first      = shift;
    my $min_length = length($first);
    foreach (@_) {
    	$min_length = length($_) if length($_) < $min_length;
    }
INDEX: foreach my $ch ( split //, $first ) {
    	last INDEX unless $i < $min_length;
    	foreach my $string (@_) {
    		last INDEX if substr($string, $i, 1) ne $ch;
    	}
    }
    continue { $i++ }
    return substr $first, 0, $i;
}

# Roy's implementation
sub LCP2 {
    return '' unless @_;
    my $prefix = shift;
    for (@_) {
        chop $prefix while (! /^\Q$prefix\E/);
        }
    return $prefix;
}

1;

Test suite:

#!/usr/bin/env perl

use strict;
use warnings;

Test::LCP->runtests;

package Test::LCP;

use base 'Test::Class';
use Test::More;
use Benchmark qw(:all :hireswallclock);

sub test_use : Test(startup => 1) {
    use_ok('LCP');
}

sub test_lcp : Test(6) {
    is( LCP::LCP(),      '',    'Without parameters' );
    is( LCP::LCP('abc'), 'abc', 'One parameter' );
    is( LCP::LCP( 'abc', 'xyz' ), '', 'None of common prefix' );
    is( LCP::LCP( 'abcdefgh', ('abcdefgh') x 15, 'abcdxyz' ),
    	'abcd', 'Some common prefix' );
    my @str = map { chomp; $_ } <DATA>;
    is( LCP::LCP(@str),
    	'file:///home/gms8994/Music/', 'Test data prefix' );
    is( LCP::LCP2(@str),
    	'file:///home/gms8994/Music/', 'Test data prefix by LCP2' );
    my $t = countit( 1, sub{LCP::LCP(@str)} );
    diag("LCP: ${\($t->iters)} iterations took ${\(timestr($t))}");
    $t = countit( 1, sub{LCP::LCP2(@str)} );
    diag("LCP2: ${\($t->iters)} iterations took ${\(timestr($t))}");
}

__DATA__
file:///home/gms8994/Music/t.A.T.u./
file:///home/gms8994/Music/nina%20sky/
file:///home/gms8994/Music/A%20Perfect%20Circle/

Test suite result:

1..7
ok 1 - use LCP;
ok 2 - Without parameters
ok 3 - One parameter
ok 4 - None of common prefix
ok 5 - Some common prefix
ok 6 - Test data prefix
ok 7 - Test data prefix by LCP2
# LCP: 22635 iterations took 1.09948 wallclock secs ( 1.09 usr +  0.00 sys =  1.09 CPU) @ 20766.06/s (n=22635)
# LCP2: 17919 iterations took 1.06787 wallclock secs ( 1.07 usr +  0.00 sys =  1.07 CPU) @ 16746.73/s (n=17919)

That means that pure Perl solution using substr is about 20% faster than Roy's solution at your test case and one prefix finding takes about 50us. There is not necessary using XS unless your data or performance expectations are bigger.