Glen Solsberry - 1 year ago 45
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.

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.

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