http://qs321.pair.com?node_id=11118304

Thanks for the nice script !

The example should just illustrate the problem. The strings in the list can be completely different and your solution will only work with this specific example.

There even might exists a better solution (with a lower score (\$len)) for this example as the given one. I am looking for an idea how to come to a good and fast solution, accepting that this solution is not the best one, which can be found. (See also the answer of Bill)

• Comment on Re^2: Divide a list of string into substrings (updated)

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Re^3: Divide a list of string into substrings (updated)
by tybalt89 (Prior) on Jun 21, 2020 at 16:16 UTC

I suspect there is no "good and fast" solution. This problem, to me, has the flavor of a permutation or "traveling salesman" problem, and is probably NP-hard (or one of the other NP classes).

So here's an attempt (with a cheat) that at least gets a solution in a sort of reasonable time for this problem.

```#!/usr/bin/perl

use strict; # https://perlmonks.org/?node_id=11118281
use warnings;

my @list=("set abcde-efghi 12345",
"set abcde-ijkl 12345",
"clr abcde-efghi+123",
"clr abcde-ijkl 12345");

#my @expected_substrings=("set","clr"," abcde-","efghi",
#                         "ijkl"," 12345","+123");
#my \$len=@expected_substrings*2;
#\$len+=length(\$_) foreach @expected_substrings;

\$_ = join "\n", @list;
my \$max = 3; ########################################### BIG CHEAT FOR
+ RUNTIME

sub score { 2 * @_ + length join '', @_; }
my \$best = score( @list );

try( \$_ );
print "\n";

sub try
{
(local \$_, my @sofar) = @_;
if( !/[ -~]/ )
{
my \$score = score @sofar;
if( \$score < \$best )
{
print "\n";
use Data::Dump 'dd'; dd \$score, @sofar;
\$best = \$score;
}
return;
}
score(@sofar) >= \$best and return;
for my \$n ( reverse 0 .. \$#list )
{
my %d;
/([ -~]{3,})(?:.*?\1){\$n}(?{ \$d{\$1}++ })(*FAIL)/s;
my @d = sort { length \$b <=> length \$a } sort keys %d;
@d > \$max and \$#d = \$max - 1;
for my \$string ( @d )
{
try( s/\Q\$string\E/\t/gr, @sofar, \$string );
}
}
}
[download]```

Outputs:

```(46, " abcde-", " 12345", "efghi", "ijkl", "clr", "set", "+123")
[download]```

where the "46" is the score and the rest are the substrings. As it finds better scores, it will print them, but so far always seems to find a best solution first.

Many thanks for this nice solution.

Especially I like the way you search for equal substrings: /([ -~]{3,})(?:.*?\1){\$n}(?{ \$d{\$1}++})(*FAIL)/s;

I will use a lot of your ideas in my solution ...