Dear Monks,

I have a table which has two columns: the number and the previous number. There can be more than two items in the chain. I build a hash where the key is the number and the value is the very first item in the chain.

Below is my code by now (a minimal example to play with). It just works, but it is fragile, since it relies on the assumption that there is at most 5 items in the chain. In the real task it should be so, but it is not guaranteed. What would be the better way to do it?

Thanks in advance!

#!/perl
use strict;
use warnings FATAL => qw(all);

use Text::CSV_XS;
use Data::Dump;

my $csv_par = 
{ 
 binary => 1, auto_diag => 1, allow_whitespace => 1,    
 sep_char => ';', eol => $/, quote_char => undef, 
};

my $csv = Text::CSV_XS->new($csv_par);
my @data = @{ $csv->getline_all(*DATA) };
shift @data; # simply throw away the header.

my %main;

my %first_pass;
for my $row ( @data )
{
  $first_pass{$row->[0]} = $row->[1];
  $main{$row->[0]} = $row->[1];
}

my %second_pass;
for my $row (@data)
{
  if ( $first_pass{$row->[1]} )
    {
      $second_pass{$row->[0]} = $row->[1];
      $main{ $row->[0] } = $main{ $row->[1] } || $row->[1];
    }
}

my %third_pass;
for my $row (@data)
{
  if ( $second_pass{$row->[1]} )
  {
    $third_pass{$row->[0]} = $row->[1];
    $main{ $row->[0] } = $main{ $row->[1] } || $row->[1];
  }
}

my %fourth_pass;
for my $row (@data)
{
  if ( $third_pass{$row->[1]} )
  {
    $fourth_pass{$row->[0]} = $row->[1];
    $main{ $row->[0] } = $main{ $row->[1] } || $row->[1];
  }
}

for my $row ( @data )
{
  $main{$row->[1]} = "" 
    unless exists $main{$row->[1]} or $row->[1] eq "";
}

for my $num ( sort {$a <=> $b} keys %main )
{
  $main{$num} = $num unless length($main{$num});  
  print "$num => $main{$num}$/";
}

# Sample chains:

# 123-234-345-456-567
# 117-228-339
# 131
# 213-324-435
# 372

__DATA__
NUM;NUMPRED
567;456
456;345
345;234
234;123
339;228
228;117
131;
435;324
324;213
372;
[download]

117 => 117
123 => 123
131 => 131
213 => 213
228 => 117
234 => 123
324 => 213
339 => 117
345 => 123
372 => 372
435 => 213
456 => 123
567 => 123
[download]