I guess this would be the hash approach you speak of: 
#!/cadappl/bin/perl -w -Ilib

use strict;
use Data::Dumper;

my @r = ( 
  [ 5, 3, 5, 5 ],
  [ 1, 1, 2, 1 ],
  [ 2, 1, 2, 3 ],
  [ 5, 5, 1, 5 ],
  [ 1, 5, 1, 1 ],
  [ 2, 3, 5, 3 ]
);

my %lineTo;
foreach my $ref (@r)
{
        $lineTo{"$$ref[0],$$ref[1]"} = "$$ref[2],$$ref[3]";
}
my $start = "1,1";
my $curr = $start;
print "$curr\n";
while (($curr = $lineTo{$curr}) ne $start)
{
        print "$curr\n";
}

[download]
Output: 
1,1
2,1
2,3
5,3
5,5
1,5

[download]

In reply to Re: reordering segments to form a polygon by Jaap
in thread reordering segments to form a polygon by dada

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