http://www.perlmonks.org?node_id=433913

Before I start, please note this is not strictly a Perl question. It is more of a general algorithms questions, and that's why I posted to Meditations.

A friend of mine has come up with a neat algorithm for creating a hierarchy from a messy bag of keywords. I was wondering if this was something novel or not, so I thought I would ask the Monks if anyone recognizes it.

Another reason I'm posting is simply because I like this algorithm. Whether it's new or not, it's simple and intuitive. It creates hierarchies in a way that seems natural and "right." I would like to hear other people's thoughts about it. Maybe there are some things I am overlooking.

The idea is to start with a list of items that have keywords assigned to them. The keywords are not necessarily chosen with a hierarchy in mind, yet by grouping commonly used keywords, a hierarchy can be synthesized. It goes something like this:

```for each item in items:
for each keyword in item.keywords:
frequency{keyword}++

for each item in items:
sort keywords by frequency
push hierarchy{sorted keywords}, item

Thus, given the following items:

```  item = 1, keywords = a b c
item = 2, keywords = b c
item = 3, keywords = d b

The frequencies would be:

```a = 1, b = 3, c = 2, d = 1

And the hierarchy would end up looking like:

```b/c/2
b/c/a/1
b/d/3

Here's my quick and dirty Perl implementation, so there's something to actually play with:

```my @items;

push @items, { name => 1, keywords => [ qw( b   ) ] },
{ name => 2, keywords => [ qw( b c ) ] },
{ name => 3, keywords => [ qw( b c ) ] },
{ name => 4, keywords => [ qw( b d ) ] },
{ name => 5, keywords => [ qw( e   ) ] },
{ name => 6, keywords => [ qw( e f ) ] };

my %freq;
for (@items) {
for (@{ \$_->{keywords} }) {
\$freq{\$_}++;
}
}

my @nodes;
for (@items) {
my @keywords = sort { \$freq{\$b} <=> \$freq{\$a} }
@{ \$_->{keywords} };
my \$path = join "/", @keywords, \$_->{name};
push @nodes, \$path;
}

print "\$_\n" for sort @nodes