New function cartesian() for calculating cartesian products.

This function helps in defining iptables, where one may want
to create rules for e.g. several IP addresses, ports and/or
protocols, but otherwise similar.

This was originally commit 946039108a47 in nsc-puppet-utils.

plugins/puppet/parser/functions/cartesian.rb

+module Puppet::Parser::Functions
+    newfunction(:cartesian, :type => :rvalue, :doc => "\
+        Return the cartesian product of the arrays passed as arguments.
+
+        This can be used to create multiple variants of a string.  It
+        works similar to the {} construct in some shells.
+
+        Example::
+
+            $in  = [ 'eth0', 'eth3' ]
+            $out = [ 'eth2', 'eth7' ]
+            $cmd = cartesian('iptables -i ', $in, ' -o ', $out, ' -j REJECT')
+
+        $cmd now holds the list::
+
+            [ 'iptables -i eth0 -o eth2 -j REJECT',
+              'iptables -i eth0 -o eth7 -j REJECT',
+              'iptables -i eth3 -o eth2 -j REJECT',
+              'iptables -i eth3 -o eth7 -j REJECT' ]
+
+        A simple string as an argument is equivalent to a one element
+        long list of that string.
+
+        If any of the arguments is an empty list, the return value will
+        be an empty list.
+    ") \
+    do |args|
+        result = [ "" ]
+        args.each do |a|
+            if not a.is_a?(Array)
+                a = [a]
+            end
+            newres = []
+            result.each do |p1|
+                a.each do |p2|
+                    newres << p1+p2
+                end
+            end
+            result = newres
+        end
+        return result
+    end
+end