I recently needed to write a Combinatorial Iterator for a project I'm working on. That is a class which when given a size and a collection of items (normally a set), returns all the possible unique combinations of elements from the collection of the given size. For instance the combinations of size 3 of the set [1,2,3,4] are [1,2,3], [1,2,4], [1,3,4] and [2,3,4]. There is a nice discussion of this problem on StackOverflow. The number of possible combinations can explode in size quickly, there are 2,598,960 combinations of five cards from a standard pack of 52 cards. Thus I wanted to handle each combination in turn as required, rather than calculate all combinations upfront. Having recently started learning Scala and Ruby, I decided to implement my solution in those languages, plus Java (which I use every day at work).
