A generic groupby function that should work similarily to ruby's

GroupBy.swift

/**
Group all items in a collection according to the value returned by a block.

:param: collection The collection whose items you want to group.
:param: groupBlock A block that returns a key for that value to be grouped by.

:returns: Returns a dictionary whose keys are the values returned by the groupBlock.
*/
func groupBy<V, K : protocol<Hashable, Equatable>, C : _ExtensibleCollectionType where C.Generator.Element == V>(collection: C, groupBlock: (V) -> K) -> [K: [V]] {
	typealias ValueGroup = [V]
	
	var grouped = [K: ValueGroup]()
	
	for value in collection {
		var key = groupBlock(value)
		var group = grouped[key]
		
		if let existingGroup = group {
			group = existingGroup + [value]
		}
		else {
			group = [value]
		}
		
		grouped[key] = group
	}
	
	return grouped
}