extension Collection {

    func groupBy<GroupingType: Hashable>(key: (Element) -> (GroupingType)) -> [[Element]] {
        var groups: [GroupingType: [Element]] = [:]
        var groupsOrder: [GroupingType] = []

        forEach { element in
            let key = key(element)

            if case nil = groups[key]?.append(element) {
                groups[key] = [element]
                groupsOrder.append(key)
            }
        }

        return groupsOrder.map { groups[$0] ?? [] }
    }

}