group.swift

public extension Sequence {
    func group<U: Hashable>(by key: (Iterator.Element) -> U?) -> [U:[Iterator.Element]] {
        var categories: [U: [Iterator.Element]] = [:]
        for element in self {
            let key = key(element)
            if case nil = categories[key!]?.append(element) {
                categories[key!] = [element]
            }
        }
        return categories
    }
}