categorise.swift

public extension SequenceType {
    
    /// Categorises elements of self into a dictionary, with the keys given by keyFunc
    
    func categorise<U : Hashable>(@noescape keyFunc: Generator.Element -> U) -> [U:[Generator.Element]] {
        var dict: [U:[Generator.Element]] = [:]
        for el in self {
            let key = keyFunc(el)
            if case nil = dict[key]?.append(el) { dict[key] = [el] }
        }
        return dict
    }
}