Urn.swift

public struct Urn {

    public enum Namespace : String {
        case SoundCloud = "soundcloud"
    }

    public enum Collection : String {
        case Playlists = "playlists"
        case Tracks = "tracks"
        case Users = "users"
        case TrackStations = "track-stations"
    }

    // MARK: -

    public let namespace:Namespace
    public let collection:Collection
    public let identifier:UInt

    // MARK: -

    public init(namespace:Namespace = .SoundCloud, collection:Collection, identifier:UInt) {
        self.namespace = namespace
        self.collection = collection
        self.identifier = identifier
    }

    public init(string:String) {
        let components = string.characters.split(":", maxSplit:3).map(String.init)
        self.namespace = Namespace(rawValue:components[0])!
        self.collection = Collection(rawValue:components[1])!
        self.identifier = UInt(components[2])!
    }
}

// MARK: - Hashable

extension Urn : Hashable {
    public var hashValue:Int {
        return description.hashValue
    }
}

public func == (lhs:Urn, rhs:Urn) -> Bool {
    return lhs.hashValue == rhs.hashValue
}

// MARK: - CustomStringConvertible

extension Urn.Collection : CustomStringConvertible {
    public var description:String {
        return self.rawValue
    }
}

extension Urn.Namespace : CustomStringConvertible {
    public var description:String {
        return self.rawValue
    }
}

extension Urn : CustomStringConvertible {
    public var description:String {
        return "<Urn \(namespace):\(collection):\(identifier)>"
    }
}