Swift UIColor extension that parses a hexadecimal color string

UIColor-hex.swift

extension UIColor {
    enum Error: ErrorType {
        case Parsing(String)
    }

    convenience init(hex input: String) throws {
        let hex = input.stringByTrimmingCharactersInSet(NSCharacterSet.alphanumericCharacterSet().invertedSet)
        var int = UInt32()
        guard NSScanner(string: hex).scanHexInt(&int) else {
            throw Error.Parsing("Unable to scan hexadecimal integer")
        }
        let a, r, g, b: UInt32
        switch hex.characters.count {
        case 3: // RGB (12-bit)
            (a, r, g, b) = (255, (int >> 8) * 17, (int >> 4 & 0xF) * 17, (int & 0xF) * 17)
        case 6: // RGB (24-bit)
            (a, r, g, b) = (255, int >> 16, int >> 8 & 0xFF, int & 0xFF)
        case 8: // ARGB (32-bit)
            (a, r, g, b) = (int >> 24, int >> 16 & 0xFF, int >> 8 & 0xFF, int & 0xFF)
        default:
            throw Error.Parsing("Invalid hexadecimal size: \(hex.characters.count)")
        }
        self.init(red: CGFloat(r) / 255, green: CGFloat(g) / 255, blue: CGFloat(b) / 255, alpha: CGFloat(a) / 255)
    }
}