Swift: Conversion to different number bases (radix)

toBase.swift

extension IntegerType {
    public func toBase(b:Int) -> String
    {
        guard b > 1 && b < 37 else {
            fatalError("base out of range")
        }
        let digits = ["0","1","2","3","4","5","6","7","8","9","A",
                      "B","C","D","E","F","G","H","I","J","K","L",
                      "M","N","O","P","Q","R","S","T","U","V","W",
                      "X","Y","Z"]
        var result = ""
        
        if let v = self as? Int {
            var value = abs(v)
            repeat {
                result = digits[value % b] + result
                value = value / b
            } while (value > 0)
        }
        return self > 0 ? result : "-" + result
    }
}