Decomposing Double into sign, mantissa and exponent in swift: check my work

Question

I've added this extension to Double below to decompose a Double into integer mantissa and exponent in Swift. I've tested it for a number of values, including DBL_MAX and -DBL_MAX. My questions are: (1) Is there a built in method or function somewhere that already does this? (2) Is this correct, for all environments, with all Doubles? Is there any way it could fail? (3) Is there a way to make it more efficient but still portable?

Thanks for your advice.

extension Double {
    func decomp () -> (mantissa:Int64,exponent:Int64) {
        var (a,b) = frexp(self)

        var exponent = Int64(b)

        while floor(a) != a {
            a *= 2
            exponent--
        }
        let mantissa = Int64(floor(a))

        return (mantissa, exponent)
    }
}

Answer 1

As far as I can tell this should be easy but it doesn't work,

The FloatingPoint protocol, as defined https://github.com/apple/swift/blob/master/stdlib/public/core/FloatingPoint.swift.gyb , to which the Double struct complies already defines an exponent and a significand variable (on line 207 and 231 respectively).

And the Double struct https://github.com/apple/swift/blob/master/stdlib/public/core/FloatingPointTypes.swift.gyb , defines the exponent on line 389 and the significand on line 398.