Decimal.doubleValue

extension Decimal {
    var doubleValue: Double {
        var d = 0.0
        var i = _length
        if _length == 0 && _isNegative != 0 { return Double.nan }
        while i > 0 {
            var m: UInt16
            switch i - 1 {
            case 0:
                m = _mantissa.0
                break
            case 1:
                m = _mantissa.1
                break
            case 2:
                m = _mantissa.2
                break
            case 3:
                m = _mantissa.3
                break
            case 4:
                m = _mantissa.4
                break
            case 5:
                m = _mantissa.5
                break
            case 6:
                m = _mantissa.6
                break
            case 7:
                m = _mantissa.7
                break
            default:
                fatalError()
            }
            d = d * 65536.0 + Double(m)
            i -= 1
        }
        
        if _exponent < 0 {
            i = UInt32(-_exponent)
            while i > 0 {
                d = d / 10.0
                i -= 1
            }
        } else {
            i = UInt32(_exponent)
            while i > 0 {
                d = d * 10.0
                i -= 1
            }
        }
        
        return _isNegative != 0 ? -d : d
    }
}