NSDecimalNumber Math Error in the deep decimals [duplicate]

问题

This question already has answers here:

Mathematical integrity of NSDecimalNumber (2 answers)

How to store 1.66 in NSDecimalNumber (1 answer)

Closed last year.

I have a Swift extension on NSDecimalNumber:

extension NSDecimalNumber {

    var asRawValue: NSDecimalNumber {
        return self.multiplying(byPowerOf10: 30)
    }

}

And a test:

let t = NSDecimalNumber(value: 335).asRawValue
let u = NSDecimalNumber(value: 0.00001).asRawValue

// fails
XCTAssert(t.subtracting(u).compare((NSDecimalNumber(value: 334.99999)).asRawValue) == .orderedSame)

// console 
po NSDecimalNumber(value: 334.99999).asRawValue
334999990000000051200000000000000

Why is there a random 512 in the middle of the long tail of zeroes of my NSDecimalNumber? How can I avoid this so I can rely on the math of very small numbers?

Thanks

来源：https://stackoverflow.com/questions/48329601/nsdecimalnumber-math-error-in-the-deep-decimals