Converting very large decimal numbers to hexadecimal in swift

Question

We can use String Format specifier to convert an integer value or a long value to hexadecimal notation.

Int Example:
print(String(format:"%x", 1111))
//result:457

Long Example:        
print(String(format:"%lx", 11111111111111))
//result:a1b01d4b1c7

But, what if we try to convert a very large decimal that is larger than uint64.max? //18446744073709551615

What is the right way to convert in this case?

Answer 1

One possible solution is to use NSDecimalNumber to hold the large value. But it doesn't have any built in way to convert the number into a string other than base 10.

The following is an extension to NSDecimalNumber that will convert the number into any base from 2 to 16. And it also includes a convenience init that takes a string in a given base.

extension NSDecimalNumber {
    convenience init(string: String, base: Int) {
        guard base >= 2 && base <= 16 else { fatalError("Invalid base") }

        let digits = "0123456789ABCDEF"
        let baseNum = NSDecimalNumber(value: base)

        var res = NSDecimalNumber(value: 0)
        for ch in string {
            let index = digits.index(of: ch)!
            let digit = digits.distance(from: digits.startIndex, to: index)
            res = res.multiplying(by: baseNum).adding(NSDecimalNumber(value: digit))
        }

        self.init(decimal: res.decimalValue)
    }

    func toBase(_ base: Int) -> String {
        guard base >= 2 && base <= 16 else { fatalError("Invalid base") }

        // Support higher bases by added more digits
        let digits = "0123456789ABCDEF"
        let rounding = NSDecimalNumberHandler(roundingMode: .down, scale: 0, raiseOnExactness: false, raiseOnOverflow: false, raiseOnUnderflow: false, raiseOnDivideByZero: false)
        let baseNum = NSDecimalNumber(value: base)

        var res = ""
        var val = self
        while val.compare(0) == .orderedDescending {
            let next = val.dividing(by: baseNum, withBehavior: rounding)
            let round = next.multiplying(by: baseNum)
            let diff = val.subtracting(round)
            let digit = diff.intValue
            let index = digits.index(digits.startIndex, offsetBy: digit)
            res.insert(digits[index], at: res.startIndex)

            val = next
        }

        return res
    }
}

Test:

let bigNum = NSDecimalNumber(string: "18446744073709551615")
print(bigNum.toBase(16))
print(bigNum.toBase(10)) // or just print(bigNum)
print(NSDecimalNumber(string: "B7", base: 16))
print(NSDecimalNumber(string: NSDecimalNumber(string: "18446744073709551615").toBase(16), base: 16))

Output:

FFFFFFFFFFFFFFFF
18446744073709551615
183
18446744073709551615