How to decode a large number from JSON in Swift - json

Question

How do I parse JSON like this:
let json = "{\"key\":18446744073709551616}"
struct Foo: Decodable {
let key: UInt64
}
let coder = JSONDecoder()
let test = try! coder.decode(Foo.self, from: json.data(using: .utf8)!)
The problem is that that number is too big for UInt64. I know of no larger integer types in Swift.
Parsed JSON number <18446744073709551616> does not fit in UInt64
I wouldn't mind getting it as String or Data, but that's not allowed because JSONDecoder knows it's supposed to be a number:
Expected to decode String but found a number instead.

Answer 1

You can use Decimal instead:
let json = "{\"key\":184467440737095516160000001}"
struct Foo: Decodable {
let key: Decimal
}
let coder = JSONDecoder()
let test = try! coder.decode(Foo.self, from: json.data(using: .utf8)!)
print(test) // Foo(key: 184467440737095516160000001)
Decimal is the Swift overlay type of NSDecimalNumber which
... can represent any number that can be expressed as mantissa x 10^exponent where mantissa is a decimal integer up to 38 digits long, and exponent is an integer from –128 through 127.
You could also parse it as a Double if the full precision is not needed:
struct Foo: Decodable {
let key: Double
}
let coder = JSONDecoder()
let test = try! coder.decode(Foo.self, from: json.data(using: .utf8)!)
print(test) // Foo(key: 1.8446744073709552e+36)

Answer 2

It seems to be the case that JSONDecoder is using NSDecimalNumber behind the scenes
struct Foo: Decodable {
let key: Int
}
// this is 1 + the mantissa of NSDecimalNumber.maximum
let json = "{\"key\":340282366920938463463374607431768211456}"
let coder = JSONDecoder()
let test = try! coder.decode(Foo.self, from: json.data(using: .utf8)!)
Even in the DecodingError, the number is not accurately represented:
Parsed JSON number <340282366920938463463374607431768211450> does not fit in Int.
So use Decimal if you want to be able to decode at maximum precision (though you still might silently lose precision). Otherwise you just need to yell at whoever is sending you that JSON.
Note that while the documentation says
mantissa is a decimal integer up to 38 digits
It's actually a 128-bit unsigned integer, so it can represent some 39 digit numbers as well, as seen above.