Java BigDecimal in Swift

This may not be what you want, as you are saying swift-native solution. But in Swift 3, an old C-struct based NSDecimal is imported as Decimal, and a big re-arrangement for it has been done as to say "it is nearly Swift-native".

let deca = 1.23 as Decimal //<- This actually may produce some conversion error, while `ExpressibleByFloatLiteral` uses `Double` as an intermediate value.
let decb = 0.01 as Decimal
print(deca + decb == 1.24) //->true

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UPDATE Added a simple example, where you can find a calculation error in Double (binary floating point system). (Tested in Xcode 8 beta 6.)

let dblc = 0.000001
let dbld = 100 as Double
let dble = 0.0001
print(dblc * dbld == dble) //->false (as Double cannot represent decimal fractions precisely)

let decc = Decimal(string: "0.000001")! //<- avoiding conversion error
let decd = 100 as Decimal //<- integer literal may not generate conversion error
let dece = Decimal(string: "0.0001")!
print(decc * decd == dece) //->true

You can also use j2obj, a Google project that translates Java code to Objective C, including all the Java types like BigInteger and BigDecimal. Using this I was able to make an XCFramework that incorporates support for those native Java types, including all their functionality from Java, into Swift. Pretty cool, although it's not a small framework.

Basically you just clone j2objc and build locally (cd to its directory and "make dist -j8"). Then make a new Xcode project with a framework target. Follow the steps on Google's j2objc site to add your J2OBJC_HOME to build settings and add the library and header search paths to your new framework project.

Now in your framework's public header you'll want to import "java/math/BigDecimal.h", and you'll want to add to your framework's headers a copy of all the headers needed by BigDecimal.m and its dependencies.

Lastly archive your framework for hardware platforms and build for simulator, then make an XCFramework including all those built .frameworks from the archives and your DerivedData.

Once you have the XCFramework you can add it to any other project whose architectures it supports. And from there use it in code. Note you have to initialize j2bjc classes before making an instance of them.