macOS Define a class, which implements arithmetic with arbitrary precision

Hi everyone!
I'm currently doing some exercises in a objective-c book. One of them is: define a class, which implements arithmetic (+, -, /, *, %) with arbitrary precision. This is a new type with just some arithmetic operations. We can do some arithmetic with an infinite precision.
I'm aware that there exist libraries to do this, but I need to attempt to implement it myself. Is there anyone can help?
Thanks in advanced!

It is just a lab book at my school. I've already read through all these. The task is to implement a class to support arbitrary precision integer arithmetic. In other words, write code to do addition, subtraction, division, multiplication, and modulus for integers at any length for the following interface:

@interface MPInteger : NSObject

-(id) initWithString: (NSString *) x;
-(NSString *) description;

-(MPInteger *) add: (MPInteger *) x;
-(MPInteger *) subtract: (MPInteger *) x;
-(MPInteger *) multiply: (MPInteger *) x;
-(MPInteger *) divideBy: (MPInteger *) x;
-(MPInteger *) modulus: (MPInteger *) x;

-(BOOL) isLessThan: (MPInteger *) x;

Is there anyone can help? much appreciated if someone can give me some idea to start!