How to do math with a number represented as an array in C?

Question

I have some unsigned char array. I want to represent a big number and add a number to this big number.

So for example I have these six elements:

0x00, 0x00, 0x00, 0x00, 0x00, 0xdf

I want to add 0x11 and get

0x00, 0x00, 0x00, 0x00, 0x00, 0xf0

So, if I add 0x10 after, I should have

0x00, 0x00, 0x00, 0x01, 0x00

Could I do it with binary operations or something another but without loops? My array could be much larger than six elements.

Answer 1

You sum each pair of bytes from the source arrays and if the result is larger then 0xFF you carry 1 to the next byte. A loop is required.

//Adds B to A, Len is the amount of bytes that will be added.
//Make sure that Len <= size of A
void Add(unsigned char *A, unsigned char *B, int Len)
{
    int Carry = 0;

    //Work backwards because the high byte of your array holds the least significant byte.
    for (int i = Len - 1; i >= 0; i--)
    {
        int Temp = (int) A[i] + B[i];   
        A[i] = (Temp + Carry) & 0xFF;
        if ((Temp + Carry) > 0xFF)
            Carry = 1;
        else
            Carry = 0;
    }
}