Converting arbritrary size strings into arbritrary precision integers (bigints)

Question

Hi,

I'm trying to implement the Solovoy-Strassen primality test for arbritrary large integers. I will also be writing a bignum (cannot use 3rd party implementation as this is an academic project). I have decided on the following structure for the bignum:

struct {
  uint64_t *tab;
  int size; // number of limbs
  int sign;
}

I will be using base-32 for my digits (hence uint64_t, for partial products, at least I assume they will be partial products). This decision was based on a previous question asked.

I'm at a standstill. I cannot conceive how one can take a string represented as an arbitrary size decimal and convert it into the bignum structure above.

Could someone please enlighten me. Even a smaller example would be nice, such as converting maybe an arbitrary string into octal digits which would be stored in a uint16_t array.

Thanks.

Answer 1

You need to do the arithmetic, calling your routines. For example, if the string is "2013" (representing 2013 in decimal), do: a=0; a=10*a+2; a=10*a+0; a=10*a+1; a=10*a+3.