How to convert a 128-bit integer to a decimal ascii string in C?

Question

I\'m trying to convert a 128-bit unsigned integer stored as an array of 4 unsigned ints to the decimal string representation in C:

unsigned int src[] = { 0x1         


        

Answer 1

A slow but simple approach is to just printing digits from most significant to least significant using subtraction. Basically you need a function for checking if x >= y and another for computing x -= y when that is the case. Then you can start counting how many times you can subtract 10^38 (and this will be most significant digit), then how many times you can subtract 10^37 ... down to how many times you can subtract 1.

The following is a full implementation of this approach:

#include 

typedef unsigned ui128[4];

int ge128(ui128 a, ui128 b)
{
    int i = 3;
    while (i >= 0 && a[i] == b[i])
        --i;
    return i < 0 ? 1 : a[i] >= b[i];
}

void sub128(ui128 a, ui128 b)
{
    int i = 0;
    int borrow = 0;
    while (i < 4)
    {
        int next_borrow = (borrow && a[i] <= b[i]) || (!borrow && a[i] < b[i]);
        a[i] -= b[i] + borrow;
        borrow = next_borrow;
        i += 1;
    }
}

ui128 deci128[] = {{1u,0u,0u,0u},
                   {10u,0u,0u,0u},
                   {100u,0u,0u,0u},
                   {1000u,0u,0u,0u},
                   {10000u,0u,0u,0u},
                   {100000u,0u,0u,0u},
                   {1000000u,0u,0u,0u},
                   {10000000u,0u,0u,0u},
                   {100000000u,0u,0u,0u},
                   {1000000000u,0u,0u,0u},
                   {1410065408u,2u,0u,0u},
                   {1215752192u,23u,0u,0u},
                   {3567587328u,232u,0u,0u},
                   {1316134912u,2328u,0u,0u},
                   {276447232u,23283u,0u,0u},
                   {2764472320u,232830u,0u,0u},
                   {1874919424u,2328306u,0u,0u},
                   {1569325056u,23283064u,0u,0u},
                   {2808348672u,232830643u,0u,0u},
                   {2313682944u,2328306436u,0u,0u},
                   {1661992960u,1808227885u,5u,0u},
                   {3735027712u,902409669u,54u,0u},
                   {2990538752u,434162106u,542u,0u},
                   {4135583744u,46653770u,5421u,0u},
                   {2701131776u,466537709u,54210u,0u},
                   {1241513984u,370409800u,542101u,0u},
                   {3825205248u,3704098002u,5421010u,0u},
                   {3892314112u,2681241660u,54210108u,0u},
                   {268435456u,1042612833u,542101086u,0u},
                   {2684354560u,1836193738u,1126043566u,1u},
                   {1073741824u,1182068202u,2670501072u,12u},
                   {2147483648u,3230747430u,935206946u,126u},
                   {0u,2242703233u,762134875u,1262u},
                   {0u,952195850u,3326381459u,12621u},
                   {0u,932023908u,3199043520u,126217u},
                   {0u,730304488u,1925664130u,1262177u},
                   {0u,3008077584u,2076772117u,12621774u},
                   {0u,16004768u,3587851993u,126217744u},
                   {0u,160047680u,1518781562u,1262177448u}};

void print128(ui128 x)
{
    int i = 38;
    int z = 0;
    while (i >= 0)
    {
        int c = 0;
        while (ge128(x, deci128[i]))
        {
            c++; sub128(x, deci128[i]);
        }
        if (i==0 || z || c > 0)
        {
            z = 1; putchar('0' + c);
        }
        --i;
    }
}

int main(int argc, const char *argv[])
{
    ui128 test = { 0x12345678, 0x90abcdef, 0xfedcba90, 0x8765421 };
    print128(test);
    return 0;
}

That number in the problem text in decimal becomes

11248221411398543556294285637029484152

and Python agrees this is the correct value (this of course doesn't mean the code is correct!!! ;-) )