optimized itoa function

Question

I am thinking on how to implement the conversion of an integer (4byte, unsigned) to string with SSE instructions. The usual routine is to divide the number and store it in a loc

Answer 1

@Inge Henriksen

I believe your code has a bug:

IntToStr(2701987) == "2701987" //Correct
IntToStr(27001987) == "2701987" //Incorrect

Here's why your code is wrong:

modVal = i % numElem10Radix;
switch (reverseArrayLen[modVal])
{
    case 5:
        *--p = reverseArray[modVal][4];
    case 4:
        *--p = reverseArray[modVal][3];
    case 3:
        *--p = reverseArray[modVal][2];
    case 2:
        *--p = reverseArray[modVal][1];
    default:
        *--p = reverseArray[modVal][0];
}

i /= numElem10Radix;

There should be a leading 0 before "1987", which is "01987". But after the first iteration, you get 4 digits instead of 5.

So,

IntToStr(27000000) = "2700" //Incorrect

Answer 2

For unsigned 0 to 9,999,999 with terminating null. (99,999,999 without)

void itoa(uint64_t u, char *out) // up to 9,999,999 with terminating zero
{
    *out = 0;
    do { 
        uint64_t n0 = u;
        *((uint64_t *)out) = (*((uint64_t *)out) << 8) | (n0 + '0' - (u /= 10) * 10);
    } while (u);
}