Silverhalide - 6 months ago 37

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[] = { 0x12345678, 0x90abcdef, 0xfedcba90, 0x8765421 };`

printf("%s", some_func(src)); // gives "53072739890371098123344"

(The input and output examples above are completely fictional; I have no idea what that input would produce.)

If I was going to hex, binary or octal, this would be a simple matter of masks and bit shifts to peel of the least significant characters. However, it seems to me that I need to do base-10 division. Unfortunately, I can't remember how to do that across multiple ints, and the system I'm using doesn't support data types larger than 32-bits, so using a 128-bit type is not possible. Using a different language is also out, and I'd rather avoid a big number library just for this one operation.

Answer

Division is not necessary:

```
#include <string.h>
#include <stdio.h>
typedef unsigned long uint32;
/* N[0] - contains least significant bits, N[3] - most significant */
char* Bin128ToDec(const uint32 N[4])
{
// log10(x) = log2(x) / log2(10) ~= log2(x) / 3.322
static char s[128 / 3 + 1 + 1];
uint32 n[4];
char* p = s;
int i;
memset(s, '0', sizeof(s) - 1);
s[sizeof(s) - 1] = '\0';
memcpy(n, N, sizeof(n));
for (i = 0; i < 128; i++)
{
int j, carry;
carry = (n[3] >= 0x80000000);
// Shift n[] left, doubling it
n[3] = ((n[3] << 1) & 0xFFFFFFFF) + (n[2] >= 0x80000000);
n[2] = ((n[2] << 1) & 0xFFFFFFFF) + (n[1] >= 0x80000000);
n[1] = ((n[1] << 1) & 0xFFFFFFFF) + (n[0] >= 0x80000000);
n[0] = ((n[0] << 1) & 0xFFFFFFFF);
// Add s[] to itself in decimal, doubling it
for (j = sizeof(s) - 2; j >= 0; j--)
{
s[j] += s[j] - '0' + carry;
carry = (s[j] > '9');
if (carry)
{
s[j] -= 10;
}
}
}
while ((p[0] == '0') && (p < &s[sizeof(s) - 2]))
{
p++;
}
return p;
}
int main(void)
{
static const uint32 testData[][4] =
{
{ 0, 0, 0, 0 },
{ 1048576, 0, 0, 0 },
{ 0xFFFFFFFF, 0, 0, 0 },
{ 0, 1, 0, 0 },
{ 0x12345678, 0x90abcdef, 0xfedcba90, 0x8765421 }
};
printf("%s\n", Bin128ToDec(testData[0]));
printf("%s\n", Bin128ToDec(testData[1]));
printf("%s\n", Bin128ToDec(testData[2]));
printf("%s\n", Bin128ToDec(testData[3]));
printf("%s\n", Bin128ToDec(testData[4]));
return 0;
}
```

Output:

```
0
1048576
4294967295
4294967296
11248221411398543556294285637029484152
```