Arbitrary precision number to ascii in c [duplicate] - c

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Converting a big integer to decimal string
(4 answers)
Closed 5 years ago.
I've a uint8_t * array which contains an arbitrary precision number bigendian encoded.
I'd like to get its decimal Ascii representation. So I'd need to write a function that returns a char *.
The environment i'm using does not allow me to import any arbitrary precision library due to hardware limitations.
I'm sure there is something i can read to easily implement it.
For example the number defined by the following hex d53ceb9d32c6ca06 should be represented by 15365415089075571206.

Answer 1

Here's a method that should work. Note that it destructively modifies the bigint that you pass to it, so if you care about the value there, copy it to a temporary scratch buffer before calling the method.
Also, this isn't the most optimized version you could write, but if you're asking how to do it on here you probably aren't concerned yet about micro-optimization with this.
#include <stdint.h>
#include <stdbool.h>
#include <stdio.h>
#include <stdlib.h>
bool is_zero(uint8_t *bigi_data, int bigi_size) {
int i = 0;
while((i < bigi_size) && (bigi_data[i] == 0)) {
i++;
}
return (i >= bigi_size);
}
uint8_t bigdivmod(uint8_t *bigi_data, int bigi_size, uint8_t divisor) {
int i = 0;
uint16_t ans = 0;
while((i < bigi_size) && (bigi_data[i] == 0)) {
i++;
}
for (; i < bigi_size; i++) {
ans = ans*256 + bigi_data[i];
bigi_data[i] = ans / divisor;
ans = ans % divisor;
}
return (uint8_t)ans;
}
static const char *digits = "0123456789abcdefghijklmnopqrstuvwxyz";
char *bigitoa(uint8_t *bigi_data, int bigi_size, char *out, int base) {
/* Assumes that "out" has enough room. DESTRUCTIVE TO BIGI, so copy */
/* if you care about the value */
/* Only really works for non-negative values */
int i = 0;
uint8_t swp;
int j;
if ((base < 2) || (base > 36)) {
return NULL;
}
if (is_zero(bigi_data, bigi_size)) {
out[0] = '0';
out[1] = '\0';
return out;
}
while (!is_zero(bigi_data, bigi_size)) {
out[i++] = digits[bigdivmod(bigi_data, bigi_size, base)];
}
out[i] = 0;
for (j = 0; j < i/2; j++) {
swp = out[i - 1 - j];
out[i - 1 - j] = out[j];
out[j] = swp;
}
return out;
}
int main(int argc, char *argv[]) {
uint8_t test_data[] = { 0xd5, 0x3c, 0xeb, 0x9d, 0x32, 0xc6, 0xca, 0x06 };
int test_data_len = sizeof(test_data);
char *p;
/* Times 3 because we can use three digits to represent 256. If changing */
/* the base below from "10", change this factor. */
p = malloc(3*test_data_len + 1);
printf("Test data works out to %s\n",
bigitoa(test_data, test_data_len, p, 10));
return 0;
}