Convert Base 16 (Hexadecimal) to Base 36 String in C

Question

In C, what's an efficient way to convert a 64 character hexadecimal number (as a string) into a base 36 string?

I mean, is it as easy as combining a few GLIB2 functions (on Linux), or Standard Library functions? Or, do I have to do it all custom?

Answer 1

This is easy with a library like GMP:

mpz_t nr;
mpz_init(nr);
mpz_set_str(nr, hexstr, 16);
printf("%s\n", mpz_get_str(NULL, 36, nr));

On debian-based systems you can install the GMP-library with:

apt-get install libgmp-dev

Full example:

#include <stdio.h>
#include <gmp.h>

int main(int argc, char **argv) {   
    mpz_t nr;
    mpz_init(nr);
    mpz_set_str(nr, argv[1], 16);
    printf("%s\n", mpz_get_str(NULL, 36, nr));

    return 0;
}

Compiled with:

gcc -O2 conv.c -o conv -lgmp

Performs quite well:

$ time ./conv 2eb1a3e346933962bdfbb7b118404b68b967d44006986d4b1e88ec23e433de12
15wa17qx942cddy4n5q5px1fw6yi9llw0lzxjg2ahh2q0w9amq

real    0m0.001s
user    0m0.000s
sys     0m0.000s

Answer 2

Convert Base 16 (Hexadecimal) to Base 36 String

Easy to do using a few standard C functions.
Form a "string times" function.

#include <stdio.h>
#include <string.h>
#include <stdlib.h>

static char *str_times(char *s, int base, int times, int carry) {
  size_t len = strlen(s);
  for (size_t i = len; i > 0;) {
    i--;
    //        Convert character digit into into value
    //        |-------------------------------------|
    int acc = strtol((char[2] ) { s[i], 0 }, 0, base) * times + carry;
    s[i] = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ"[acc % base];
    carry = acc / base;
  }
  while (carry) {
    memmove(&s[1], &s[0], ++len);
    s[0] = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ"[carry % base];
    carry /= base;
  }
  return s;
}

char *str_base_convert(char *dest, int base_dest, const char *src, int base_src) {
  strcpy(dest, "0");
  while (*src) {
    str_times(dest, base_dest, base_src, strtol((char [2]) {*src,0}, 0, base_src));
    src++;
  }
  return dest;
}

char *str16_to_str36(char *dest, const char *src) {
  return str_base_convert(dest, 36, src, 16);
}

Sample usage

int main(void) {
  char dest[51];  // Size for 64-digit hexadecimal number in base 36
  puts(str16_to_str36(dest, "1"));
  puts(str16_to_str36(dest, "24"));
  puts(str16_to_str36(dest, "FF"));
  puts(str16_to_str36(dest, "FFFFffff"));
  puts(str16_to_str36(dest,
      "FFFFffffFFFFffffFFFFffffFFFFffffFFFFffffFFFFffffFFFFffffFFFFffff"));
  return 0;
}

Output

1
10
73
1Z141Z3
6DP5QCB22IM238NR3WVP0IC7Q99W035JMY2IW7I6N43D37JTOF

Answer 3

That 64-character hex requirement is a killer, that's a 256-bit integer which is larger than any native C integer type can handle.

Since converting to base-36 involves computing the modulo of the number and 36, I think you're going to need a bignum library (such as GNU MP) to do this.

Once you have that, implementing the algorithm to emit base-36 (from e.g. Wikipedia) should be trivial.