Decimal to Base36

gistfile1.c

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

char *base36 = "0123456789abcdefghijklmnopqrstuvwxyz";

void encode36(char *buffer, uint64_t num) {
    for(num=num; num > 0; num /= 36) {
        *buffer++ = base36[num % 36];
    }
}

void flip(char *buffer) {
    size_t x, y;
    
    for(x=0,y=strlen(buffer) - 1; x < y; x++, y--) {
        buffer[x] ^= buffer[y];
        buffer[y] ^= buffer[x];
        buffer[x] ^= buffer[y];
    }
}

int main (int argc, const char * argv[])
{
    char output[14] = {0};
    
    encode36(output, 1000000000000);
    flip(output);
    
    printf("%s\n", output);
    
    return 0;
}