Creates a base36 random string in the shell

randstr.c

#include <stdio.h>
#include <stdlib.h>
#include <time.h>
#include <string.h>
#include <math.h>
#include <ctype.h>

int wal_stricmp(const char *a, const char *b) {
  int ca, cb;
  do {
     ca = *((unsigned char *) a++);
     cb = *((unsigned char *) b++);
     ca = tolower(toupper(ca));
     cb = tolower(toupper(cb));
   } while (ca == cb && ca != '\0');
   return ca - cb;
}

void generate_random_string(char *str, size_t len) {
  static const char charset[] = 
    "0123456789"
    "abcdefghijklmnopqrstuvwxyz"
    "ABCDEFGHIJKLMNOPQRSTUVWXYZ";

  static int limit = RAND_MAX - (RAND_MAX % strlen(charset));
    
  for (size_t i = 0; i < len; i++) {
    int index = (int)((rand() * rand() + i) % strlen(charset));
    str[i] = charset[index];
    rand();
  }
}

int main(int argc, char **argv) {
  size_t len = 12;
  size_t times = 1;
  char *random_str = NULL;
  unsigned int i = 0;
  unsigned int cur_time = 0;

  if (argc == 2 && !wal_stricmp(argv[1], "help")) {
    printf("%s\n",
      "Usage: \x1b[36mrandstr\x1b[39m <\x1b[33mcount\x1b[39m> <\x1b[33mrepeat\x1b[39m>\n"
      "where\n"
      "  \x1b[33mcount\x1b[39m  - number of digits; defaults to 12\n"
      "  \x1b[33mrepeat\x1b[39m - repeat number of times; defaults to 1\n"
    );
    
    return 0;
  }

  if (argc > 1) {
    int requested_len = atoi(argv[1]);
    if      (requested_len < 1) len = 1;
    else if (requested_len > 100) len = 100;
    else    len = requested_len;   
  }

  if (argc > 2) {
    int requested_repeats = atoi(argv[2]);
    if      (requested_repeats < 1) times = 1;
    else if (requested_repeats > 100) times = 100;
    else    times = requested_repeats;
  }

  random_str = (char*)calloc(len + 1, sizeof(char));
  if (!random_str) {
    printf("Could not allocate memory for string\n");
    
    return 1;
  }

  cur_time = (unsigned int)time(NULL);
  srand(cur_time);

  for (i = 0; i < times; i++) {
    generate_random_string(random_str, len);  
    printf("%s%s", i > 0 ? " " : "", random_str);
  }

  printf("\n");

  if (random_str)
    free(random_str);
  
  return 0;
}