259.c

// challenge: https://s.mita.me/baag

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

int conv_from_nos(const char nos_num[]) {
  // nos_num: the NOS encoded number ending with a null byte
  size_t nos_len = strlen(nos_num);

  int result = 0;
  for (unsigned int i = 0; i < nos_len; i++) {
    if (nos_num[i] == '0')
      result += i + 1;
    else if (nos_num[i] == '1')
      result -= i + 1;
    else if (nos_num[i] == '2')
      result *= i + 1;
    else {
      printf("%c\n", nos_num[i]);
      printf("%s is not a valid NOS encoded number.\n", nos_num);
      return -1;
    }
  }
  return result;
}

int main(int argc, char const *argv[]) {
  if (argc != 2){
    printf("Syntax: `%s <NOS encoded number>'\n", argv[0]);
    exit(-1);
  }
  int result = conv_from_nos(argv[1]);
  printf("Result: %i\n", result);
  return 0;
}