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

int read_in_base(int);
void skip_rest_of_input_line (void);

int main(void) {
  int i, n, base;

  printf("THIS PROGRAM CONVERTS NUMBERS IN A " 
         "GIVEN BASE (FROM 2 TO 36) TO A DECIMAL NUMBER.\n\n");
  do {
    printf("Enter number base (0 terminates the program, max 36): ");
    scanf("%d", &base); skip_rest_of_input_line();
    if (base > 0){
      printf("Enter a non-negative number to be converted " 
             "to decimal notation: ");
      n = read_in_base(base);
      if (n >= 0)
        printf("The decimal number is: %d\n", n);
      else {
        printf("You have typed an illegal ciffer - TRY AGAIN\n"); 
        skip_rest_of_input_line();
      }
    }
  } while (base > 0);
  
  return 0;
}

int calculate_ciffer_number(char ciffer){
  int ciffer_number;

  if (ciffer >= '0' && ciffer <= '9')
    ciffer_number = ciffer - '0';
  else if (ciffer >= 'a' && ciffer <= 'z')
    ciffer_number = ciffer - 'a' + 10;
  else {
    printf("Should not happen. Bye\n");
    exit(EXIT_FAILURE);
  }
  return ciffer_number;
}

/* Read a number in base and return the corresponding decimal number.
   Return -1 if erroneous input is encountered. */
int read_in_base(int base){
  int ciffer_number, res = 0, done = 0;
  char ciffer;

  do {
    ciffer = getchar();

    if ((ciffer >= '0' && ciffer <= '9') ||  (ciffer >= 'a' && ciffer <= 'z'))
      ciffer_number = calculate_ciffer_number(ciffer);
    else if (ciffer == '\n')
      done = 1; 
    else return -1;

    if (ciffer_number >= 0 && ciffer_number < base){
      if (!done) res = res * base + ciffer_number;
    }
    else return -1;
  }
  while (!done);

  return res;
}     

void skip_rest_of_input_line (void){
  char c;

  c = getchar();
  while (c != '\n') c = getchar();
}