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

// max length input string
#define MAXSTR 25

// convert input binary string to a number
int bindec(char*, size_t);

int main() {

   // user input string
   char s[MAXSTR+3]; 

   // prompt for input
   printf("> ");

   // read input string; at most MAXSTR+1 chars accepted
   // Note: this is tricky. If we accept only MAXSTR chars,
   // we can't see if user entered more chars and they are
   // being dropped by fgets.
   fgets(s, MAXSTR+3, stdin);

   // check input length; n does not include final carriage return
   int n = strlen(s)-1;  
   if (n > MAXSTR) {
     printf("input cannot be more than %d characters\n", MAXSTR);
     exit(1);
   }

   for(size_t i = 0; i < n; i++) {
	   if(s[i] == '0' || s[i] == '1') {
		   continue;
	   }
	   else {
		   printf("input must contain only zeros and ones\n");
		   exit(1);
	   }
   }
   if(!n) {
	   printf("No input given only hit enter\n");
	   exit(1);
   }

   int output = bindec(s, MAXSTR);
   printf("%d\n", output);

   return 0;
}

int bindec(char* buf, size_t len) {
	int ret = 0;
	for(size_t i = 0; i < len; i++) {
		if(buf[i] == '1') {
			ret <<= 1;
			ret++;
		}
		else {
			ret <<= 0;
		}
	}
	return ret;
}