/* 
 * Program to get an integer from stdin without using scanf.
 *
 * (reading one character at a time and using recursion)
 * revised to skip leading whitespace and return error indicator
 */
#include <stdio.h>
#include <stdlib.h>
#include <ctype.h>
#include <stdbool.h>

int getint(bool *error);

int main(void) {
	printf("enter a non-negative number\n");
	bool err;
	int n = getint(&err);
	if (err) {
		printf("not a number\n");
		return 1;
	}
	int next_ch = getchar();
	if ((next_ch != ' ') && (next_ch != '\n')) {
		printf("not a number\n");
		return 1;
	}
	printf("value is %d\n", n);
	return 0;
}

/* 
 * for non-main functions, I just define them before first use rather
 * than having separate declarations and definitions
 */

/* 
 * convert character to digit, returning -1 if character does not 
 * represent a digit
 */
int char2digit (char c) {
	if ((c < '0') || (c > '9'))
		return -1;
	else
		return c - '0';

}

/* 
 * return integer that is 10 * previous_digits + rest of input 
 */
int getint_r(int previous_digits) {
	int input = getchar();
	int next = char2digit(input);
	if (next == -1) {
		ungetc(input, stdin);
		return previous_digits;
	} 
	else {
		int temp = 10 * previous_digits + next;
		return getint_r(temp);
	}
}

/* 
 * skip to first non-WS char and return it
 */
int skip_to_first_nonws(void) {
	int input = getchar();
	if (!isspace(input)) 
		return input;
	else 
		return skip_to_first_nonws();
}


/* 
 * get an integer char by char from stdin, skipping any leading
 * whitespace and stopping when next char is not a digit
 * sets *error to true if first non-whitespace character not a digit,
 *   else false
 */
int getint(bool *error) {
	int input = skip_to_first_nonws();
	int first = char2digit(input);
	if (first == -1) {
		*error = true;
		return 0;
	} else {
		*error = false;
		return getint_r(first);
	}
}