/* 
 * Program to get an integer from stdin without using scanf.
 *
 * (reading one character at a time and using a loop)
 */
#include <stdio.h>
#include <stdlib.h>
#include <stdbool.h>
#include <ctype.h>

/* 
 * get an integer char by char from stdin, stopping when next char
 * not a digit, and skipping leading white space (spaces, newlines, etc.)
 * *bool will be true if no integer found, false otherwise
 */
int getint(bool *error);

int main(void) {
	printf("enter a non-negative number\n");
	bool err;
	int n = getint(&err);
	if (err) {
		printf("not an integer\n");
		return 1;
	}
	int next_ch = getchar();
	if ((next_ch != ' ') && (next_ch != '\n')) {
		printf("not an integer\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';

}

/* 
 * get an integer char by char from stdin, skipping leading ws,
 * stopping when next char * and set *err to true if no digits
 */
int getint(bool *error) {
	*error = false;

	/* read input until a non-whitespace is found */
	int input;
	input = getchar();
	while (isspace(input)) { 
		input = getchar();
	}
	/* input is first non-space */

	/* check whether first non-space is a digit */
	int first = char2digit(input);
	if (first == -1) {
		*error = true;
		return 0;
	}

	/* now read and accumulate digits until non-digit found */
	int to_return = first;
	do {
		input = getchar();
		first = char2digit(input);
		if (first != -1) {
			to_return = to_return * 10 + first;
		}
	} while (first != -1);

	/* put character back in stdin for caller */
	ungetc(input, stdin);
	return to_return;
}