/*
 * Program to compute elements of the Fibonacci sequence using loop.
 *
 * (more efficient than recursion, but also a little harder to write and 
 * understand?)
 */
#include <stdio.h>

int fibonacci(int n);

int main(void) {

	int n;
	printf("enter n\n");
	if (scanf("%d", &n) != 1) {
		printf("not number\n");
		return 1;
	}
	if (n < 0) {
		printf("negative\n");
		return 1;
	}
	printf("fibonacci(%d) is %d\n", n, fibonacci(n));
	return 0;
}

int fibonacci(int n) {
	/* 
	 * for n == 0, 1
	 * for n == 1, 1
	 * otherwise fibonacci(n-1) + fibonacci(n-2)
	 */
	if (n <= 1) {
		return 1;
	} 
	else {
		/* 
		 * i is main loop variable 
		 * one_before is fibonacci(i-1)
		 * two_before is fibonacci(i-2)
		 */
		int i = 2;
		int one_before = 1;
		int two_before = 1;
		int current;
		for ( ; i <= n; ++i) {
			current = one_before + two_before;
			two_before = one_before;
			one_before = current;
		}
		return current;
	}

}