/*
 * Simple example of loop -- loop version of fibonacci.c
 */
#include <stdio.h>

int fib(int n);

int main(void) {
	int n;
	printf("enter integer value\n");
	if (scanf("%d", &n) == 1) {
		printf("fibonacci(%d) is %d\n", n, fib(n));
		return 0;
	}
	else {
		printf("not an integer value\n");
		return 1;
	}
}

int fib(int n) {
	if (n <= 1) return 1;
	int current;
	int prev = 1;
	int prevprev = 1;
	for (int index = 2; index <= n; ++index) {
		/* 
		 * at this point:
		 * prev == fib(index-1)
		 * prevprev == fib(index-2)
		 */
		current = prevprev + prev;
		/*
		 * now current == fib(index)
		 */
		prevprev = prev;
		prev = current;
	}
	return current;
	/* 
	 * sample trace of execution of "for" loop, with n == 4
	 * 
	 * at start of first iteration:
	 * index = 2
	 * prevprev = 1
	 * prev = 1
	 * at end of iteration:
	 * current = 2 (note that this is fib(2))
	 * prevprev = 1
	 * prev = 2
	 *
	 * at start of next iteration:
	 * index = 3
	 * prevprev = 1
	 * prev = 2
	 * at end of iteration:
	 * current = 3 (note that this is fib(3))
	 * prevprev = 2
	 * prev = 3
	 *
	 * at start of next iteration:
	 * index = 4
	 * prevprev = 2
	 * prev = 3
	 * at end of iteration:
	 * current = 5 (note that this is fib(4))
	 * prevprev = 3
	 * prev = 5
	 *
	 * loop stops because index becomes 5, which is not <= 4
	 */
}