Huge fibonacci modulo m C++ - c++

Question

I'm trying to calculate Fn mod m, where Fn is the nth Fibonacci number. n may be really huge, so its really not efficient to calculate Fn in a straightforward way (matrix exponentiation would work, though). The problem statement asks us to do this without calculating Fn, using the distributive property of the modulo:
(a+b)mod m = [a mod m + b mod m] mod m
(Before anyone asks me, I looked up answers to this same problem. I'd like an answer to my specific question, however, since I'm not asking about the algorithm to solve this problem)
Using this and the fact that the nth Fibonacci number is just the sum of the previous two, I don't need to store Fibonacci numbers, but rather only the results of calculating successive modulo operations. In that sense, I should have an array F of size n which has in it stored the results of iteratively calculating Fn mod m using the above property. I have managed to solve this problem using the following code. However, upon reviewing it, I stumbled upon something that rather confused me.
long long get_fibonacci_huge_mod(long long n, long long m) {
long long Fib[3] = {0, 1, 1};
long long result;
long long index;
long long period;
long long F[n+1];
F[0] = 0;
F[1] = 1;
F[2] = 1;
for (long long i = 3; i <= n; i++) {
F[i] = (F[i-2] + F[i-1]) % m;
if (F[i] == 0 && F[i+1] == 1 && F[i+2] == 1) {
period = i;
break;
}
}
index = n % period;
result = F[index];
return result;
}
This solution outputs correct results for any n and m, even if they are quite large. It might get a little bit slow when n is huge, but I'm not worried about that right now. I'm interested in specifically solving the problem this way. I'll try solving it using matrix exponentiation or any other much faster algorithm later.
So my question is as follows. At the beginning of the code, I create an array F of size n+1. Then I iterate through this array calculating Fn mod m using the distributive property. One thing that confused me after writing this loop was the fact that, since F was initialized to all zeros, how is it correctly using F[i+2], F[i+1], if they haven't been calculated yet? I assume that they are being correctly used since the algorithm outputs correct results every time. Perhaps this assumption is wrong?
My question isn't about the algorithm per se, I'm asking about what's going on inside the loop.
Thank you

Answer 1

This is a faulty implementation of a correct algorithm. Let us look at the corrected version first.
long long get_fibonacci_huge_mod(long long n, long long m) {
long long result;
long long index;
long long period = n+1;
long long sz = min (n+1,m*m+1); // Bound for period
long long *F = new long long[sz];
F[0] = 0;
F[1] = 1;
F[2] = 1;
for (long long i = 3; i < sz; i++) {
F[i] = (F[i-2] + F[i-1]) % m;
if (F[i] == 1 && F[i-1] == 0) { // we have got back to where we started
period = i-1;
break;
}
}
index = n % period;
result = F[index];
delete[]F;
return result;
}
So why does the original code work? Because you got lucky. The checks for i+1 and i+2 never evaluated to true because of the lucky garbage the array was initialized to. As a result this reduced to the naive evaluation of F(n) without incorporating periodicity at all.