Better Fibonacci sequence calculation

I attempted to make better Fibonacci sequence calculation algorithm in C++. This was the best I could:

constexpr unsigned fibonacci(unsigned n) {
    unsigned result[2] = {1, 0};
    for (unsigned i = 1; i < n; ++i)
        result[i%2] = result[0] + result[1];
    return result[1 - n%2];
}

...which runs at O(n) time complexity and O(1) space complexity.

Is there any better?

(Uh-oh: better - better define a quality measure!)

• Your code doesn't tell what it's about.

• constexpr looks good - "my" environment complains with C++11.

-
"The array&index manipulation" where "simultaneous assignment" is wanted is hard to read (could be worse: the elements of result[] could be non-interchangeable).

Alas, what I found for "modern C++" is ghastly compared to python's a, b = b, a + b. I appreciate the attempt to avoid avoidable assignments; I'm mildly curious if it makes any difference in the code generated by an optimising compiler.

-
Is there any better? Well, with output size limited by a constant, there's a tighter limit:

the runtime of your code is in O(1), just as any other.

In a comment, you express concern about the complexity of multiplication. If you accept ("bit-wise") "shift" as a (very) cheap operation, you can take three steps in the Fibonacci sequence at once without an increase in "logic gate complexity":

#include
#include
#include

/// Iterates an a,b = b,a+b sequence in steps of three.
//constexpr
static unsigned long tri(int previous, int current, const unsigned int n) {
if (n

(For variants using arrays of precomputed elements in stead of "the setup-loop" (and a main()), consult the edit history.)

(b`*Phi³ coincidentally can be computed with just two summands (and no other power up to 2³² can).)

StackExchange Code Review Q#163132, answer score: 3