nth fibonacci

can someone explain to me how this program calculates nth fibonacci number? I scribbled for 2 hours now and stuck :)
hints greatly appreciated.

Code:

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int f (unsigned int n) {
    int x = 0, y = 1, a = 1, b = 0, t;
    for (;;) {
        if (n & 1) {
            t = a;
            a = a * x + b * y;
            b = t * y + b * (x + y);
        }
        n >>= 1;
        if (n == 0) return b;
        t = x;
        x = x * x + y * y;
        y = t * y + y * (t + y);
    }
}

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Thanks.

Looks like nobody has a clue. :)

To the hints,

Quote:

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if (n & 1)

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checks to see if n is odd number. And

Quote:

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n >>= 1;

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is like n /= 2;

So it goes.