fibonnacci.cpp

// Calculates the Fibonnacci sequence by fast exponentiation:
// http://rosettacode.org/wiki/Fibonacci_sequence#C.2B.2B
#include <iostream>
 
inline void fibmul(int* f, int* g)
{
  int tmp = f[0]*g[0] + f[1]*g[1];
  f[1] = f[0]*g[1] + f[1]*(g[0] + g[1]);
  f[0] = tmp;
}
 
int fibonacci(int n)
{
  int f[] = { 1, 0 };
  int g[] = { 0, 1 };
  while (n > 0)
  {
    if (n & 1) // n odd
    {
      fibmul(f, g);
      --n;
    }
    else
    {
      fibmul(g, g);
      n >>= 1;
    }
  }
  return f[1];
}
 
int main()
{
  for (int i = 0; i < 9; ++i)
    cout << "fib(" << i << "): " << fibonacci(i) << endl;
  return 0;
}