fibonacci

gistfile1.cs

/// <summary>
/// Gives the Fibonacci number.
/// </summary>
/// <remarks>using System.Numerics;</remarks>
/// <param name="n">element to return.</param>
/// <returns>Fibonacci and Negafibonacci numbers.</returns>
public static BigInteger fibonacci(int n)
{
	if (n == 0)
		return BigInteger.Zero;

	int an = Math.Abs(n);
	int nr = 2;
	BigInteger[] result = { BigInteger.One, BigInteger.One };
	BigInteger[] tmp = new BigInteger[nr];

	for (int i = 1; i < an; i++)
	{
		tmp[0] = BigInteger.Add(result[0], result[1]);
		tmp[1] = result[0];
		Array.Copy(tmp, 0, result, 0, 2);
	}

	return ((n < 0) && (n % 2 == 0)) ? -result[1] : result[1];
}