fibonacci

gistfile1.java

/**
 * Gives the Fibonacci number.
 * 
 * @param n
 *            element to return.
 * @return Fibonacci and Negafibonacci numbers.
 */
public static BigInteger fibonacci(final int n) {
	if (n == 0)
		return BigInteger.ZERO;

	int an = Math.abs(n);
	BigInteger[] result = { BigInteger.ONE, BigInteger.ONE };
	BigInteger[] tmp = new BigInteger[2];

	for (int i = 1; i < an; i++) {
		tmp[0] = result[0].add(result[1]);
		tmp[1] = result[0];
		result = Arrays.copyOf(tmp, tmp.length);
	}

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