fib_blog_4.go

func calcFib(n int) *big.Int {
	// (1+√5)/2
	a := big.NewRat(1, 2)
	b := big.NewRat(1, 2)
	x := fibNum{a, b}

	// (1-√5)/2
	c := big.NewRat(1, 2)
	d := big.NewRat(-1, 2)
	y := fibNum{c, d}

	// (1/√5)
	e := big.NewRat(0, 999)
	f := big.NewRat(1, 5)
	z := fibNum{e, f}

	tmp := fibMin(fibExp(x, n), fibExp(y, n))

	// *(1/√5)
	r := fibMul(tmp, z)
	return r.num.Num()

}