package fibonacci

import (
	"errors"
	"fmt"
	"math/big"
)

// Fibonacci calculates Fibonacci number.
// This function generated correct values from 0 to 93 sequence number.
// For bigger values use FibonacciBig function.
func Fibonacci(n uint) uint64 {
	if n <= 1 {
		return uint64(n)
	}

	var n2, n1 uint64 = 0, 1

	for i := uint(2); i < n; i++ {
		n2, n1 = n1, n1+n2
	}

	return n2 + n1
}

// FibonacciBig calculates Fibonacci number using bit.Int.
// For the sequence numbers below 94, it is recommended to use Fibonacci function as it is more efficient.
func FibonacciBig(n uint) *big.Int {
	if n <= 1 {
		return big.NewInt(int64(n))
	}

	var n2, n1 = big.NewInt(0), big.NewInt(1)

	for i := uint(1); i < n; i++ {
		n2.Add(n2, n1)
		n1, n2 = n2, n1
	}

	return n1
}

func FibonacciFromString(str string) (*big.Int, error) {

	n := new(big.Int)
	n, ok := n.SetString(str, 10)

	if !ok {
		return nil, errors.New("ConvertError")
	}

	if n.Sign() != 1 {
		return big.NewInt(int64(n.Int64())), nil
	}

	// Initialize two big ints with the first two numbers in the sequence.
	a := big.NewInt(0)
	b := big.NewInt(1)

	// Loop while a is smaller than 1e100.
	for i := int64(1); i <= n.Int64(); i++ {
		// Compute the next Fibonacci number, storing it in a.
		a.Add(a, b)
		// Swap a and b so that b is the next number in the sequence.
		a, b = b, a
	}

	fmt.Println(a) // 100-digit Fibonacci number

	// Test a for primality.
	// (ProbablyPrimes' argument sets the number of Miller-Rabin
	// rounds to be performed. 20 is a good value.)
	fmt.Println(a.ProbablyPrime(20))

	return a, nil
}