package tail
import scala.annotation.tailrec
import scala.math.BigInt

final object Fibonacci {
    def fibonacci(n : Int) : BigInt = {
        @tailrec
        def fib_sum(n : Int, part_sum : BigInt, sum : BigInt) : BigInt = n match {
            case 0 | 1 => part_sum
            case _ => fib_sum(n - 1, part_sum + sum, part_sum)
        }
        fib_sum(n, 1, 1)
    }
}