// Fibonacci using BigInt with LazyList.foldLeft optimized for GC (Scala v2.13 and above)
// Does not run out of memory for very large Fibonacci numbers
def fib(n: Int): BigInt = {

  def series(i: BigInt, j: BigInt): LazyList[BigInt] = i #:: series(j, i + j)

  series(1, 0).take(n).foldLeft(BigInt("0"))(_ + _)
}

// Small test
(0 to 13) foreach {n => print(fib(n).toString + " ")}

// result: 0 1 1 2 3 5 8 13 21 34 55 89 144 233