math.swift

import Foundation

/// Calculates the `n`th Fibonacci number
///
/// This function returns `0` for all negative numbers.
func fib(_ n: Int) -> Int {
  guard n > 0 else {
    return 0
  }

  if n == 1 {
    return 1
  } else {
    return fib(n - 1) + fib(n - 2)
  }
}