fib.rb

def fib_iter(prev2, prev, remain)
  current = prev2 + prev
  remain > 0 ? fib_iter(prev, current, remain - 1) : current
end

# n == 0 => 0, n == 1 => 1, n == 2 => 1, n == 3 => 2, n == 4 => 3, n == 5 => 5 ...
def fib(n)
  return n if [0, 1].include?(n)
  return fib_iter(0, 1, n - 2)
end

def main
  fib(0) # => 0
  fib(1) # => 1
  fib(2) # => 1
  fib(3) # => 2
  fib(4) # => 3
  fib(10) # => 55
end