Nth fibonacci (incomplete)

fibonacci.rb

# Given: 8 
# Possible solutions - 
# a + b = given
# 7 - 1 = 6
# 6 - 2 = 4 # <- subtraction should be less than either
# 5 - 3 = 2
# 4 - 4 = 0 # <- but not equal to 0
# 
# Given: 13 
# Possible solutions - 
# a + b = given
# 12 - 1 = 11
# 11 - 2 = 9
# 10 - 3 = 7
# 9 - 4 = 5
# 8 - 5 = 3 # <- subtraction should be less than either
# 7 - 6 = 1 
# 
# So on the first time its less than both it the correct sequence

# 0 1 1 2 3 5 8 
def nth_fibonacci(fib_num, check_num = 0)
  if fib_num == 0
    return fib_num
  else
    a = fib_num - 1
    b = check_num + 1
    if a < fib_num and b < fib_num
      
    else
      nth_fibonacci fib_num, check_num +1
    end
  end
end