Euler Plaindrome

palin.exs

defmodule PalindromeNum do
  def largest_palindrome(upper_bound, lower_bound) do
    find_largest({upper_bound, upper_bound}, 0, lower_bound)
  end

  defp find_largest({x, _}, largest_num, lower_bound) when x <= lower_bound, do: largest_num
  defp find_largest({x, y}, largest_num, lower_bound) when y <= lower_bound do
    find_largest({x-1, x-1}, largest_num, lower_bound)
  end
  defp find_largest({x, y}, largest_num, _) when x == y and x * y < largest_num, do: largest_num
  defp find_largest({x, y}, largest_num, lower_bound) when x * y < largest_num do
    find_largest({x-1, x-1}, largest_num, lower_bound)
  end
  defp find_largest({x, y}, largest_num, lower_bound) do
    test_num = x*y
    if palindrome?(test_num) do
      find_largest({x-1,x-1}, test_num, lower_bound)
    else
      find_largest({x, y-1}, largest_num, lower_bound) 
    end
  end

  def palindrome?(num) do
    list = Integer.digits(num)
    list == Enum.reverse(list)
  end
end

IO.inspect PalindromeNum.largest_palindrome(999, 100)