defmodule GuessingGame do
  # tell user our guess
  # "yes" game over
  # "bigger" -> bigger( low, high )
  # "smaller" -> smaller( low, high )
  # anything else -> tell user to enter a valid response

  def guess(a, b) when a > b, do: guess(b, a) # guard, matches only, when a > b
  
  def guess(low, high) do
    answer = IO.gets "Hmm... maybe you're thinking of #{mid(low, high)}?\n"
    case String.trim(answer) do
      "bigger" -> bigger(low, high)
      "smaller" -> smaller(low, high)
      "yes" -> "I knew I could guess your number!"
      _ ->
        IO.puts ~s(Type "bigger", "smaller" or "yes")
	guess(low, high)
    end
  end
  
  def mid( low, high) do
    div low + high, 2
  end

  def bigger(low, high) do
    new_low = min( high, mid(low, high) + 1)
    guess(new_low, high)
  end

  def smaller(low, high) do
    new_high = max(low, mid(low, high) - 1)
    guess(low, new_high)
  end  
end