Answer to Best of Ruby Quiz, quiz 4 "the animal quiz"

$answers = [[["an elephent", true]]]
$stack = []

def input
  gets.chomp
end

# convert_answer_to_question
def c_a_t_q(key)
  "Is it a/an #{key}? (y or n)"
end

def agent_answer
  case gets.chomp
  when "y"
    true
  when "n"
    false
  else
    raise "Invalid input, please input 'y' or 'n'"
  end
end

def play_again?
  $stack.clear
  puts "Thanks.\nPlay again? (y or n)"
  if agent_answer
    main_process
  else
    exit
  end
end

#convert_question_to_answer
def c_q_t_a(question)
  question.match(/Is it (.*)\?$/) ? $1 : raise("Invalid input, please input the sentance 'Is it xxx?'")
end

# warning 
def final_answer?
  $answers[$stack[0]].size == ($stack[1] + 1)
end

def header_process
  puts "Think of an animal..."
  rand_num = rand($answers.size)  
  $stack << rand_num << 0
end

# push common stack to $stack
def p_c_s
  $answers << []
  ($stack[1]).times do |i|
    $answers.last << $answers[$stack[0]][i]
  end
end

def body_process
  current_pair = $answers[$stack[0]][$stack[1]]
  puts c_a_t_q(current_pair[0])
  if agent_answer == current_pair[1]
    if final_answer?
      puts "I win. Pretty smart, aren't I?"
    else
      $stack[1] += 1
      body_process
    end
  else
    puts "You win. Help me learn from my mistake before you go... \nWhat animal were you thinking of?"
    real_answer = input
    puts "Give me a question to distinguish #{real_answer} from #{current_pair[0]}."
    distinguish_question = c_q_t_a(input)
    puts "For #{real_answer}, what is the answer to your question? (y or n)"
    distinguish_answer = agent_answer
    p_c_s
    $answers.last << [distinguish_question, distinguish_answer] << [real_answer, true]
  end

end

def main_process
  header_process
  body_process
  play_again?
rescue => e
  puts e.message
  play_again?
end

main_process