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# coding=utf-8
# Copyright 2019 The SEED Authors
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"""A class implementing minimal Atari 2600 preprocessing.
Adapted from SEED RL, originally adapted from Dopamine.
"""

import cv2
from gymnasium.spaces.box import Box
import gymnasium as gym
import numpy as np


class AtariPreprocessing:
  """A class implementing image preprocessing for Atari 2600 agents.
  Specifically, this provides the following subset from the JAIR paper
  (Bellemare et al., 2013) and Nature DQN paper (Mnih et al., 2015):
    * Frame skipping (defaults to 4).
    * Terminal signal when a life is lost (off by default).
    * Grayscale and max-pooling of the last two frames.
    * Downsample the screen to a square image (defaults to 84x84).
  More generally, this class follows the preprocessing guidelines set down in
  Machado et al. (2018), "Revisiting the Arcade Learning Environment:
  Evaluation Protocols and Open Problems for General Agents".
  It also provides random starting no-ops, which are used in the Rainbow, Apex
  and R2D2 papers.
  """

  def __init__(self, environment: gym.Env, frame_skip=4, terminal_on_life_loss=False,
               screen_size=84, max_random_noops=0):
    """Constructor for an Atari 2600 preprocessor.
    Args:
      environment: Gym environment whose observations are preprocessed.
      frame_skip: int, the frequency at which the agent experiences the game.
      terminal_on_life_loss: bool, If True, the step() method returns
        is_terminal=True whenever a life is lost. See Mnih et al. 2015.
      screen_size: int, size of a resized Atari 2600 frame.
      max_random_noops: int, maximum number of no-ops to apply at the beginning
        of each episode to reduce determinism. These no-ops are applied at a
        low-level, before frame skipping.
    Raises:
      ValueError: if frame_skip or screen_size are not strictly positive.
    """
    if frame_skip <= 0:
      raise ValueError('Frame skip should be strictly positive, got {}'.
                       format(frame_skip))
    if screen_size <= 0:
      raise ValueError('Target screen size should be strictly positive, got {}'.
                       format(screen_size))

    self.environment = environment
    self.terminal_on_life_loss = terminal_on_life_loss
    self.frame_skip = frame_skip
    self.screen_size = screen_size
    self.max_random_noops = max_random_noops

    obs_dims = self.environment.observation_space
    # Stores temporary observations used for pooling over two successive
    # frames.
    self.screen_buffer = [
        np.empty((obs_dims.shape[0], obs_dims.shape[1]), dtype=np.uint8),
        np.empty((obs_dims.shape[0], obs_dims.shape[1]), dtype=np.uint8)
    ]

    self.game_over = False
    self.lives = 0  # Will need to be set by reset().

  @property
  def observation_space(self):
    # Return the observation space adjusted to match the shape of the processed
    # observations.
    return Box(low=0, high=255, shape=(self.screen_size, self.screen_size, 1),
               dtype=np.uint8)

  @property
  def action_space(self):
    return self.environment.action_space

  @property
  def reward_range(self):
    return self.environment.reward_range

  @property
  def metadata(self):
    return self.environment.metadata

  def close(self):
    return self.environment.close()

  def apply_random_noops(self):
    """Steps self.environment with random no-ops."""
    if self.max_random_noops <= 0:
      return
    # Other no-ops implementations actually always do at least 1 no-op. We
    # follow them.
    no_ops = self.environment.np_random.randint(1, self.max_random_noops + 1)
    for _ in range(no_ops):
      _, _, game_over, _, _ = self.environment.step(0)
      if game_over:
        self.environment.reset()

  def reset(self):
    """Resets the environment.
    Returns:
      observation: numpy array, the initial observation emitted by the
        environment.
    """
    self.environment.reset()
    self.apply_random_noops()

    self.lives = self.environment.unwrapped.ale.lives()
    self._fetch_grayscale_observation(self.screen_buffer[0])
    self.screen_buffer[1].fill(0)
    return self._pool_and_resize()

  def render(self, mode):
    """Renders the current screen, before preprocessing.
    This calls the Gym API's render() method.
    Args:
      mode: Mode argument for the environment's render() method.
        Valid values (str) are:
          'rgb_array': returns the raw ALE image.
          'human': renders to display via the Gym renderer.
    Returns:
      if mode='rgb_array': numpy array, the most recent screen.
      if mode='human': bool, whether the rendering was successful.
    """
    return self.environment.render(mode)

  def step(self, action):
    """Applies the given action in the environment.
    Remarks:
      * If a terminal state (from life loss or episode end) is reached, this may
        execute fewer than self.frame_skip steps in the environment.
      * Furthermore, in this case the returned observation may not contain valid
        image data and should be ignored.
    Args:
      action: The action to be executed.
    Returns:
      observation: numpy array, the observation following the action.
      reward: float, the reward following the action.
      is_terminal: bool, whether the environment has reached a terminal state.
        This is true when a life is lost and terminal_on_life_loss, or when the
        episode is over.
      info: Gym API's info data structure.
    """
    accumulated_reward = 0.

    for time_step in range(self.frame_skip):
      # We bypass the Gym observation altogether and directly fetch the
      # grayscale image from the ALE. This is a little faster.
      _, reward, game_over, _, info = self.environment.step(action)
      accumulated_reward += reward

      if self.terminal_on_life_loss:
        new_lives = self.environment.unwrapped.ale.lives()
        is_terminal = game_over or new_lives < self.lives
        self.lives = new_lives
      else:
        is_terminal = game_over

      if is_terminal:
        break
      # We max-pool over the last two frames, in grayscale.
      elif time_step >= self.frame_skip - 2:
        t = time_step - (self.frame_skip - 2)
        self._fetch_grayscale_observation(self.screen_buffer[t])

    # Pool the last two observations.
    observation = self._pool_and_resize()

    self.game_over = game_over
    return observation, accumulated_reward, is_terminal, info

  def _fetch_grayscale_observation(self, output):
    """Returns the current observation in grayscale.
    The returned observation is stored in 'output'.
    Args:
      output: numpy array, screen buffer to hold the returned observation.
    Returns:
      observation: numpy array, the current observation in grayscale.
    """
    self.environment.unwrapped.ale.getScreenGrayscale(output)
    return output

  def _pool_and_resize(self):
    """Transforms two frames into a Nature DQN observation.
    For efficiency, the transformation is done in-place in self.screen_buffer.
    Returns:
      transformed_screen: numpy array, pooled, resized screen.
    """
    # Pool if there are enough screens to do so.
    if self.frame_skip > 1:
      np.maximum(self.screen_buffer[0], self.screen_buffer[1],
                 out=self.screen_buffer[0])

    transformed_image = cv2.resize(self.screen_buffer[0],
                                   (self.screen_size, self.screen_size),
                                   interpolation=cv2.INTER_LINEAR)
    int_image = np.asarray(transformed_image, dtype=np.uint8)
    return np.expand_dims(int_image, axis=2)
