"""Wrapper that stacks frames."""
from collections import deque
from typing import Union

import numpy as np

import gym
from gym.error import DependencyNotInstalled
from gym.spaces import Box


class LazyFrames:
    """Ensures common frames are only stored once to optimize memory use.

    To further reduce the memory use, it is optionally to turn on lz4 to compress the observations.

    Note:
        This object should only be converted to numpy array just before forward pass.
    """

    __slots__ = ("frame_shape", "dtype", "shape", "lz4_compress", "_frames")

    def __init__(self, frames: list, lz4_compress: bool = False):
        """Lazyframe for a set of frames and if to apply lz4.

        Args:
            frames (list): The frames to convert to lazy frames
            lz4_compress (bool): Use lz4 to compress the frames internally

        Raises:
            DependencyNotInstalled: lz4 is not installed
        """
        self.frame_shape = tuple(frames[0].shape)
        self.shape = (len(frames),) + self.frame_shape
        self.dtype = frames[0].dtype
        if lz4_compress:
            try:
                from lz4.block import compress
            except ImportError:
                raise DependencyNotInstalled(
                    "lz4 is not installed, run `pip install gym[other]`"
                )

            frames = [compress(frame) for frame in frames]
        self._frames = frames
        self.lz4_compress = lz4_compress

    def __array__(self, dtype=None):
        """Gets a numpy array of stacked frames with specific dtype.

        Args:
            dtype: The dtype of the stacked frames

        Returns:
            The array of stacked frames with dtype
        """
        arr = self[:]
        if dtype is not None:
            return arr.astype(dtype)
        return arr

    def __len__(self):
        """Returns the number of frame stacks.

        Returns:
            The number of frame stacks
        """
        return self.shape[0]

    def __getitem__(self, int_or_slice: Union[int, slice]):
        """Gets the stacked frames for a particular index or slice.

        Args:
            int_or_slice: Index or slice to get items for

        Returns:
            np.stacked frames for the int or slice

        """
        if isinstance(int_or_slice, int):
            return self._check_decompress(self._frames[int_or_slice])  # single frame
        return np.stack(
            [self._check_decompress(f) for f in self._frames[int_or_slice]], axis=0
        )

    def __eq__(self, other):
        """Checks that the current frames are equal to the other object."""
        return self.__array__() == other

    def _check_decompress(self, frame):
        if self.lz4_compress:
            from lz4.block import decompress

            return np.frombuffer(decompress(frame), dtype=self.dtype).reshape(
                self.frame_shape
            )
        return frame


class FrameStack(gym.ObservationWrapper):
    """Observation wrapper that stacks the observations in a rolling manner.

    For example, if the number of stacks is 4, then the returned observation contains
    the most recent 4 observations. For environment 'Pendulum-v1', the original observation
    is an array with shape [3], so if we stack 4 observations, the processed observation
    has shape [4, 3].

    Note:
        - To be memory efficient, the stacked observations are wrapped by :class:`LazyFrame`.
        - The observation space must be :class:`Box` type. If one uses :class:`Dict`
          as observation space, it should apply :class:`FlattenObservation` wrapper first.
          - After :meth:`reset` is called, the frame buffer will be filled with the initial observation. I.e. the observation returned by :meth:`reset` will consist of ``num_stack`-many identical frames,

    Example:
        >>> import gym
        >>> env = gym.make('CarRacing-v1')
        >>> env = FrameStack(env, 4)
        >>> env.observation_space
        Box(4, 96, 96, 3)
        >>> obs = env.reset()
        >>> obs.shape
        (4, 96, 96, 3)
    """

    def __init__(
        self,
        env: gym.Env,
        num_stack: int,
        lz4_compress: bool = False,
    ):
        """Observation wrapper that stacks the observations in a rolling manner.

        Args:
            env (Env): The environment to apply the wrapper
            num_stack (int): The number of frames to stack
            lz4_compress (bool): Use lz4 to compress the frames internally
        """
        super().__init__(env)
        self.num_stack = num_stack
        self.lz4_compress = lz4_compress

        self.frames = deque(maxlen=num_stack)

        low = np.repeat(self.observation_space.low[np.newaxis, ...], num_stack, axis=0)
        high = np.repeat(
            self.observation_space.high[np.newaxis, ...], num_stack, axis=0
        )
        self.observation_space = Box(
            low=low, high=high, dtype=self.observation_space.dtype
        )

    def observation(self, observation):
        """Converts the wrappers current frames to lazy frames.

        Args:
            observation: Ignored

        Returns:
            :class:`LazyFrames` object for the wrapper's frame buffer,  :attr:`self.frames`
        """
        assert len(self.frames) == self.num_stack, (len(self.frames), self.num_stack)
        return LazyFrames(list(self.frames), self.lz4_compress)

    def step(self, action):
        """Steps through the environment, appending the observation to the frame buffer.

        Args:
            action: The action to step through the environment with

        Returns:
            Stacked observations, reward, terminated, truncated, and information from the environment
        """
        observation, reward, terminated, truncated, info = self.env.step(action)
        self.frames.append(observation)
        return self.observation(None), reward, terminated, truncated, info

    def reset(self, **kwargs):
        """Reset the environment with kwargs.

        Args:
            **kwargs: The kwargs for the environment reset

        Returns:
            The stacked observations
        """
        obs, info = self.env.reset(**kwargs)

        [self.frames.append(obs) for _ in range(self.num_stack)]

        return self.observation(None), info
