import warnings
from typing import Dict, Tuple, Union

import numpy as np
import torch as th
from gym import spaces
from torch.nn import functional as F


def is_image_space_channels_first(observation_space: spaces.Box) -> bool:
    """
    Check if an image observation space (see ``is_image_space``)
    is channels-first (CxHxW, True) or channels-last (HxWxC, False).

    Use a heuristic that channel dimension is the smallest of the three.
    If second dimension is smallest, raise an exception (no support).

    :param observation_space:
    :return: True if observation space is channels-first image, False if channels-last.
    """
    smallest_dimension = np.argmin(observation_space.shape).item()
    if smallest_dimension == 1:
        warnings.warn("Treating image space as channels-last, while second dimension was smallest of the three.")
    return smallest_dimension == 0


def is_image_space(
    observation_space: spaces.Space,
    check_channels: bool = False,
) -> bool:
    """
    Check if a observation space has the shape, limits and dtype
    of a valid image.
    The check is conservative, so that it returns False if there is a doubt.

    Valid images: RGB, RGBD, GrayScale with values in [0, 255]

    :param observation_space:
    :param check_channels: Whether to do or not the check for the number of channels.
        e.g., with frame-stacking, the observation space may have more channels than expected.
    :return:
    """
    if isinstance(observation_space, spaces.Box) and len(observation_space.shape) == 3:
        # Check the type
        if observation_space.dtype != np.uint8:
            return False

        # Check the value range
        if np.any(observation_space.low != 0) or np.any(observation_space.high != 255):
            return False

        # Skip channels check
        if not check_channels:
            return True
        # Check the number of channels
        if is_image_space_channels_first(observation_space):
            n_channels = observation_space.shape[0]
        else:
            n_channels = observation_space.shape[-1]
        # RGB, RGBD, GrayScale
        return n_channels in [1, 3, 4]
    return False


def maybe_transpose(observation: np.ndarray, observation_space: spaces.Space) -> np.ndarray:
    """
    Handle the different cases for images as PyTorch use channel first format.

    :param observation:
    :param observation_space:
    :return: channel first observation if observation is an image
    """
    # Avoid circular import
    from stable_baselines3.common.vec_env import VecTransposeImage

    if is_image_space(observation_space):
        if not (observation.shape == observation_space.shape or observation.shape[1:] == observation_space.shape):
            # Try to re-order the channels
            transpose_obs = VecTransposeImage.transpose_image(observation)
            if transpose_obs.shape == observation_space.shape or transpose_obs.shape[1:] == observation_space.shape:
                observation = transpose_obs
    return observation


def preprocess_obs(
    obs: th.Tensor,
    observation_space: spaces.Space,
    normalize_images: bool = True
) -> Union[th.Tensor, Dict[str, th.Tensor]]:
    """
    Preprocess observation to be to a neural network.
    For images, it normalizes the values by dividing them by 255 (to have values in [0, 1])
    For discrete observations, it create a one hot vector.

    :param obs: Observation
    :param observation_space:
    :param normalize_images: Whether to normalize images or not
        (True by default)
    :return:
    """
    if isinstance(observation_space, spaces.Box):
        if is_image_space(observation_space) and normalize_images:
            return obs.float() / 255.0
        return obs.float()

    elif isinstance(observation_space, spaces.Discrete):
        # One hot encoding and convert to float to avoid errors
        return F.one_hot(obs.long(), num_classes=observation_space.n).float()

    elif isinstance(observation_space, spaces.MultiDiscrete):
        # Tensor concatenation of one hot encodings of each Categorical sub-space
        return th.cat(
            [
                F.one_hot(obs_.long(), num_classes=int(observation_space.nvec[idx])).float()
                for idx, obs_ in enumerate(th.split(obs.long(), 1, dim=1))
            ],
            dim=-1,
        ).view(obs.shape[0], sum(observation_space.nvec))

    elif isinstance(observation_space, spaces.MultiBinary):
        return obs.float()

    elif isinstance(observation_space, spaces.Dict):
        # Do not modify by reference the original observation
        preprocessed_obs = {}
        for key, _obs in obs.items():
            if observation_space.spaces.__contains__(key):
                preprocessed_obs[key] = preprocess_obs(_obs, observation_space[key], normalize_images=normalize_images)
        return preprocessed_obs

    else:
        raise NotImplementedError(f"Preprocessing not implemented for {observation_space}")


def get_obs_shape(
    observation_space: spaces.Space,
) -> Union[Tuple[int, ...], Dict[str, Tuple[int, ...]]]:
    """
    Get the shape of the observation (useful for the buffers).

    :param observation_space:
    :return:
    """
    if isinstance(observation_space, spaces.Box):
        return observation_space.shape
    elif isinstance(observation_space, spaces.Discrete):
        # Observation is an int
        return (1,)
    elif isinstance(observation_space, spaces.MultiDiscrete):
        # Number of discrete features
        return (int(len(observation_space.nvec)),)
    elif isinstance(observation_space, spaces.MultiBinary):
        # Number of binary features
        return (int(observation_space.n),)
    elif isinstance(observation_space, spaces.Dict):
        return {key: get_obs_shape(subspace) for (key, subspace) in observation_space.spaces.items()}

    else:
        raise NotImplementedError(f"{observation_space} observation space is not supported")


def get_flattened_obs_dim(observation_space: spaces.Space) -> int:
    """
    Get the dimension of the observation space when flattened.
    It does not apply to image observation space.

    Used by the ``FlattenExtractor`` to compute the input shape.

    :param observation_space:
    :return:
    """
    # See issue https://github.com/openai/gym/issues/1915
    # it may be a problem for Dict/Tuple spaces too...
    if isinstance(observation_space, spaces.MultiDiscrete):
        return sum(observation_space.nvec)
    else:
        # Use Gym internal method
        return spaces.utils.flatdim(observation_space)


def get_action_dim(action_space: spaces.Space) -> int:
    """
    Get the dimension of the action space.

    :param action_space:
    :return:
    """
    if isinstance(action_space, spaces.Box):
        return int(np.prod(action_space.shape))
    elif isinstance(action_space, spaces.Discrete):
        # Action is an int
        return 1
    elif isinstance(action_space, spaces.MultiDiscrete):
        # Number of discrete actions
        return int(len(action_space.nvec))
    elif isinstance(action_space, spaces.MultiBinary):
        # Number of binary actions
        return int(action_space.n)
    else:
        raise NotImplementedError(f"{action_space} action space is not supported")


def check_for_nested_spaces(obs_space: spaces.Space):
    """
    Make sure the observation space does not have nested spaces (Dicts/Tuples inside Dicts/Tuples).
    If so, raise an Exception informing that there is no support for this.

    :param obs_space: an observation space
    :return:
    """
    if isinstance(obs_space, (spaces.Dict, spaces.Tuple)):
        sub_spaces = obs_space.spaces.values() if isinstance(obs_space, spaces.Dict) else obs_space.spaces
        for sub_space in sub_spaces:
            if isinstance(sub_space, (spaces.Dict, spaces.Tuple)):
                raise NotImplementedError(
                    "Nested observation spaces are not supported (Tuple/Dict space inside Tuple/Dict space)."
                )
