import warnings
from abc import ABC, abstractmethod
from typing import Dict, Generator, Optional, Union

import numpy as np
import torch as th
from gym import spaces

try:
    # Check memory used by replay buffer when possible
    import psutil
except ImportError:
    psutil = None

from stable_baselines3.common.preprocessing import get_action_dim, get_obs_shape
from stable_baselines3.common.type_aliases import ReplayBufferSamples, RolloutBufferSamples
from stable_baselines3.common.vec_env import VecNormalize


class BaseBuffer(ABC):
    """
    Base class that represent a buffer (rollout or replay)

    :param buffer_size: Max number of element in the buffer
    :param observation_space: Observation space
    :param action_space: Action space
    :param device: PyTorch device
        to which the values will be converted
    :param n_envs: Number of parallel environments
    """

    def __init__(
        self,
        buffer_size: int,
        observation_space: spaces.Space,
        action_space: spaces.Space,
        device: Union[th.device, str] = "cpu",
        n_envs: int = 1,
    ):
        super(BaseBuffer, self).__init__()
        self.buffer_size = buffer_size
        self.observation_space = observation_space
        self.action_space = action_space
        self.obs_shape = get_obs_shape(observation_space)
        self.action_dim = get_action_dim(action_space)
        self.pos = 0
        self.full = False
        self.device = device
        self.n_envs = n_envs

    @staticmethod
    def swap_and_flatten(arr: np.ndarray) -> np.ndarray:
        """
        Swap and then flatten axes 0 (buffer_size) and 1 (n_envs)
        to convert shape from [n_steps, n_envs, ...] (when ... is the shape of the features)
        to [n_steps * n_envs, ...] (which maintain the order)

        :param arr:
        :return:
        """
        shape = arr.shape
        if len(shape) < 3:
            shape = shape + (1,)
        return arr.swapaxes(0, 1).reshape(shape[0] * shape[1], *shape[2:])

    def size(self) -> int:
        """
        :return: The current size of the buffer
        """
        if self.full:
            return self.buffer_size
        return self.pos

    def add(self, *args, **kwargs) -> None:
        """
        Add elements to the buffer.
        """
        raise NotImplementedError()

    def extend(self, *args, **kwargs) -> None:
        """
        Add a new batch of transitions to the buffer
        """
        # Do a for loop along the batch axis
        for data in zip(*args):
            self.add(*data)

    def reset(self) -> None:
        """
        Reset the buffer.
        """
        self.pos = 0
        self.full = False

    def sample(self, batch_size: int, env: Optional[VecNormalize] = None):
        """
        :param batch_size: Number of element to sample
        :param env: associated gym VecEnv
            to normalize the observations/rewards when sampling
        :return:
        """
        upper_bound = self.buffer_size if self.full else self.pos
        batch_inds = np.random.randint(0, upper_bound, size=batch_size)
        return self._get_samples(batch_inds, env=env)

    @abstractmethod
    def _get_samples(
        self, batch_inds: np.ndarray, env: Optional[VecNormalize] = None
    ) -> Union[ReplayBufferSamples, RolloutBufferSamples]:
        """
        :param batch_inds:
        :param env:
        :return:
        """
        raise NotImplementedError()

    def to_torch(self, array: np.ndarray, copy: bool = True) -> th.Tensor:
        """
        Convert a numpy array to a PyTorch tensor.
        Note: it copies the data by default

        :param array:
        :param copy: Whether to copy or not the data
            (may be useful to avoid changing things be reference)
        :return:
        """
        if copy:
            return th.tensor(array).to(self.device)
        return th.as_tensor(array).to(self.device)

    @staticmethod
    def _normalize_obs(
        obs: Union[np.ndarray, Dict[str, np.ndarray]], env: Optional[VecNormalize] = None
    ) -> Union[np.ndarray, Dict[str, np.ndarray]]:
        if env is not None:
            return env.normalize_obs(obs)
        return obs

    @staticmethod
    def _normalize_reward(reward: np.ndarray, env: Optional[VecNormalize] = None) -> np.ndarray:
        if env is not None:
            return env.normalize_reward(reward).astype(np.float32)
        return reward


class ReplayBuffer(BaseBuffer):
    """
    Replay buffer used in off-policy algorithms like SAC/TD3.

    :param buffer_size: Max number of element in the buffer
    :param observation_space: Observation space
    :param action_space: Action space
    :param device:
    :param n_envs: Number of parallel environments
    :param optimize_memory_usage: Enable a memory efficient variant
        of the replay buffer which reduces by almost a factor two the memory used,
        at a cost of more complexity.
        See https://github.com/DLR-RM/stable-baselines3/issues/37#issuecomment-637501195
        and https://github.com/DLR-RM/stable-baselines3/pull/28#issuecomment-637559274
    """

    def __init__(
        self,
        buffer_size: int,
        observation_space: spaces.Space,
        action_space: spaces.Space,
        device: Union[th.device, str] = "cpu",
        n_envs: int = 1,
        optimize_memory_usage: bool = False,
    ):
        super(ReplayBuffer, self).__init__(buffer_size, observation_space, action_space, device, n_envs=n_envs)

        assert n_envs == 1, "Replay buffer only support single environment for now"

        # Check that the replay buffer can fit into the memory
        if psutil is not None:
            mem_available = psutil.virtual_memory().available

        self.optimize_memory_usage = optimize_memory_usage
        self.observations = np.zeros((self.buffer_size, self.n_envs) + self.obs_shape, dtype=observation_space.dtype)
        if optimize_memory_usage:
            # `observations` contains also the next observation
            self.next_observations = None
        else:
            self.next_observations = np.zeros((self.buffer_size, self.n_envs) + self.obs_shape, dtype=observation_space.dtype)
        self.actions = np.zeros((self.buffer_size, self.n_envs, self.action_dim), dtype=action_space.dtype)
        self.rewards = np.zeros((self.buffer_size, self.n_envs), dtype=np.float32)
        self.dones = np.zeros((self.buffer_size, self.n_envs), dtype=np.float32)

        if psutil is not None:
            total_memory_usage = self.observations.nbytes + self.actions.nbytes + self.rewards.nbytes + self.dones.nbytes
            if self.next_observations is not None:
                total_memory_usage += self.next_observations.nbytes

            if total_memory_usage > mem_available:
                # Convert to GB
                total_memory_usage /= 1e9
                mem_available /= 1e9
                warnings.warn(
                    "This system does not have apparently enough memory to store the complete "
                    f"replay buffer {total_memory_usage:.2f}GB > {mem_available:.2f}GB"
                )

    def add(self, obs: np.ndarray, next_obs: np.ndarray, action: np.ndarray, reward: np.ndarray, done: np.ndarray) -> None:
        # Copy to avoid modification by reference
        self.observations[self.pos] = np.array(obs).copy()
        if self.optimize_memory_usage:
            self.observations[(self.pos + 1) % self.buffer_size] = np.array(next_obs).copy()
        else:
            self.next_observations[self.pos] = np.array(next_obs).copy()

        self.actions[self.pos] = np.array(action).copy()
        self.rewards[self.pos] = np.array(reward).copy()
        self.dones[self.pos] = np.array(done).copy()

        self.pos += 1
        if self.pos == self.buffer_size:
            self.full = True
            self.pos = 0

    def sample(self, batch_size: int, env: Optional[VecNormalize] = None) -> ReplayBufferSamples:
        """
        Sample elements from the replay buffer.
        Custom sampling when using memory efficient variant,
        as we should not sample the element with index `self.pos`
        See https://github.com/DLR-RM/stable-baselines3/pull/28#issuecomment-637559274

        :param batch_size: Number of element to sample
        :param env: associated gym VecEnv
            to normalize the observations/rewards when sampling
        :return:
        """
        if not self.optimize_memory_usage:
            return super().sample(batch_size=batch_size, env=env)
        # Do not sample the element with index `self.pos` as the transitions is invalid
        # (we use only one array to store `obs` and `next_obs`)
        if self.full:
            batch_inds = (np.random.randint(1, self.buffer_size, size=batch_size) + self.pos) % self.buffer_size
        else:
            batch_inds = np.random.randint(0, self.pos, size=batch_size)
        return self._get_samples(batch_inds, env=env)

    def _get_samples(self, batch_inds: np.ndarray, env: Optional[VecNormalize] = None) -> ReplayBufferSamples:
        if self.optimize_memory_usage:
            next_obs = self._normalize_obs(self.observations[(batch_inds + 1) % self.buffer_size, 0, :], env)
        else:
            next_obs = self._normalize_obs(self.next_observations[batch_inds, 0, :], env)

        data = (
            self._normalize_obs(self.observations[batch_inds, 0, :], env),
            self.actions[batch_inds, 0, :],
            next_obs,
            self.dones[batch_inds],
            self._normalize_reward(self.rewards[batch_inds], env),
        )
        return ReplayBufferSamples(*tuple(map(self.to_torch, data)))


class RolloutBuffer(BaseBuffer):
    """
    Rollout buffer used in on-policy algorithms like A2C/PPO.
    It corresponds to ``buffer_size`` transitions collected
    using the current policy.
    This experience will be discarded after the policy update.
    In order to use PPO objective, we also store the current value of each state
    and the log probability of each taken action.

    The term rollout here refers to the model-free notion and should not
    be used with the concept of rollout used in model-based RL or planning.
    Hence, it is only involved in policy and value function training but not action selection.

    :param buffer_size: Max number of element in the buffer
    :param observation_space: Observation space
    :param action_space: Action space
    :param device:
    :param gae_lambda: Factor for trade-off of bias vs variance for Generalized Advantage Estimator
        Equivalent to classic advantage when set to 1.
    :param gamma: Discount factor
    :param n_envs: Number of parallel environments
    """

    def __init__(
        self,
        buffer_size: int,
        observation_space: spaces.Space,
        action_space: spaces.Space,
        device: Union[th.device, str] = "cpu",
        gae_lambda: float = 1,
        gamma: float = 0.99,
        n_envs: int = 1,
    ):

        super(RolloutBuffer, self).__init__(buffer_size, observation_space, action_space, device, n_envs=n_envs)
        self.gae_lambda = gae_lambda
        self.gamma = gamma
        self.observations, self.actions, self.rewards, self.advantages = None, None, None, None
        self.returns, self.dones, self.values, self.log_probs = None, None, None, None
        self.generator_ready = False
        self.reset()

    def reset(self) -> None:
        self.observations = np.zeros((self.buffer_size, self.n_envs) + self.obs_shape, dtype=np.float32)
        self.actions = np.zeros((self.buffer_size, self.n_envs, self.action_dim), dtype=np.float32)
        self.rewards = np.zeros((self.buffer_size, self.n_envs), dtype=np.float32)
        self.returns = np.zeros((self.buffer_size, self.n_envs), dtype=np.float32)
        self.dones = np.zeros((self.buffer_size, self.n_envs), dtype=np.float32)
        self.values = np.zeros((self.buffer_size, self.n_envs), dtype=np.float32)
        self.log_probs = np.zeros((self.buffer_size, self.n_envs), dtype=np.float32)
        self.advantages = np.zeros((self.buffer_size, self.n_envs), dtype=np.float32)
        self.generator_ready = False
        super(RolloutBuffer, self).reset()

    def compute_returns_and_advantage(self, last_values: th.Tensor, dones: np.ndarray) -> None:
        """
        Post-processing step: compute the returns (sum of discounted rewards)
        and GAE advantage.
        Adapted from Stable-Baselines PPO2.

        Uses Generalized Advantage Estimation (https://arxiv.org/abs/1506.02438)
        to compute the advantage. To obtain vanilla advantage (A(s) = R - V(S))
        where R is the discounted reward with value bootstrap,
        set ``gae_lambda=1.0`` during initialization.

        :param last_values:
        :param dones:

        """
        # convert to numpy
        last_values = last_values.clone().cpu().numpy().flatten()

        last_gae_lam = 0
        for step in reversed(range(self.buffer_size)):
            if step == self.buffer_size - 1:
                next_non_terminal = 1.0 - dones
                next_values = last_values
            else:
                next_non_terminal = 1.0 - self.dones[step + 1]
                next_values = self.values[step + 1]
            delta = self.rewards[step] + self.gamma * next_values * next_non_terminal - self.values[step]
            last_gae_lam = delta + self.gamma * self.gae_lambda * next_non_terminal * last_gae_lam
            self.advantages[step] = last_gae_lam
        self.returns = self.advantages + self.values

    def add(
        self, obs: np.ndarray, action: np.ndarray, reward: np.ndarray, done: np.ndarray, value: th.Tensor, log_prob: th.Tensor
    ) -> None:
        """
        :param obs: Observation
        :param action: Action
        :param reward:
        :param done: End of episode signal.
        :param value: estimated value of the current state
            following the current policy.
        :param log_prob: log probability of the action
            following the current policy.
        """
        if len(log_prob.shape) == 0:
            # Reshape 0-d tensor to avoid error
            log_prob = log_prob.reshape(-1, 1)

        self.observations[self.pos] = np.array(obs).copy()
        self.actions[self.pos] = np.array(action).copy()
        self.rewards[self.pos] = np.array(reward).copy()
        self.dones[self.pos] = np.array(done).copy()
        self.values[self.pos] = value.clone().cpu().numpy().flatten()
        self.log_probs[self.pos] = log_prob.clone().cpu().numpy()
        self.pos += 1
        if self.pos == self.buffer_size:
            self.full = True

    def get(self, batch_size: Optional[int] = None) -> Generator[RolloutBufferSamples, None, None]:
        assert self.full, ""
        indices = np.random.permutation(self.buffer_size * self.n_envs)
        # Prepare the data
        if not self.generator_ready:
            for tensor in ["observations", "actions", "values", "log_probs", "advantages", "returns"]:
                self.__dict__[tensor] = self.swap_and_flatten(self.__dict__[tensor])
            self.generator_ready = True

        # Return everything, don't create minibatches
        if batch_size is None:
            batch_size = self.buffer_size * self.n_envs

        start_idx = 0
        while start_idx < self.buffer_size * self.n_envs:
            yield self._get_samples(indices[start_idx : start_idx + batch_size])
            start_idx += batch_size

    def _get_samples(self, batch_inds: np.ndarray, env: Optional[VecNormalize] = None) -> RolloutBufferSamples:
        data = (
            self.observations[batch_inds],
            self.actions[batch_inds],
            self.values[batch_inds].flatten(),
            self.log_probs[batch_inds].flatten(),
            self.advantages[batch_inds].flatten(),
            self.returns[batch_inds].flatten(),
        )
        return RolloutBufferSamples(*tuple(map(self.to_torch, data)))