import random
from typing import Any, Dict, List, Optional
import numpy as np

# Import ray before psutil will make sure we use psutil's bundled version
import ray  # noqa F401
import psutil  # noqa E402

from ray.rllib.execution.segment_tree import SumSegmentTree, MinSegmentTree
from ray.rllib.policy.sample_batch import SampleBatch
from ray.rllib.utils.annotations import override
from ray.rllib.utils.metrics.window_stat import WindowStat
from ray.rllib.utils.replay_buffers.replay_buffer import ReplayBuffer
from ray.rllib.utils.typing import SampleBatchType
from ray.util.annotations import DeveloperAPI


@DeveloperAPI
class PrioritizedReplayBuffer(ReplayBuffer):
    """This buffer implements Prioritized Experience Replay

    The algorithm has been described by Tom Schaul et. al. in "Prioritized
    Experience Replay". See https://arxiv.org/pdf/1511.05952.pdf for
    the full paper.
    """

    def __init__(
        self,
        capacity: int = 10000,
        storage_unit: str = "timesteps",
        alpha: float = 1.0,
        **kwargs
    ):
        """Initializes a PrioritizedReplayBuffer instance.

        Args:
            capacity: Max number of timesteps to store in the FIFO
                buffer. After reaching this number, older samples will be
                dropped to make space for new ones.
            storage_unit: Either 'timesteps', 'sequences' or
                'episodes'. Specifies how experiences are stored.
            alpha: How much prioritization is used
                (0.0=no prioritization, 1.0=full prioritization).
            ``**kwargs``: Forward compatibility kwargs.
        """
        ReplayBuffer.__init__(self, capacity, storage_unit, **kwargs)

        assert alpha > 0
        self._alpha = alpha

        # Segment tree must have capacity that is a power of 2
        it_capacity = 1
        while it_capacity < self.capacity:
            it_capacity *= 2

        self._it_sum = SumSegmentTree(it_capacity)
        self._it_min = MinSegmentTree(it_capacity)
        self._max_priority = 1.0
        self._prio_change_stats = WindowStat("reprio", 1000)

    @DeveloperAPI
    @override(ReplayBuffer)
    def _add_single_batch(self, item: SampleBatchType, **kwargs) -> None:
        """Add a batch of experiences to self._storage with weight.

        An item consists of either one or more timesteps, a sequence or an
        episode. Differs from add() in that it does not consider the storage
        unit or type of batch and simply stores it.

        Args:
            item: The item to be added.
            ``**kwargs``: Forward compatibility kwargs.
        """
        weight = kwargs.get("weight", None)

        if weight is None:
            weight = self._max_priority

        self._it_sum[self._next_idx] = weight**self._alpha
        self._it_min[self._next_idx] = weight**self._alpha

        ReplayBuffer._add_single_batch(self, item)

    def _sample_proportional(self, num_items: int) -> List[int]:
        res = []
        for _ in range(num_items):
            # TODO(szymon): should we ensure no repeats?
            mass = random.random() * self._it_sum.sum(0, len(self._storage))
            idx = self._it_sum.find_prefixsum_idx(mass)
            res.append(idx)
        return res

    @DeveloperAPI
    @override(ReplayBuffer)
    def sample(
        self, num_items: int, beta: float, **kwargs
    ) -> Optional[SampleBatchType]:
        """Sample `num_items` items from this buffer, including prio. weights.

        Samples in the results may be repeated.

        Examples for storage of SamplesBatches:
        - If storage unit `timesteps` has been chosen and batches of
        size 5 have been added, sample(5) will yield a concatenated batch of
        15 timesteps.
        - If storage unit 'sequences' has been chosen and sequences of
        different lengths have been added, sample(5) will yield a concatenated
        batch with a number of timesteps equal to the sum of timesteps in
        the 5 sampled sequences.
        - If storage unit 'episodes' has been chosen and episodes of
        different lengths have been added, sample(5) will yield a concatenated
        batch with a number of timesteps equal to the sum of timesteps in
        the 5 sampled episodes.

        Args:
            num_items: Number of items to sample from this buffer.
            beta: To what degree to use importance weights (0 - no corrections,
            1 - full correction).
            ``**kwargs``: Forward compatibility kwargs.

        Returns:
            Concatenated SampleBatch of items including "weights" and
            "batch_indexes" fields denoting IS of each sampled
            transition and original idxes in buffer of sampled experiences.
        """
        assert beta >= 0.0

        if len(self) == 0:
            raise ValueError("Trying to sample from an empty buffer.")

        idxes = self._sample_proportional(num_items)

        weights = []
        batch_indexes = []
        p_min = self._it_min.min() / self._it_sum.sum()
        max_weight = (p_min * len(self)) ** (-beta)

        for idx in idxes:
            p_sample = self._it_sum[idx] / self._it_sum.sum()
            weight = (p_sample * len(self)) ** (-beta)
            count = self._storage[idx].count
            # If zero-padded, count will not be the actual batch size of the
            # data.
            if (
                isinstance(self._storage[idx], SampleBatch)
                and self._storage[idx].zero_padded
            ):
                actual_size = self._storage[idx].max_seq_len
            else:
                actual_size = count
            weights.extend([weight / max_weight] * actual_size)
            batch_indexes.extend([idx] * actual_size)
            self._num_timesteps_sampled += count
        batch = self._encode_sample(idxes)

        # Note: prioritization is not supported in multi agent lockstep
        if isinstance(batch, SampleBatch):
            batch["weights"] = np.array(weights)
            batch["batch_indexes"] = np.array(batch_indexes)

        return batch

    @DeveloperAPI
    def update_priorities(self, idxes: List[int], priorities: List[float]) -> None:
        """Update priorities of items at given indices.

        Sets priority of item at index idxes[i] in buffer
        to priorities[i].

        Args:
            idxes: List of indices of items
            priorities: List of updated priorities corresponding to items at the
            idxes denoted by variable `idxes`.
        """
        # Making sure we don't pass in e.g. a torch tensor.
        assert isinstance(
            idxes, (list, np.ndarray)
        ), "ERROR: `idxes` is not a list or np.ndarray, but {}!".format(
            type(idxes).__name__
        )
        assert len(idxes) == len(priorities)
        for idx, priority in zip(idxes, priorities):
            assert priority > 0
            assert 0 <= idx < len(self._storage)
            delta = priority**self._alpha - self._it_sum[idx]
            self._prio_change_stats.push(delta)
            self._it_sum[idx] = priority**self._alpha
            self._it_min[idx] = priority**self._alpha

            self._max_priority = max(self._max_priority, priority)

    @DeveloperAPI
    @override(ReplayBuffer)
    def stats(self, debug: bool = False) -> Dict:
        """Returns the stats of this buffer.

        Args:
            debug: If true, adds sample eviction statistics to the returned stats dict.

        Returns:
            A dictionary of stats about this buffer.
        """
        parent = ReplayBuffer.stats(self, debug)
        if debug:
            parent.update(self._prio_change_stats.stats())
        return parent

    @DeveloperAPI
    @override(ReplayBuffer)
    def get_state(self) -> Dict[str, Any]:
        """Returns all local state.

        Returns:
            The serializable local state.
        """
        # Get parent state.
        state = super().get_state()
        # Add prio weights.
        state.update(
            {
                "sum_segment_tree": self._it_sum.get_state(),
                "min_segment_tree": self._it_min.get_state(),
                "max_priority": self._max_priority,
            }
        )
        return state

    @DeveloperAPI
    @override(ReplayBuffer)
    def set_state(self, state: Dict[str, Any]) -> None:
        """Restores all local state to the provided `state`.

        Args:
            state: The new state to set this buffer. Can be obtained by calling
            `self.get_state()`.
        """
        super().set_state(state)
        self._it_sum.set_state(state["sum_segment_tree"])
        self._it_min.set_state(state["min_segment_tree"])
        self._max_priority = state["max_priority"]
