import math
from typing import TypeVar, Optional, Iterator, Sequence

import torch
from torch.utils.data import Dataset
import torch.distributed as dist


T_co = TypeVar('T_co', covariant=True)


class DistributedWeightedSampler(torch.utils.data.distributed.DistributedSampler):
    r"""Extension of pytorch's native distributed sampler, but supports weighted sampling
    .. note::
        Dataset is assumed to be of constant size.
    Arguments:
        dataset: Dataset used for sampling.
        num_replicas (int, optional): Number of processes participating in
            distributed training. By default, :attr:`rank` is retrieved from the
            current distributed group.
        rank (int, optional): Rank of the current process within :attr:`num_replicas`.
            By default, :attr:`rank` is retrieved from the current distributed
            group.
            indices.
        seed (int, optional): random seed used to shuffle the sampler if
            :attr:`shuffle=True`. This number should be identical across all
            processes in the distributed group. Default: ``0``.
        drop_last (bool, optional): if ``True``, then the sampler will drop the
            tail of the data to make it evenly divisible across the number of
            replicas. If ``False``, the sampler will add extra indices to make
            the data evenly divisible across the replicas. Default: ``False``.
    .. warning::
        In distributed mode, calling the :meth:`set_epoch` method at
        the beginning of each epoch **before** creating the :class:`DataLoader` iterator
        is necessary to make shuffling work properly across multiple epochs. Otherwise,
        the same ordering will be always used.
    Example::
        >>> sampler = DistributedSampler(dataset) if is_distributed else None
        >>> loader = DataLoader(dataset, shuffle=(sampler is None),
        ...                     sampler=sampler)
        >>> for epoch in range(start_epoch, n_epochs):
        ...     if is_distributed:
        ...         sampler.set_epoch(epoch)
        ...     train(loader)
    """

    def __init__(self, dataset: Dataset, weights: Sequence[float],
                 replacement: bool = True, generator=None, num_replicas: Optional[int] = None,
                 rank: Optional[int] = None,
                 seed: int = 0, drop_last: bool = False) -> None:
        if num_replicas is None:
            if not dist.is_available():
                raise RuntimeError("Requires distributed package to be available")
            num_replicas = dist.get_world_size()
        if rank is None:
            if not dist.is_available():
                raise RuntimeError("Requires distributed package to be available")
            rank = dist.get_rank()
        self.dataset = dataset
        self.num_replicas = num_replicas
        self.rank = rank
        self.epoch = 0
        self.drop_last = drop_last
        self.weights = torch.Tensor(weights)
        self.replacement = replacement
        self.generator = generator
        # If the dataset length is evenly divisible by # of replicas, then there
        # is no need to drop any data, since the dataset will be split equally.
        if self.drop_last and len(self.dataset) % self.num_replicas != 0:
            # Split to nearest available length that is evenly divisible.
            # This is to ensure each rank receives the same amount of data when
            # using this Sampler.
            self.num_samples = math.ceil(
                (len(self.dataset) - self.num_replicas) / self.num_replicas
            )
        else:
            self.num_samples = math.ceil(len(self.dataset) / self.num_replicas)
        self.total_size = self.num_samples * self.num_replicas
        self.seed = seed

    def __iter__(self) -> Iterator[T_co]:
        indices = list(range(len(self.dataset)))

        if not self.drop_last:
            # add extra samples to make it evenly divisible
            indices += indices[:(self.total_size - len(indices))]
        else:
            # remove tail of data to make it evenly divisible.
            indices = indices[:self.total_size]
        assert len(indices) == self.total_size

        # subsample
        indices = indices[self.rank:self.total_size:self.num_replicas]
        weights = self.weights[self.rank:self.total_size:self.num_replicas]

        assert len(indices) == self.num_samples

        rand_tensor = torch.multinomial(self.weights, self.num_samples, self.replacement, generator=self.generator)
        return iter(rand_tensor)