# Copyright (c) MONAI Consortium
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
#     http://www.apache.org/licenses/LICENSE-2.0
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.

# based on Pytorch DistributedSampler and WeightedRandomSampler combined

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

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

__all__ = ["DistributedWeightedSampler"]

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


class DistributedWeightedSampler(Sampler[T_co]):
    def __init__(
        self,
        dataset: Dataset,
        weights: Sequence[float],
        num_samples: int,
        num_replicas: Optional[int] = None,
        rank: Optional[int] = None,
        shuffle: bool = True,
        seed: int = 0,
        drop_last: bool = False,
    ) -> None:
        if not isinstance(num_samples, int) or isinstance(num_samples, bool) or num_samples <= 0:
            raise ValueError(f"num_samples should be a positive integer value, but got num_samples={num_samples}")

        weights_tensor = torch.as_tensor(weights, dtype=torch.float)
        if len(weights_tensor.shape) != 1:
            raise ValueError(
                "weights should be a 1d sequence but given " f"weights have shape {tuple(weights_tensor.shape)}"
            )

        self.weights = weights_tensor
        self.num_samples = num_samples

        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()
        if rank >= num_replicas or rank < 0:
            raise ValueError(f"Invalid rank {rank}, rank should be in the interval [0, {num_replicas - 1}]")
        self.dataset = dataset
        self.num_replicas = num_replicas
        self.rank = rank
        self.epoch = 0
        self.drop_last = drop_last
        self.shuffle = shuffle

        if self.shuffle:
            self.num_samples = int(math.ceil(self.num_samples / self.num_replicas))
        else:
            # this is not used, as we always shuffle, the only reason to use this class

            # 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:  # type: ignore[arg-type]
                # 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  # type: ignore[arg-type]
                )
            else:
                self.num_samples = math.ceil(len(self.dataset) / self.num_replicas)  # type: ignore[arg-type]

        self.total_size = self.num_samples * self.num_replicas
        self.shuffle = shuffle
        self.seed = seed

    def __iter__(self) -> Iterator[T_co]:
        if self.shuffle:
            # deterministically shuffle based on epoch and seed
            g = torch.Generator()
            g.manual_seed(self.seed + self.epoch)
            indices = torch.multinomial(input=self.weights, num_samples=self.total_size, replacement=True, generator=g).tolist()  # type: ignore[arg-type]
        else:
            # this is not used, as we always shuffle, the only reason to use this class
            indices = list(range(len(self.dataset)))  # type: ignore[arg-type]
            if not self.drop_last:
                # add extra samples to make it evenly divisible
                padding_size = self.total_size - len(indices)
                if padding_size <= len(indices):
                    indices += indices[:padding_size]
                else:
                    indices += (indices * math.ceil(padding_size / len(indices)))[:padding_size]
            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]
        assert len(indices) == self.num_samples

        return iter(indices)

    def __len__(self) -> int:
        return self.num_samples

    def set_epoch(self, epoch: int) -> None:
        self.epoch = epoch