import math

import torch
from torch.utils.data import DistributedSampler as _DistributedSampler


class DistributedSampler(_DistributedSampler):

    def __init__(self, dataset, num_replicas=None, rank=None, shuffle=True):
        super().__init__(dataset, num_replicas=num_replicas, rank=rank)
        self.shuffle = shuffle

    def __iter__(self):
        # deterministically shuffle based on epoch
        if self.shuffle:
            g = torch.Generator()
            g.manual_seed(self.epoch)
            indices = torch.randperm(len(self.dataset), generator=g).tolist()
        else:
            indices = torch.arange(len(self.dataset)).tolist()

        # add extra samples to make it evenly divisible
        # in case that indices is shorter than half of total_size
        indices = (indices *
                   math.ceil(self.total_size / len(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)
