# Copyright (c) 2021, NVIDIA CORPORATION.  All rights reserved.
#
# 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.

"""Dataloaders."""


import torch

from nemo.utils import logging


class MegatronPretrainingSampler:
    def __init__(
        self, total_samples, consumed_samples, micro_batch_size, data_parallel_rank, data_parallel_size, drop_last=True
    ):
        # Keep a copy of input params for later use.
        self.total_samples = total_samples
        self.consumed_samples = consumed_samples
        self.micro_batch_size = micro_batch_size
        self.data_parallel_rank = data_parallel_rank
        self.micro_batch_times_data_parallel_size = self.micro_batch_size * data_parallel_size
        self.drop_last = drop_last

        logging.info(
            f'Instantiating MegatronPretrainingSampler with total_samples: {total_samples} and consumed_samples: {consumed_samples}'
        )

        # Sanity checks.
        assert self.total_samples > 0, 'no sample to consume: {}'.format(self.total_samples)
        assert self.consumed_samples < self.total_samples, 'no samples left to consume: {}, {}'.format(
            self.consumed_samples, self.total_samples
        )
        assert self.micro_batch_size > 0
        assert data_parallel_size > 0
        assert self.data_parallel_rank < data_parallel_size, (
            'data_parallel_rank should be smaller than data size: {}, '
            '{}'.format(self.data_parallel_rank, data_parallel_size)
        )

    def __len__(self):
        return (self.total_samples - self.consumed_samples - 1) // self.micro_batch_times_data_parallel_size + 1

    def get_start_end_idx(self):
        start_idx = self.data_parallel_rank * self.micro_batch_size
        end_idx = start_idx + self.micro_batch_size
        return start_idx, end_idx

    def __iter__(self):
        batch = []
        # Last batch will be dropped if drop_last is not set False
        for idx in range(self.consumed_samples, self.total_samples):
            batch.append(idx)
            if len(batch) == self.micro_batch_times_data_parallel_size:
                start_idx, end_idx = self.get_start_end_idx()
                yield batch[start_idx:end_idx]
                batch = []

        # Check the last partial batch and see drop_last is set
        if len(batch) > 0 and not self.drop_last:
            start_idx, end_idx = self.get_start_end_idx()
            yield batch[start_idx:end_idx]


class MegatronPretrainingRandomSampler:
    def __init__(self, total_samples, consumed_samples, micro_batch_size, data_parallel_rank, data_parallel_size):
        # Keep a copy of input params for later use.
        self.total_samples = total_samples
        self.consumed_samples = consumed_samples
        self.micro_batch_size = micro_batch_size
        self.data_parallel_rank = data_parallel_rank
        self.data_parallel_size = data_parallel_size
        self.micro_batch_times_data_parallel_size = self.micro_batch_size * data_parallel_size
        self.last_batch_size = self.total_samples % self.micro_batch_times_data_parallel_size

        # Sanity checks.
        assert self.total_samples > 0, 'no sample to consume: {}'.format(self.total_samples)
        assert self.micro_batch_size > 0
        assert data_parallel_size > 0
        assert self.data_parallel_rank < data_parallel_size, (
            'data_parallel_rank should be smaller than data size: {}, '
            '{}'.format(self.data_parallel_rank, data_parallel_size)
        )

    def __len__(self):
        return self.total_samples

    def __iter__(self):
        active_total_samples = self.total_samples - self.last_batch_size
        self.epoch = self.consumed_samples // active_total_samples
        current_epoch_samples = self.consumed_samples % active_total_samples
        assert current_epoch_samples % self.micro_batch_times_data_parallel_size == 0

        # data sharding and random sampling
        bucket_size = (self.total_samples // self.micro_batch_times_data_parallel_size) * self.micro_batch_size
        bucket_offset = current_epoch_samples // self.data_parallel_size
        start_idx = self.data_parallel_rank * bucket_size

        g = torch.Generator()
        g.manual_seed(self.epoch)
        random_idx = torch.randperm(bucket_size, generator=g).tolist()
        idx_range = [start_idx + x for x in random_idx[bucket_offset:]]

        batch = []
        # Last batch if not complete will be dropped.
        for idx in idx_range:
            batch.append(idx)
            if len(batch) == self.micro_batch_size:
                self.consumed_samples += self.micro_batch_times_data_parallel_size
                yield batch
                batch = []
