#   Copyright (c) 2020 PaddlePaddle Authors. 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.


class _DatasetFetcher:
    def __init__(self, dataset, auto_collate_batch, collate_fn, drop_last):
        self.dataset = dataset
        self.auto_collate_batch = auto_collate_batch
        self.collate_fn = collate_fn
        self.drop_last = drop_last

    # NOTE: fetch function here perform the whole pipeline of dataset
    #       reading and data transforms of a batch in each calling, this
    #       may take a long time inside, if DataLoader is exit outside,
    #       fetch need to perceive exit situation, so we pass done_event
    #       here for fetch to check exit status
    # NOTE: if DataLoader exit by `break`, performing GPU tensor operations,
    #       e.g. to_tensor may cause SIGSEGV in thread, so we pass the
    #       done_event argument to check DataLoader exit status between
    #       each sample processing in the batch
    def fetch(self, batch_indices, done_event=None):
        raise NotImplementedError(
            f"'fetch' not implement for class {self.__class__.__name__}"
        )


class _IterableDatasetFetcher(_DatasetFetcher):
    def __init__(self, dataset, auto_collate_batch, collate_fn, drop_last):
        super().__init__(dataset, auto_collate_batch, collate_fn, drop_last)
        self.dataset_iter = iter(dataset)

    def fetch(self, batch_indices, done_event=None):
        if self.auto_collate_batch:
            data = []
            for _ in batch_indices:
                if done_event is None or not done_event.is_set():
                    try:
                        data.append(next(self.dataset_iter))
                    except StopIteration:
                        break
                else:
                    return None

            if len(data) == 0 or (
                self.drop_last and len(data) < len(batch_indices)
            ):
                raise StopIteration

        else:
            data = next(self.dataset_iter)

        if self.collate_fn:
            data = self.collate_fn(data)
        return data


class _MapDatasetFetcher(_DatasetFetcher):
    def __init__(self, dataset, auto_collate_batch, collate_fn, drop_last):
        super().__init__(dataset, auto_collate_batch, collate_fn, drop_last)

    def fetch(self, batch_indices, done_event=None):
        if self.auto_collate_batch:
            data = []
            for idx in batch_indices:
                if done_event is None or not done_event.is_set():
                    data.append(self.dataset[idx])
                else:
                    return None

        else:
            data = self.dataset[batch_indices]

        if self.collate_fn:
            data = self.collate_fn(data)
        return data