text: #!/usr/bin/env python3
  # Copyright (c) Facebook, Inc. and its affiliates. All Rights Reserved.
  
  """Data loader."""
  
  import itertools
  import numpy as np
  import torch
  from torch.utils.data._utils.collate import default_collate
  from torch.utils.data.distributed import DistributedSampler
  from torch.utils.data.sampler import RandomSampler
  
  from slowfast.datasets.multigrid_helper import ShortCycleBatchSampler
  
  from . import utils as utils
  from .build import build_dataset
  from .samplers import RASampler
  
  
  def detection_collate(batch):
      """
      Collate function for detection task. Concatanate bboxes, labels and
      metadata from different samples in the first dimension instead of
      stacking them to have a batch-size dimension.
      Args:
          batch (tuple or list): data batch to collate.
      Returns:
          (tuple): collated detection data batch.
      """
      inputs, labels, video_idx, extra_data = zip(*batch)
      inputs, video_idx = default_collate(inputs), default_collate(video_idx)
      labels = torch.tensor(np.concatenate(labels, axis=0)).float()
  
      collated_extra_data = {}
      for key in extra_data[0].keys():
          data = [d[key] for d in extra_data]
          if key == "boxes" or key == "ori_boxes":
              # Append idx info to the bboxes before concatenating them.
              bboxes = [
                  np.concatenate(
                      [np.full((data[i].shape[0], 1), float(i)), data[i]], axis=1
                  )
                  for i in range(len(data))
              ]
              bboxes = np.concatenate(bboxes, axis=0)
              collated_extra_data[key] = torch.tensor(bboxes).float()
          elif key == "metadata":
              collated_extra_data[key] = torch.tensor(
                  list(itertools.chain(*data))
              ).view(-1, 2)
          else:
              collated_extra_data[key] = default_collate(data)
  
      return inputs, labels, video_idx, collated_extra_data
  
  
  def construct_loader(cfg, split, is_precise_bn=False):
      """
      Constructs the data loader for the given dataset.
      Args:
          cfg (CfgNode): configs. Details can be found in
              slowfast/config/defaults.py
          split (str): the split of the data loader. Options include `train`,
              `val`, and `test`.
      """
      assert split in ["train", "val", "test"]
      if split in ["train"]:
          dataset_name = cfg.TRAIN.DATASET
          batch_size = int(cfg.TRAIN.BATCH_SIZE / max(1, cfg.NUM_GPUS))
          shuffle = True
          drop_last = True
      elif split in ["val"]:
          dataset_name = cfg.TRAIN.DATASET
          batch_size = int(cfg.TRAIN.BATCH_SIZE / max(1, cfg.NUM_GPUS))
          shuffle = False
          drop_last = False
      elif split in ["test"]:
          dataset_name = cfg.TEST.DATASET
          batch_size = int(cfg.TEST.BATCH_SIZE / max(1, cfg.NUM_GPUS))
          shuffle = False
          drop_last = False
  
      # Construct the dataset
      dataset = build_dataset(dataset_name, cfg, split)
  
      if isinstance(dataset, torch.utils.data.IterableDataset):
          loader = torch.utils.data.DataLoader(
              dataset,
              batch_size=batch_size,
              num_workers=cfg.DATA_LOADER.NUM_WORKERS,
              pin_memory=cfg.DATA_LOADER.PIN_MEMORY,
              drop_last=drop_last,
              collate_fn=detection_collate if cfg.DETECTION.ENABLE else None,
              worker_init_fn=utils.loader_worker_init_fn(dataset),
          )
      else:
          if (
              cfg.MULTIGRID.SHORT_CYCLE
              and split in ["train"]
              and not is_precise_bn
          ):
              # Create a sampler for multi-process training
              sampler = utils.create_sampler(dataset, shuffle, cfg)
              batch_sampler = ShortCycleBatchSampler(
                  sampler, batch_size=batch_size, drop_last=drop_last, cfg=cfg
              )
              # Create a loader
              loader = torch.utils.data.DataLoader(
                  dataset,
                  batch_sampler=batch_sampler,
                  num_workers=cfg.DATA_LOADER.NUM_WORKERS,
                  pin_memory=cfg.DATA_LOADER.PIN_MEMORY,
                  worker_init_fn=utils.loader_worker_init_fn(dataset),
              )
          else:
              # Create a sampler for multi-process training
              sampler = utils.create_sampler(dataset, shuffle, cfg)
              # Create a loader
              loader = torch.utils.data.DataLoader(
                  dataset,
                  batch_size=batch_size,
                  shuffle=(False if sampler else shuffle),
                  sampler=sampler,
                  num_workers=cfg.DATA_LOADER.NUM_WORKERS,
                  pin_memory=cfg.DATA_LOADER.PIN_MEMORY,
                  drop_last=drop_last,
                  collate_fn=detection_collate if cfg.DETECTION.ENABLE else None,
                  worker_init_fn=utils.loader_worker_init_fn(dataset),
              )
      return loader
  
  
  def shuffle_dataset(loader, cur_epoch):
      """ "
      Shuffles the data.
      Args:
          loader (loader): data loader to perform shuffle.
          cur_epoch (int): number of the current epoch.
      """
      if (
          loader._dataset_kind
          == torch.utils.data.dataloader._DatasetKind.Iterable
      ):
          if hasattr(loader.dataset, "sampler"):
              sampler = loader.dataset.sampler
          else:
              raise RuntimeError(
                  "Unknown sampler for IterableDataset when shuffling dataset"
              )
      else:
          sampler = (
              loader.batch_sampler.sampler
              if isinstance(loader.batch_sampler, ShortCycleBatchSampler)
              else loader.sampler
          )
      assert isinstance(
          sampler, (RandomSampler, DistributedSampler, RASampler)
      ), "Sampler type '{}' not supported".format(type(sampler))
      # RandomSampler handles shuffling automatically
      if isinstance(sampler, DistributedSampler) or isinstance(sampler, RASampler):
          # DistributedSampler shuffles data based on epoch
          sampler.set_epoch(cur_epoch)
  