import torch


class Prefetcher():
    """
    Modified from the data_prefetcher in https://github.com/NVIDIA/apex/blob/master/examples/imagenet/main_amp.py
    """
    def __init__(self, loader):
        self.orig_loader = loader
        self.stream = torch.cuda.Stream()
        self.next_sample = None

    def preload(self):
        try:
            self.next_sample = next(self.loader)
        except StopIteration:
            self.next_sample = None
            return

        with torch.cuda.stream(self.stream):
            for key, value in self.next_sample.items():
                if isinstance(value, torch.Tensor):
                    self.next_sample[key] = value.cuda(non_blocking=True)

    def __next__(self):
        torch.cuda.current_stream().wait_stream(self.stream)
        sample = self.next_sample
        if sample is not None:
            for key, value in sample.items():
                if isinstance(value, torch.Tensor):
                    sample[key].record_stream(torch.cuda.current_stream())
            self.preload()
        else:
            # throw stop exception if there is no more data to perform as a default dataloader
            raise StopIteration("No samples in loader. example: `iterator = iter(Prefetcher(loader)); "
                                "data = next(iterator)`")
        return sample

    def __iter__(self):
        self.loader = iter(self.orig_loader)
        self.preload()
        return self
