import torch


def sequence_mask(lengths, max_len=None):
    """
    Creates a boolean mask from sequence lengths.
    """
    batch_size = lengths.numel()
    max_len = max_len or lengths.max()
    return (torch.arange(0, max_len, device=lengths.device, requires_grad=False)
            .type_as(lengths)
            .repeat(batch_size, 1)
            .lt(lengths.unsqueeze(1)))