# -*- coding: utf-8 -*-

"""Network related utility tools."""

import logging
from typing import Dict, List, Tuple

import numpy as np
import torch


def to_device(m, x):
    """Send tensor into the device of the module.

    Args:
        m (torch.nn.Module): Torch module.
        x (Tensor): Torch tensor.

    Returns:
        Tensor: Torch tensor located in the same place as torch module.

    """
    if isinstance(m, torch.nn.Module):
        device = next(m.parameters()).device
    elif isinstance(m, torch.Tensor):
        device = m.device
    else:
        raise TypeError("Expected torch.nn.Module or torch.tensor, " f"bot got: {type(m)}")
    return x.to(device)


def pad_list(xs, pad_value):
    """Perform padding for the list of tensors.

    Args:
        xs (List): List of Tensors [(T_1, `*`), (T_2, `*`), ..., (T_B, `*`)].
        pad_value (float): Value for padding.

    Returns:
        Tensor: Padded tensor (B, Tmax, `*`).

    Examples:
        >>> x = [torch.ones(4), torch.ones(2), torch.ones(1)]
        >>> x
        [tensor([1., 1., 1., 1.]), tensor([1., 1.]), tensor([1.])]
        >>> pad_list(x, 0)
        tensor([[1., 1., 1., 1.],
                [1., 1., 0., 0.],
                [1., 0., 0., 0.]])

    """
    n_batch = len(xs)
    max_len = max(x.size(0) for x in xs)
    pad = xs[0].new(n_batch, max_len, *xs[0].size()[1:]).fill_(pad_value)

    for i in range(n_batch):
        pad[i, : xs[i].size(0)] = xs[i]

    return pad


def pad_list_all_dim(xs, pad_value):
    """Perform padding for the list of tensors.

    Args:
        xs (List): List of Tensors [(T_1, `*`), (T_2, `*`), ..., (T_B, `*`)].
        pad_value (float): Value for padding.

    Returns:
        Tensor: Padded tensor (B, Tmax, `*`).

    Examples:
        >>> x = [torch.ones(4), torch.ones(2), torch.ones(1)]
        >>> x
        [tensor([1., 1., 1., 1.]), tensor([1., 1.]), tensor([1.])]
        >>> pad_list(x, 0)
        tensor([[1., 1., 1., 1.],
                [1., 1., 0., 0.],
                [1., 0., 0., 0.]])

    """
    n_batch = len(xs)
    num_dim = len(xs[0].shape)
    max_len_all_dim = []
    for i in range(num_dim):
        max_len_all_dim.append(max(x.size(i) for x in xs))
    pad = xs[0].new(n_batch, *max_len_all_dim).fill_(pad_value)

    for i in range(n_batch):
        if num_dim == 1:
            pad[i, : xs[i].size(0)] = xs[i]
        elif num_dim == 2:
            pad[i, : xs[i].size(0), : xs[i].size(1)] = xs[i]
        elif num_dim == 3:
            pad[i, : xs[i].size(0), : xs[i].size(1), : xs[i].size(2)] = xs[i]
        else:
            raise ValueError(
                "pad_list_all_dim only support 1-D, 2-D and 3-D tensors, not {}-D.".format(num_dim)
            )

    return pad


def make_pad_mask(lengths, xs=None, length_dim=-1, maxlen=None):
    """Make mask tensor containing indices of padded part.

    Args:
        lengths (LongTensor or List): Batch of lengths (B,).
        xs (Tensor, optional): The reference tensor.
            If set, masks will be the same shape as this tensor.
        length_dim (int, optional): Dimension indicator of the above tensor.
            See the example.

    Returns:
        Tensor: Mask tensor containing indices of padded part.
                dtype=torch.uint8 in PyTorch 1.2-
                dtype=torch.bool in PyTorch 1.2+ (including 1.2)

    Examples:
        With only lengths.

        >>> lengths = [5, 3, 2]
        >>> make_pad_mask(lengths)
        masks = [[0, 0, 0, 0 ,0],
                 [0, 0, 0, 1, 1],
                 [0, 0, 1, 1, 1]]

        With the reference tensor.

        >>> xs = torch.zeros((3, 2, 4))
        >>> make_pad_mask(lengths, xs)
        tensor([[[0, 0, 0, 0],
                 [0, 0, 0, 0]],
                [[0, 0, 0, 1],
                 [0, 0, 0, 1]],
                [[0, 0, 1, 1],
                 [0, 0, 1, 1]]], dtype=torch.uint8)
        >>> xs = torch.zeros((3, 2, 6))
        >>> make_pad_mask(lengths, xs)
        tensor([[[0, 0, 0, 0, 0, 1],
                 [0, 0, 0, 0, 0, 1]],
                [[0, 0, 0, 1, 1, 1],
                 [0, 0, 0, 1, 1, 1]],
                [[0, 0, 1, 1, 1, 1],
                 [0, 0, 1, 1, 1, 1]]], dtype=torch.uint8)

        With the reference tensor and dimension indicator.

        >>> xs = torch.zeros((3, 6, 6))
        >>> make_pad_mask(lengths, xs, 1)
        tensor([[[0, 0, 0, 0, 0, 0],
                 [0, 0, 0, 0, 0, 0],
                 [0, 0, 0, 0, 0, 0],
                 [0, 0, 0, 0, 0, 0],
                 [0, 0, 0, 0, 0, 0],
                 [1, 1, 1, 1, 1, 1]],
                [[0, 0, 0, 0, 0, 0],
                 [0, 0, 0, 0, 0, 0],
                 [0, 0, 0, 0, 0, 0],
                 [1, 1, 1, 1, 1, 1],
                 [1, 1, 1, 1, 1, 1],
                 [1, 1, 1, 1, 1, 1]],
                [[0, 0, 0, 0, 0, 0],
                 [0, 0, 0, 0, 0, 0],
                 [1, 1, 1, 1, 1, 1],
                 [1, 1, 1, 1, 1, 1],
                 [1, 1, 1, 1, 1, 1],
                 [1, 1, 1, 1, 1, 1]]], dtype=torch.uint8)
        >>> make_pad_mask(lengths, xs, 2)
        tensor([[[0, 0, 0, 0, 0, 1],
                 [0, 0, 0, 0, 0, 1],
                 [0, 0, 0, 0, 0, 1],
                 [0, 0, 0, 0, 0, 1],
                 [0, 0, 0, 0, 0, 1],
                 [0, 0, 0, 0, 0, 1]],
                [[0, 0, 0, 1, 1, 1],
                 [0, 0, 0, 1, 1, 1],
                 [0, 0, 0, 1, 1, 1],
                 [0, 0, 0, 1, 1, 1],
                 [0, 0, 0, 1, 1, 1],
                 [0, 0, 0, 1, 1, 1]],
                [[0, 0, 1, 1, 1, 1],
                 [0, 0, 1, 1, 1, 1],
                 [0, 0, 1, 1, 1, 1],
                 [0, 0, 1, 1, 1, 1],
                 [0, 0, 1, 1, 1, 1],
                 [0, 0, 1, 1, 1, 1]]], dtype=torch.uint8)

    """
    if length_dim == 0:
        raise ValueError("length_dim cannot be 0: {}".format(length_dim))

    if not isinstance(lengths, list):
        lengths = lengths.tolist()
    bs = int(len(lengths))
    if maxlen is None:
        if xs is None:
            maxlen = int(max(lengths))
        else:
            maxlen = xs.size(length_dim)
    else:
        assert xs is None
        assert maxlen >= int(max(lengths))

    seq_range = torch.arange(0, maxlen, dtype=torch.int64)
    seq_range_expand = seq_range.unsqueeze(0).expand(bs, maxlen)
    seq_length_expand = seq_range_expand.new(lengths).unsqueeze(-1)
    mask = seq_range_expand >= seq_length_expand

    if xs is not None:
        assert xs.size(0) == bs, (xs.size(0), bs)

        if length_dim < 0:
            length_dim = xs.dim() + length_dim
        # ind = (:, None, ..., None, :, , None, ..., None)
        ind = tuple(slice(None) if i in (0, length_dim) else None for i in range(xs.dim()))
        mask = mask[ind].expand_as(xs).to(xs.device)
    return mask


def make_non_pad_mask(lengths, xs=None, length_dim=-1):
    """Make mask tensor containing indices of non-padded part.

    Args:
        lengths (LongTensor or List): Batch of lengths (B,).
        xs (Tensor, optional): The reference tensor.
            If set, masks will be the same shape as this tensor.
        length_dim (int, optional): Dimension indicator of the above tensor.
            See the example.

    Returns:
        ByteTensor: mask tensor containing indices of padded part.
                    dtype=torch.uint8 in PyTorch 1.2-
                    dtype=torch.bool in PyTorch 1.2+ (including 1.2)

    Examples:
        With only lengths.

        >>> lengths = [5, 3, 2]
        >>> make_non_pad_mask(lengths)
        masks = [[1, 1, 1, 1 ,1],
                 [1, 1, 1, 0, 0],
                 [1, 1, 0, 0, 0]]

        With the reference tensor.

        >>> xs = torch.zeros((3, 2, 4))
        >>> make_non_pad_mask(lengths, xs)
        tensor([[[1, 1, 1, 1],
                 [1, 1, 1, 1]],
                [[1, 1, 1, 0],
                 [1, 1, 1, 0]],
                [[1, 1, 0, 0],
                 [1, 1, 0, 0]]], dtype=torch.uint8)
        >>> xs = torch.zeros((3, 2, 6))
        >>> make_non_pad_mask(lengths, xs)
        tensor([[[1, 1, 1, 1, 1, 0],
                 [1, 1, 1, 1, 1, 0]],
                [[1, 1, 1, 0, 0, 0],
                 [1, 1, 1, 0, 0, 0]],
                [[1, 1, 0, 0, 0, 0],
                 [1, 1, 0, 0, 0, 0]]], dtype=torch.uint8)

        With the reference tensor and dimension indicator.

        >>> xs = torch.zeros((3, 6, 6))
        >>> make_non_pad_mask(lengths, xs, 1)
        tensor([[[1, 1, 1, 1, 1, 1],
                 [1, 1, 1, 1, 1, 1],
                 [1, 1, 1, 1, 1, 1],
                 [1, 1, 1, 1, 1, 1],
                 [1, 1, 1, 1, 1, 1],
                 [0, 0, 0, 0, 0, 0]],
                [[1, 1, 1, 1, 1, 1],
                 [1, 1, 1, 1, 1, 1],
                 [1, 1, 1, 1, 1, 1],
                 [0, 0, 0, 0, 0, 0],
                 [0, 0, 0, 0, 0, 0],
                 [0, 0, 0, 0, 0, 0]],
                [[1, 1, 1, 1, 1, 1],
                 [1, 1, 1, 1, 1, 1],
                 [0, 0, 0, 0, 0, 0],
                 [0, 0, 0, 0, 0, 0],
                 [0, 0, 0, 0, 0, 0],
                 [0, 0, 0, 0, 0, 0]]], dtype=torch.uint8)
        >>> make_non_pad_mask(lengths, xs, 2)
        tensor([[[1, 1, 1, 1, 1, 0],
                 [1, 1, 1, 1, 1, 0],
                 [1, 1, 1, 1, 1, 0],
                 [1, 1, 1, 1, 1, 0],
                 [1, 1, 1, 1, 1, 0],
                 [1, 1, 1, 1, 1, 0]],
                [[1, 1, 1, 0, 0, 0],
                 [1, 1, 1, 0, 0, 0],
                 [1, 1, 1, 0, 0, 0],
                 [1, 1, 1, 0, 0, 0],
                 [1, 1, 1, 0, 0, 0],
                 [1, 1, 1, 0, 0, 0]],
                [[1, 1, 0, 0, 0, 0],
                 [1, 1, 0, 0, 0, 0],
                 [1, 1, 0, 0, 0, 0],
                 [1, 1, 0, 0, 0, 0],
                 [1, 1, 0, 0, 0, 0],
                 [1, 1, 0, 0, 0, 0]]], dtype=torch.uint8)

    """
    return ~make_pad_mask(lengths, xs, length_dim)


def mask_by_length(xs, lengths, fill=0):
    """Mask tensor according to length.

    Args:
        xs (Tensor): Batch of input tensor (B, `*`).
        lengths (LongTensor or List): Batch of lengths (B,).
        fill (int or float): Value to fill masked part.

    Returns:
        Tensor: Batch of masked input tensor (B, `*`).

    Examples:
        >>> x = torch.arange(5).repeat(3, 1) + 1
        >>> x
        tensor([[1, 2, 3, 4, 5],
                [1, 2, 3, 4, 5],
                [1, 2, 3, 4, 5]])
        >>> lengths = [5, 3, 2]
        >>> mask_by_length(x, lengths)
        tensor([[1, 2, 3, 4, 5],
                [1, 2, 3, 0, 0],
                [1, 2, 0, 0, 0]])

    """
    assert xs.size(0) == len(lengths)
    ret = xs.data.new(*xs.size()).fill_(fill)
    for i, l in enumerate(lengths):
        ret[i, :l] = xs[i, :l]
    return ret


def to_torch_tensor(x):
    """Change to torch.Tensor or ComplexTensor from numpy.ndarray.

    Args:
        x: Inputs. It should be one of numpy.ndarray, Tensor, ComplexTensor, and dict.

    Returns:
        Tensor or ComplexTensor: Type converted inputs.

    Examples:
        >>> xs = np.ones(3, dtype=np.float32)
        >>> xs = to_torch_tensor(xs)
        tensor([1., 1., 1.])
        >>> xs = torch.ones(3, 4, 5)
        >>> assert to_torch_tensor(xs) is xs
        >>> xs = {'real': xs, 'imag': xs}
        >>> to_torch_tensor(xs)
        ComplexTensor(
        Real:
        tensor([1., 1., 1.])
        Imag;
        tensor([1., 1., 1.])
        )

    """
    # If numpy, change to torch tensor
    if isinstance(x, np.ndarray):
        if x.dtype.kind == "c":
            # Dynamically importing because torch_complex requires python3
            from torch_complex.tensor import ComplexTensor

            return ComplexTensor(x)
        else:
            return torch.from_numpy(x)

    # If {'real': ..., 'imag': ...}, convert to ComplexTensor
    elif isinstance(x, dict):
        # Dynamically importing because torch_complex requires python3
        from torch_complex.tensor import ComplexTensor

        if "real" not in x or "imag" not in x:
            raise ValueError("has 'real' and 'imag' keys: {}".format(list(x)))
        # Relative importing because of using python3 syntax
        return ComplexTensor(x["real"], x["imag"])

    # If torch.Tensor, as it is
    elif isinstance(x, torch.Tensor):
        return x

    else:
        error = (
            "x must be numpy.ndarray, torch.Tensor or a dict like "
            "{{'real': torch.Tensor, 'imag': torch.Tensor}}, "
            "but got {}".format(type(x))
        )
        try:
            from torch_complex.tensor import ComplexTensor
        except Exception:
            # If PY2
            raise ValueError(error)
        else:
            # If PY3
            if isinstance(x, ComplexTensor):
                return x
            else:
                raise ValueError(error)


def get_subsample(train_args, mode, arch):
    """Parse the subsampling factors from the args for the specified `mode` and `arch`.

    Args:
        train_args: argument Namespace containing options.
        mode: one of ('asr', 'mt', 'st')
        arch: one of ('rnn', 'rnn-t', 'rnn_mix', 'rnn_mulenc', 'transformer')

    Returns:
        np.ndarray / List[np.ndarray]: subsampling factors.
    """
    if arch == "transformer":
        return np.array([1])

    elif mode == "mt" and arch == "rnn":
        # +1 means input (+1) and layers outputs (train_args.elayer)
        subsample = np.ones(train_args.elayers + 1, dtype=np.int32)
        logging.warning("Subsampling is not performed for machine translation.")
        logging.info("subsample: " + " ".join([str(x) for x in subsample]))
        return subsample

    elif (
        (mode == "asr" and arch in ("rnn", "rnn-t"))
        or (mode == "mt" and arch == "rnn")
        or (mode == "st" and arch == "rnn")
    ):
        subsample = np.ones(train_args.elayers + 1, dtype=np.int32)
        if train_args.etype.endswith("p") and not train_args.etype.startswith("vgg"):
            ss = train_args.subsample.split("_")
            for j in range(min(train_args.elayers + 1, len(ss))):
                subsample[j] = int(ss[j])
        else:
            logging.warning(
                "Subsampling is not performed for vgg*. "
                "It is performed in max pooling layers at CNN."
            )
        logging.info("subsample: " + " ".join([str(x) for x in subsample]))
        return subsample

    elif mode == "asr" and arch == "rnn_mix":
        subsample = np.ones(train_args.elayers_sd + train_args.elayers + 1, dtype=np.int32)
        if train_args.etype.endswith("p") and not train_args.etype.startswith("vgg"):
            ss = train_args.subsample.split("_")
            for j in range(min(train_args.elayers_sd + train_args.elayers + 1, len(ss))):
                subsample[j] = int(ss[j])
        else:
            logging.warning(
                "Subsampling is not performed for vgg*. "
                "It is performed in max pooling layers at CNN."
            )
        logging.info("subsample: " + " ".join([str(x) for x in subsample]))
        return subsample

    elif mode == "asr" and arch == "rnn_mulenc":
        subsample_list = []
        for idx in range(train_args.num_encs):
            subsample = np.ones(train_args.elayers[idx] + 1, dtype=np.int32)
            if train_args.etype[idx].endswith("p") and not train_args.etype[idx].startswith("vgg"):
                ss = train_args.subsample[idx].split("_")
                for j in range(min(train_args.elayers[idx] + 1, len(ss))):
                    subsample[j] = int(ss[j])
            else:
                logging.warning(
                    "Encoder %d: Subsampling is not performed for vgg*. "
                    "It is performed in max pooling layers at CNN.",
                    idx + 1,
                )
            logging.info("subsample: " + " ".join([str(x) for x in subsample]))
            subsample_list.append(subsample)
        return subsample_list

    else:
        raise ValueError("Invalid options: mode={}, arch={}".format(mode, arch))


def rename_state_dict(old_prefix: str, new_prefix: str, state_dict: Dict[str, torch.Tensor]):
    """Replace keys of old prefix with new prefix in state dict."""
    # need this list not to break the dict iterator
    old_keys = [k for k in state_dict if k.startswith(old_prefix)]
    if len(old_keys) > 0:
        logging.warning(f"Rename: {old_prefix} -> {new_prefix}")
    for k in old_keys:
        v = state_dict.pop(k)
        new_k = k.replace(old_prefix, new_prefix)
        state_dict[new_k] = v


class Swish(torch.nn.Module):
    """Swish activation definition.

    Swish(x) = (beta * x) * sigmoid(x)
                 where beta = 1 defines standard Swish activation.

    References:
        https://arxiv.org/abs/2108.12943 / https://arxiv.org/abs/1710.05941v1.
        E-swish variant: https://arxiv.org/abs/1801.07145.

    Args:
        beta: Beta parameter for E-Swish.
                (beta >= 1. If beta < 1, use standard Swish).
        use_builtin: Whether to use PyTorch function if available.

    """

    def __init__(self, beta: float = 1.0, use_builtin: bool = False) -> None:
        super().__init__()

        self.beta = beta

        if beta > 1:
            self.swish = lambda x: (self.beta * x) * torch.sigmoid(x)
        else:
            if use_builtin:
                self.swish = torch.nn.SiLU()
            else:
                self.swish = lambda x: x * torch.sigmoid(x)

    def forward(self, x: torch.Tensor) -> torch.Tensor:
        """Forward computation."""
        return self.swish(x)


def get_activation(act):
    """Return activation function."""

    activation_funcs = {
        "hardtanh": torch.nn.Hardtanh,
        "tanh": torch.nn.Tanh,
        "relu": torch.nn.ReLU,
        "selu": torch.nn.SELU,
        "swish": Swish,
    }

    return activation_funcs[act]()


class TooShortUttError(Exception):
    """Raised when the utt is too short for subsampling.

    Args:
        message: Error message to display.
        actual_size: The size that cannot pass the subsampling.
        limit: The size limit for subsampling.

    """

    def __init__(self, message: str, actual_size: int, limit: int) -> None:
        """Construct a TooShortUttError module."""
        super().__init__(message)

        self.actual_size = actual_size
        self.limit = limit


def check_short_utt(sub_factor: int, size: int) -> Tuple[bool, int]:
    """Check if the input is too short for subsampling.

    Args:
        sub_factor: Subsampling factor for Conv2DSubsampling.
        size: Input size.

    Returns:
        : Whether an error should be sent.
        : Size limit for specified subsampling factor.

    """
    if sub_factor == 2 and size < 3:
        return True, 7
    elif sub_factor == 4 and size < 7:
        return True, 7
    elif sub_factor == 6 and size < 11:
        return True, 11

    return False, -1


def sub_factor_to_params(sub_factor: int, input_size: int) -> Tuple[int, int, int]:
    """Get conv2D second layer parameters for given subsampling factor.

    Args:
        sub_factor: Subsampling factor (1/X).
        input_size: Input size.

    Returns:
        : Kernel size for second convolution.
        : Stride for second convolution.
        : Conv2DSubsampling output size.

    """
    if sub_factor == 2:
        return 3, 1, (((input_size - 1) // 2 - 2))
    elif sub_factor == 4:
        return 3, 2, (((input_size - 1) // 2 - 1) // 2)
    elif sub_factor == 6:
        return 5, 3, (((input_size - 1) // 2 - 2) // 3)
    else:
        raise ValueError("subsampling_factor parameter should be set to either 2, 4 or 6.")


def make_chunk_mask(
    size: int,
    chunk_size: int,
    left_chunk_size: int = 0,
    device: torch.device = None,
) -> torch.Tensor:
    """Create chunk mask for the subsequent steps (size, size).

    Reference: https://github.com/k2-fsa/icefall/blob/master/icefall/utils.py

    Args:
        size: Size of the source mask.
        chunk_size: Number of frames in chunk.
        left_chunk_size: Size of the left context in chunks (0 means full context).
        device: Device for the mask tensor.

    Returns:
        mask: Chunk mask. (size, size)

    """
    mask = torch.zeros(size, size, device=device, dtype=torch.bool)

    for i in range(size):
        if left_chunk_size < 0:
            start = 0
        else:
            start = max((i // chunk_size - left_chunk_size) * chunk_size, 0)

        end = min((i // chunk_size + 1) * chunk_size, size)
        mask[i, start:end] = True

    return ~mask


def make_source_mask(lengths: torch.Tensor) -> torch.Tensor:
    """Create source mask for given lengths.

    Reference: https://github.com/k2-fsa/icefall/blob/master/icefall/utils.py

    Args:
        lengths: Sequence lengths. (B,)

    Returns:
        : Mask for the sequence lengths. (B, max_len)

    """
    max_len = lengths.max()
    batch_size = lengths.size(0)

    expanded_lengths = torch.arange(max_len).expand(batch_size, max_len).to(lengths)

    return expanded_lengths >= lengths.unsqueeze(1)


def get_transducer_task_io(
    labels: torch.Tensor,
    encoder_out_lens: torch.Tensor,
    ignore_id: int = -1,
    blank_id: int = 0,
) -> Tuple[torch.Tensor, torch.Tensor, torch.Tensor, torch.Tensor]:
    """Get Transducer loss I/O.

    Args:
        labels: Label ID sequences. (B, L)
        encoder_out_lens: Encoder output lengths. (B,)
        ignore_id: Padding symbol ID.
        blank_id: Blank symbol ID.

    Returns:
        decoder_in: Decoder inputs. (B, U)
        target: Target label ID sequences. (B, U)
        t_len: Time lengths. (B,)
        u_len: Label lengths. (B,)

    """

    def pad_list(labels: List[torch.Tensor], padding_value: int = 0):
        """Create padded batch of labels from a list of labels sequences.

        Args:
            labels: Labels sequences. [B x (?)]
            padding_value: Padding value.

        Returns:
            labels: Batch of padded labels sequences. (B,)

        """
        batch_size = len(labels)

        padded = (
            labels[0]
            .new(batch_size, max(x.size(0) for x in labels), *labels[0].size()[1:])
            .fill_(padding_value)
        )

        for i in range(batch_size):
            padded[i, : labels[i].size(0)] = labels[i]

        return padded

    device = labels.device

    labels_unpad = [y[y != ignore_id] for y in labels]
    blank = labels[0].new([blank_id])

    decoder_in = pad_list(
        [torch.cat([blank, label], dim=0) for label in labels_unpad], blank_id
    ).to(device)

    target = pad_list(labels_unpad, blank_id).type(torch.int32).to(device)

    encoder_out_lens = list(map(int, encoder_out_lens))
    t_len = torch.IntTensor(encoder_out_lens).to(device)

    u_len = torch.IntTensor([y.size(0) for y in labels_unpad]).to(device)

    return decoder_in, target, t_len, u_len


def pad_to_len(t: torch.Tensor, pad_len: int, dim: int):
    """Pad the tensor `t` at `dim` to the length `pad_len` with right padding zeros."""
    if t.size(dim) == pad_len:
        return t
    else:
        pad_size = list(t.shape)
        pad_size[dim] = pad_len - t.size(dim)
        return torch.cat([t, torch.zeros(*pad_size, dtype=t.dtype, device=t.device)], dim=dim)
