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# pylint: disable=invalid-name
"""Sequence mask in python"""

import numpy as np


def sequence_mask(data, valid_length, mask_value, axis):
    """batch_matmul operator implemented in numpy.

    Parameters
    ----------
    data : numpy.ndarray
        N-D with shape [batch_size, MAX_LENGTH, ...] or [MAX_LENGTH, batch_size, ...]

    valid_length : numpy.ndarray
        1-D with shape [batch_size,]

    mask_value : float
        Masking value

    axis : int
        The axis of the length dimension

    Returns
    -------
    out : numpy.ndarray
        N-D with shape same as data
    """
    in_shape = data.shape
    max_length = data.shape[axis]
    val_len_expand_shape = [1 for _ in range(len(in_shape))]
    val_len_expand_shape[1 - axis] = in_shape[1 - axis]
    seq_len_expand_shape = [1 for _ in range(len(in_shape))]
    seq_len_expand_shape[axis] = in_shape[axis]
    mask = np.broadcast_to(
        np.arange(max_length).reshape(seq_len_expand_shape), in_shape
    ) >= valid_length.reshape(val_len_expand_shape)
    out = data * (1 - mask) + mask_value * mask
    return out
