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#
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
#
#     http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
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"""Keras layer that creates a self-attention mask."""

import tensorflow as tf


@tf.keras.utils.register_keras_serializable(package='Text')
class SelfAttentionMask(tf.keras.layers.Layer):
  """Create 3D attention mask from a 2D tensor mask.

    inputs[0]: from_tensor: 2D or 3D Tensor of shape
      [batch_size, from_seq_length, ...].
    inputs[1]: to_mask: int32 Tensor of shape [batch_size, to_seq_length].

    Returns:
      float Tensor of shape [batch_size, from_seq_length, to_seq_length].
  """

  def call(self, inputs, to_mask=None):
    if isinstance(inputs, list) and to_mask is None:
      to_mask = inputs[1]
      inputs = inputs[0]
    from_shape = tf.shape(inputs)
    batch_size = from_shape[0]
    from_seq_length = from_shape[1]

    to_shape = tf.shape(to_mask)
    to_seq_length = to_shape[1]

    to_mask = tf.cast(
        tf.reshape(to_mask, [batch_size, 1, to_seq_length]),
        dtype=inputs.dtype)

    return tf.broadcast_to(to_mask,
                           [batch_size, from_seq_length, to_seq_length])
