# coding=utf-8
# Copyright 2021 The Google Research Authors.
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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.
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#     http://www.apache.org/licenses/LICENSE-2.0
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# Copyright 2019 The TensorFlow Authors. All Rights Reserved.
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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
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#     http://www.apache.org/licenses/LICENSE-2.0
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# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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# ==============================================================================
"""Keras-based einsum layer.

Copied from
https://github.com/tensorflow/models/blob/master/official/nlp/modeling/layers/dense_einsum.py.
"""

# Copied from:
# https://github.com/google-research/google-research/blob/master/performer/fast_attention/tensorflow/util.py

import tensorflow as tf

_CHR_IDX = ["a", "b", "c", "d", "e", "f", "g", "h", "i", "j", "k", "l", "m"]


@tf.keras.utils.register_keras_serializable(package="Text")
class DenseEinsum(tf.keras.layers.Layer):
  """A densely connected layer that uses tf.einsum as the backing computation.

  This layer can perform einsum calculations of arbitrary dimensionality.

  Arguments:
    output_shape: Positive integer or tuple, dimensionality of the output space.
    num_summed_dimensions: The number of dimensions to sum over. Standard 2D
      matmul should use 1, 3D matmul should use 2, and so forth.
    activation: Activation function to use. If you don't specify anything, no
      activation is applied
      (ie. "linear" activation: `a(x) = x`).
    use_bias: Boolean, whether the layer uses a bias vector.
    kernel_initializer: Initializer for the `kernel` weights matrix.
    bias_initializer: Initializer for the bias vector.
    kernel_regularizer: Regularizer function applied to the `kernel` weights
      matrix.
    bias_regularizer: Regularizer function applied to the bias vector.
    activity_regularizer: Regularizer function applied to the output of the
      layer (its "activation")..
    kernel_constraint: Constraint function applied to the `kernel` weights
      matrix.
    bias_constraint: Constraint function applied to the bias vector.
  Input shape:
    N-D tensor with shape: `(batch_size, ..., input_dim)`. The most common
      situation would be a 2D input with shape `(batch_size, input_dim)`.
  Output shape:
    N-D tensor with shape: `(batch_size, ..., units)`. For instance, for a 2D
      input with shape `(batch_size, input_dim)`, the output would have shape
      `(batch_size, units)`.
  """

  def __init__(self,
               output_shape,
               num_summed_dimensions=1,
               activation=None,
               use_bias=True,
               kernel_initializer="glorot_uniform",
               bias_initializer="zeros",
               kernel_regularizer=None,
               bias_regularizer=None,
               activity_regularizer=None,
               kernel_constraint=None,
               bias_constraint=None,
               **kwargs):
    super(DenseEinsum, self).__init__(**kwargs)
    self._output_shape = output_shape if isinstance(
        output_shape, (list, tuple)) else (output_shape,)
    self._activation = tf.keras.activations.get(activation)
    self._use_bias = use_bias
    self._kernel_initializer = tf.keras.initializers.get(kernel_initializer)
    self._bias_initializer = tf.keras.initializers.get(bias_initializer)
    self._kernel_regularizer = tf.keras.regularizers.get(kernel_regularizer)
    self._bias_regularizer = tf.keras.regularizers.get(bias_regularizer)
    self._kernel_constraint = tf.keras.constraints.get(kernel_constraint)
    self._bias_constraint = tf.keras.constraints.get(bias_constraint)
    self._num_summed_dimensions = num_summed_dimensions
    self._einsum_string = None

  def _build_einsum_string(self, free_input_dims, bound_dims, output_dims):
    input_str = ""
    kernel_str = ""
    output_str = ""
    letter_offset = 0
    for i in range(free_input_dims):
      char = _CHR_IDX[i + letter_offset]
      input_str += char
      output_str += char

    letter_offset += free_input_dims
    for i in range(bound_dims):
      char = _CHR_IDX[i + letter_offset]
      input_str += char
      kernel_str += char

    letter_offset += bound_dims
    for i in range(output_dims):
      char = _CHR_IDX[i + letter_offset]
      kernel_str += char
      output_str += char

    return input_str + "," + kernel_str + "->" + output_str

  def build(self, input_shape):
    input_shape = tf.TensorShape(input_shape)
    input_rank = input_shape.rank
    free_input_dims = input_rank - self._num_summed_dimensions
    output_dims = len(self._output_shape)

    self._einsum_string = self._build_einsum_string(free_input_dims,
                                                    self._num_summed_dimensions,
                                                    output_dims)

    # This is only saved for testing purposes.
    self._kernel_shape = (
        input_shape[free_input_dims:].concatenate(self._output_shape))

    self._kernel = self.add_weight(
        "kernel",
        shape=self._kernel_shape,
        initializer=self._kernel_initializer,
        regularizer=self._kernel_regularizer,
        constraint=self._kernel_constraint,
        dtype=self.dtype,
        trainable=True)
    if self._use_bias:
      self._bias = self.add_weight(
          "bias",
          shape=self._output_shape,
          initializer=self._bias_initializer,
          regularizer=self._bias_regularizer,
          constraint=self._bias_constraint,
          dtype=self.dtype,
          trainable=True)
    else:
      self._bias = None
    super(DenseEinsum, self).build(input_shape)

  def get_config(self):
    config = {
        "output_shape":
            self._output_shape,
        "num_summed_dimensions":
            self._num_summed_dimensions,
        "activation":
            tf.keras.activations.serialize(self._activation),
        "use_bias":
            self._use_bias,
        "kernel_initializer":
            tf.keras.initializers.serialize(self._kernel_initializer),
        "bias_initializer":
            tf.keras.initializers.serialize(self._bias_initializer),
        "kernel_regularizer":
            tf.keras.regularizers.serialize(self._kernel_regularizer),
        "bias_regularizer":
            tf.keras.regularizers.serialize(self._bias_regularizer),
        "activity_regularizer":
            tf.keras.regularizers.serialize(self._activity_regularizer),
        "kernel_constraint":
            tf.keras.constraints.serialize(self._kernel_constraint),
        "bias_constraint":
            tf.keras.constraints.serialize(self._bias_constraint)
    }
    base_config = super(DenseEinsum, self).get_config()
    return dict(list(base_config.items()) + list(config.items()))

  def call(self, inputs):
    ret = tf.einsum(self._einsum_string, inputs, self._kernel)
    if self._use_bias:
      ret += self._bias
    if self._activation is not None:
      ret = self._activation(ret)
    return ret
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