# Copyright 2026 The TensorFlow Authors. All Rights Reserved.
#
# 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,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.

"""Transformer encoder."""

# pylint: disable=g-classes-have-attributes

from typing import Any, Callable, Optional, Union

from absl import logging
import tensorflow as tf, tf_keras
import tensorflow_models as tfm

from official.modeling import tf_utils

layers = tfm.nlp.layers

_Initializer = Union[str, tf_keras.initializers.Initializer]
_approx_gelu = lambda x: tf_keras.activations.gelu(x, approximate=True)


class TransformerEncoder(tf_keras.layers.Layer):
  """TransformerEncoder.

  Args:
    vocab_size: The size of the token vocabulary.
    pad_token_id: the token id for the pad token
    hidden_size: The size of the transformer hidden layers.
    num_layers: The number of transformer layers.
    num_attention_heads: The number of attention heads for each transformer. The
      hidden size must be divisible by the number of attention heads.
    max_sequence_length: The maximum sequence length that this encoder can
      consume. If None, max_sequence_length uses the value from sequence length.
      This determines the variable shape for positional embeddings.
    type_vocab_size: The number of types that the 'type_ids' input can take.
    inner_dim: The output dimension of the first Dense layer in a two-layer
      feedforward network for each transformer.
    inner_activation: The activation for the first Dense layer in a two-layer
      feedforward network for each transformer.
    output_dropout: Dropout probability for the post-attention and output
      dropout.
    attention_dropout: The dropout rate to use for the attention layers within
      the transformer layers.
    initializer: The initialzer to use for all weights in this encoder.
    output_range: The sequence output range, [0, output_range), by slicing the
      target sequence of the last transformer layer. `None` means the entire
      target sequence will attend to the source sequence, which yields the full
      output.
    embedding_width: The width of the word embeddings. If the embedding width is
      not equal to hidden size, embedding parameters will be factorized into two
      matrices in the shape of ['vocab_size', 'embedding_width'] and
      ['embedding_width', 'hidden_size'] ('embedding_width' is usually much
      smaller than 'hidden_size').
    embedding_layer: An optional Layer instance which will be called to generate
      embeddings for the input word IDs.
    norm_first: Whether to normalize inputs to attention and intermediate dense
      layers. If set False, output of attention and intermediate dense layers is
      normalized.
  """

  def __init__(
      self,
      vocab_size: int,
      hidden_size: int = 768,
      num_layers: int = 12,
      num_attention_heads: int = 12,
      max_sequence_length: int = 512,
      type_vocab_size: int = 16,
      inner_dim: int = 3072,
      inner_activation: Callable[..., Any] = _approx_gelu,
      output_dropout: float = 0.1,
      attention_dropout: float = 0.1,
      initializer: _Initializer = tf_keras.initializers.TruncatedNormal(
          stddev=0.02
      ),
      output_range: Optional[int] = None,
      embedding_width: Optional[int] = None,
      embedding_layer: Optional[tf_keras.layers.Layer] = None,
      norm_first: bool = False,
      **kwargs
  ):
    super().__init__(**kwargs)

    activation = tf_keras.activations.get(inner_activation)
    initializer = tf_keras.initializers.get(initializer)

    if embedding_width is None:
      embedding_width = hidden_size

    if embedding_layer is None:
      self._embedding_layer = layers.OnDeviceEmbedding(
          vocab_size=vocab_size,
          embedding_width=embedding_width,
          initializer=initializer,
          name='word_embeddings',
      )
    else:
      self._embedding_layer = embedding_layer

    self._position_embedding_layer = layers.PositionEmbedding(
        initializer=initializer,
        max_length=max_sequence_length,
        name='position_embedding',
    )

    self._type_embedding_layer = layers.OnDeviceEmbedding(
        vocab_size=type_vocab_size,
        embedding_width=embedding_width,
        initializer=initializer,
        use_one_hot=True,
        name='type_embeddings',
    )

    self._embedding_norm_layer = tf_keras.layers.LayerNormalization(
        name='embeddings/layer_norm', axis=-1, epsilon=1e-12, dtype=tf.float32
    )

    self._embedding_dropout = tf_keras.layers.Dropout(
        rate=output_dropout, name='embedding_dropout'
    )

    # We project the 'embedding' output to 'hidden_size' if it is not already
    # 'hidden_size'.
    self._embedding_projection = None
    if embedding_width != hidden_size:
      self._embedding_projection = tf_keras.layers.EinsumDense(
          '...x,xy->...y',
          output_shape=hidden_size,
          bias_axes='y',
          kernel_initializer=initializer,
          name='embedding_projection',
      )

    self._transformer_layers = []
    self._attention_mask_layer = layers.SelfAttentionMask(
        name='self_attention_mask'
    )
    for i in range(num_layers):
      layer = layers.TransformerEncoderBlock(
          num_attention_heads=num_attention_heads,
          inner_dim=inner_dim,
          inner_activation=inner_activation,
          output_dropout=output_dropout,
          attention_dropout=attention_dropout,
          norm_first=norm_first,
          return_attention_scores=False,
          kernel_initializer=tf_utils.clone_initializer(initializer),
          name='transformer/layer_%d' % i,
      )
      self._transformer_layers.append(layer)
    self._num_layers = num_layers

    self._pooler_layer = tf_keras.layers.Dense(
        units=hidden_size,
        activation='tanh',
        kernel_initializer=initializer,
        name='pooler_transform',
    )
    self._config = {
        'vocab_size': vocab_size,
        'hidden_size': hidden_size,
        'num_layers': num_layers,
        'num_attention_heads': num_attention_heads,
        'max_sequence_length': max_sequence_length,
        'type_vocab_size': type_vocab_size,
        'inner_dim': inner_dim,
        'inner_activation': tf_keras.activations.serialize(activation),
        'output_dropout': output_dropout,
        'attention_dropout': attention_dropout,
        'initializer': tf_keras.initializers.serialize(initializer),
        'output_range': output_range,
        'embedding_width': embedding_width,
        'embedding_layer': embedding_layer,
        'norm_first': norm_first,
    }
    self.inputs = dict(
        input_word_ids=tf_keras.Input(shape=(None,), dtype=tf.int32),
        input_mask=tf_keras.Input(shape=(None,), dtype=tf.int32),
        input_type_ids=tf_keras.Input(shape=(None,), dtype=tf.int32),
    )

  def call(self, inputs):
    word_embeddings = None

    if isinstance(inputs, dict):
      if 'input_word_ids' in inputs.keys():
        word_ids = inputs.get('input_word_ids')
        mask = inputs.get('input_mask')
        type_ids = inputs.get('input_type_ids', None)
        word_embeddings = inputs.get('input_word_embeddings', None)
      elif 'left_word_ids' in inputs.keys():
        word_ids = inputs.get('left_word_ids')
        mask = inputs.get('left_mask')
      elif 'right_word_ids' in inputs.keys():
        word_ids = inputs.get('right_word_ids')
        mask = inputs.get('right_mask')
      dense_inputs = inputs.get('dense_inputs', None)
      dense_mask = inputs.get('dense_mask', None)
      dense_type_ids = inputs.get('dense_type_ids', None)
    elif isinstance(inputs, list):
      ## Dual Encoder Tasks
      word_ids, mask = inputs
      type_ids = None
      dense_inputs, dense_mask, dense_type_ids = None, None, None
    else:
      raise ValueError('Unexpected inputs type to %s.' % self.__class__)

    if type_ids is None:
      type_ids = tf.zeros_like(mask)

    if word_embeddings is None:
      word_embeddings = self._embedding_layer(word_ids)

    if dense_inputs is not None:
      mask = tf.concat([mask, dense_mask], axis=1)

    embeddings = self._get_embeddings(
        word_ids, type_ids, word_embeddings, dense_inputs, dense_type_ids
    )
    embeddings = self._embedding_norm_layer(embeddings)
    embeddings = self._embedding_dropout(embeddings)

    if self._embedding_projection is not None:
      embeddings = self._embedding_projection(embeddings)

    attention_mask = self._attention_mask_layer(embeddings, mask)

    encoder_outputs = []
    x = embeddings
    for layer in self._transformer_layers:
      x = layer([x, attention_mask])
      encoder_outputs.append(x)

    last_encoder_output = encoder_outputs[-1]
    first_token_tensor = last_encoder_output[:, 0, :]
    pooled_output = self._pooler_layer(first_token_tensor)

    output = dict(
        sequence_output=encoder_outputs[-1],
        pooled_output=pooled_output,
        encoder_outputs=encoder_outputs,
    )

    return output

  def get_embedding_table(self):
    return self._embedding_layer.embeddings

  def get_embedding_layer(self):
    return self._embedding_layer

  def get_config(self):
    return dict(self._config)

  @property
  def transformer_layers(self):
    """List of Transformer layers in the encoder."""
    return self._transformer_layers

  @property
  def pooler_layer(self):
    """The pooler dense layer after the transformer layers."""
    return self._pooler_layer

  @classmethod
  def from_config(cls, config, custom_objects=None):
    if 'embedding_layer' in config and config['embedding_layer'] is not None:
      warn_string = (
          'You are reloading a model that was saved with a '
          'potentially-shared embedding layer object. If you contine to '
          'train this model, the embedding layer will no longer be shared. '
          'To work around this, load the model outside of the Keras API.'
      )
      print('WARNING: ' + warn_string)
      logging.warn(warn_string)

    return cls(**config)

  def _get_embeddings(
      self,
      word_ids: tf.Tensor,
      type_ids: tf.Tensor,
      word_embeddings: Optional[tf.Tensor],
      dense_inputs: Optional[tf.Tensor],
      dense_type_ids: Optional[tf.Tensor],
  ) -> tf.Tensor:
    if word_embeddings is None:
      word_embeddings = self._embedding_layer(word_ids)

    if dense_inputs is not None:
      # Concat the dense embeddings at sequence end.
      word_embeddings = tf.concat([word_embeddings, dense_inputs], axis=1)
      type_ids = tf.concat([type_ids, dense_type_ids], axis=1)

    type_embeddings = self._type_embedding_layer(type_ids)

    # absolute position embeddings.
    position_embeddings = self._position_embedding_layer(word_embeddings)
    return word_embeddings + position_embeddings + type_embeddings