"""Define the Google's Transformer model."""

import tensorflow as tf

from opennmt.models.sequence_to_sequence import SequenceToSequence, EmbeddingsSharingLevel
from opennmt.encoders.encoder import ParallelEncoder
from opennmt.encoders.self_attention_encoder import SelfAttentionEncoder_Length
from opennmt.decoders.self_attention_decoder import SelfAttentionDecoder
from opennmt.layers.position import SinusoidalPositionEncoder
from opennmt.utils.misc import merge_dict


class Transformer(SequenceToSequence):
  """Attention-based sequence-to-sequence model as described in
  https://arxiv.org/abs/1706.03762.
  """

  def __init__(self,
               source_inputter,
               target_inputter,
               num_layers,
               num_units,
               num_heads,
               ffn_inner_dim,
               dropout=0.1,
               attention_dropout=0.1,
               relu_dropout=0.1,
               position_encoder=SinusoidalPositionEncoder(),
               decoder_self_attention_type="scaled_dot",
               share_embeddings=EmbeddingsSharingLevel.NONE,
               share_encoders=False,
               alignment_file_key="train_alignments",
               name="transformer"):
    """Initializes a Transformer model.

    Args:
      source_inputter: A :class:`opennmt.inputters.inputter.Inputter` to process
        the source data. If this inputter returns parallel inputs, a multi
        source Transformer architecture will be constructed.
      target_inputter: A :class:`opennmt.inputters.inputter.Inputter` to process
        the target data. Currently, only the
        :class:`opennmt.inputters.text_inputter.WordEmbedder` is supported.
      num_layers: The shared number of layers.
      num_units: The number of hidden units.
      num_heads: The number of heads in each self-attention layers.
      ffn_inner_dim: The inner dimension of the feed forward layers.
      dropout: The probability to drop units in each layer output.
      attention_dropout: The probability to drop units from the attention.
      relu_dropout: The probability to drop units from the ReLU activation in
        the feed forward layer.
      position_encoder: A :class:`opennmt.layers.position.PositionEncoder` to
        apply on the inputs.
      decoder_self_attention_type: Type of self attention in the decoder,
        "scaled_dot" or "average" (case insensitive).
      share_embeddings: Level of embeddings sharing, see
        :class:`opennmt.models.sequence_to_sequence.EmbeddingsSharingLevel`
        for possible values.
      share_encoders: In case of multi source architecture, whether to share the
        separate encoders parameters or not.
      alignment_file_key: The data configuration key of the training alignment
        file to support guided alignment.
      name: The name of this model.
    """
    encoders = [
        SelfAttentionEncoder_Length(
            num_layers,
            num_units=num_units,
            num_heads=num_heads,
            ffn_inner_dim=ffn_inner_dim,
            dropout=dropout,
            attention_dropout=attention_dropout,
            relu_dropout=relu_dropout,
            position_encoder=position_encoder)
        for _ in range(source_inputter.num_outputs)]
    if len(encoders) > 1:
      encoder = ParallelEncoder(
          encoders,
          outputs_reducer=None,
          states_reducer=None,
          share_parameters=share_encoders)
    else:
      encoder = encoders[0]
    decoder = SelfAttentionDecoder(
        num_layers,
        num_units=num_units,
        num_heads=num_heads,
        ffn_inner_dim=ffn_inner_dim,
        dropout=dropout,
        attention_dropout=attention_dropout,
        relu_dropout=relu_dropout,
        position_encoder=position_encoder,
        self_attention_type=decoder_self_attention_type)

    self._num_units = num_units
    super(Transformer, self).__init__(
        source_inputter,
        target_inputter,
        encoder,
        decoder,
        share_embeddings=share_embeddings,
        alignment_file_key=alignment_file_key,
        daisy_chain_variables=True,
        name=name)

  def auto_config(self, num_devices=1):
    config = super(Transformer, self).auto_config(num_devices=num_devices)
    return merge_dict(config, {
        "params": {
            "average_loss_in_time": True,
            "label_smoothing": 0.1,
            "optimizer": "LazyAdamOptimizer",
            "optimizer_params": {
                "beta1": 0.9,
                "beta2": 0.998
            },
            "learning_rate": 2.0,
            "decay_type": "noam_decay_v2",
            "decay_params": {
                "model_dim": self._num_units,
                "warmup_steps": 8000
            }
        },
        "train": {
            "effective_batch_size": 25000,
            "batch_size": 3072,
            "batch_type": "tokens",
            "maximum_features_length": 100,
            "maximum_labels_length": 100,
            "keep_checkpoint_max": 8,
            "average_last_checkpoints": 8
        }
    })

  def _initializer(self, params):
    return tf.variance_scaling_initializer(
        mode="fan_avg", distribution="uniform", dtype=self.dtype)
