# Copyright 2023 The Flax Authors.
#
# 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-based language model.

Reusing decoder only model from examples/wmt.
"""

# pylint: disable=attribute-defined-outside-init
# See issue #620.
# pytype: disable=wrong-arg-count
# pytype: disable=wrong-keyword-args
# pytype: disable=attribute-error

from typing import Callable, Any, Optional

from flax import linen as nn
from flax import struct
from jax import lax
import jax.numpy as jnp
import numpy as np

@struct.dataclass
class TransformerConfig:
  """Global hyperparameters used to minimize obnoxious kwarg plumbing."""
  vocab_size: int
  output_vocab_size: int
  share_embeddings: bool = False
  logits_via_embedding: bool = False
  dtype: Any = jnp.float32
  emb_dim: int = 512
  num_heads: int = 8
  num_layers: int = 6
  qkv_dim: int = 512
  mlp_dim: int = 2048
  max_len: int = 2048
  dropout_rate: float = 0.1
  attention_dropout_rate: float = 0.1
  deterministic: bool = False
  decode: bool = False
  kernel_init: Callable = nn.initializers.xavier_uniform()
  bias_init: Callable = nn.initializers.normal(stddev=1e-6)
  posemb_init: Optional[Callable] = None


def shift_right(x, axis=1):
  """Shift the input to the right by padding and slicing on axis."""
  pad_widths = [(0, 0)] * len(x.shape)
  pad_widths[axis] = (1, 0)
  padded = jnp.pad(
      x, pad_widths, mode='constant', constant_values=x.dtype.type(0))
  return lax.dynamic_slice_in_dim(padded, 0, padded.shape[axis] - 1, axis)


def shift_inputs(x, segment_ids=None, axis=1):
  """Shift inputs and replace EOS by 0 for packed inputs."""
  shifted = shift_right(x, axis=axis)
  # For packed targets, the first shifted token of a new sequence is made
  # 0, rather than being the EOS token for the last sequence.
  if segment_ids is not None:
    shifted *= (segment_ids == shift_right(segment_ids, axis=axis))
  return shifted


def sinusoidal_init(max_len=2048,
                    min_scale=1.0,
                    max_scale=10000.0):
  """1D Sinusoidal Position Embedding Initializer.

  Args:
      max_len: maximum possible length for the input.
      min_scale: float: minimum frequency-scale in sine grating.
      max_scale: float: maximum frequency-scale in sine grating.

  Returns:
      output: init function returning `(1, max_len, d_feature)`
  """

  def init(key, shape, dtype=np.float32):
    """Sinusoidal init."""
    del key, dtype
    d_feature = shape[-1]
    pe = np.zeros((max_len, d_feature), dtype=np.float32)
    position = np.arange(0, max_len)[:, np.newaxis]
    scale_factor = -np.log(max_scale / min_scale) / (d_feature // 2 - 1)
    div_term = min_scale * np.exp(np.arange(0, d_feature // 2) * scale_factor)
    pe[:, :d_feature // 2] = np.sin(position * div_term)
    pe[:, d_feature // 2: 2 * (d_feature // 2)] = np.cos(position * div_term)
    pe = pe[np.newaxis, :, :]  # [1, max_len, d_feature]
    return jnp.array(pe)

  return init


class AddPositionEmbs(nn.Module):
  """Adds (optionally learned) positional embeddings to the inputs.

  Args:
    config: TransformerConfig dataclass containing hyperparameters.
    decode: whether to run in single-position autoregressive mode.
  """
  config: TransformerConfig
  decode: bool = False

  @nn.compact
  def __call__(self,
               inputs,
               inputs_positions=None):
    """Applies AddPositionEmbs module.

    By default this layer uses a fixed sinusoidal embedding table. If a
    learned position embedding is desired, pass an initializer to
    posemb_init in the configuration.

    Args:
      inputs: input data.
      inputs_positions: input position indices for packed sequences.

    Returns:
      output: `(bs, timesteps, in_dim)`
    """
    config = self.config
    # inputs.shape is (batch_size, seq_len, emb_dim)
    assert inputs.ndim == 3, ('Number of dimensions should be 3,'
                              ' but it is: %d' % inputs.ndim)
    length = inputs.shape[1]
    pos_emb_shape = (1, config.max_len, inputs.shape[-1])
    if config.posemb_init is None:
      # Use a fixed (non-learned) sinusoidal position embedding.
      pos_embedding = sinusoidal_init(max_len=config.max_len)(None,
                                                              pos_emb_shape,
                                                              None)
    else:
      pos_embedding = self.param('pos_embedding', config.posemb_init,
                                 pos_emb_shape)
    pe = pos_embedding[:, :length, :]

    # We use a cache position index for tracking decoding position.
    if self.decode:
      is_initialized = self.has_variable('cache', 'cache_index')
      cache_index = self.variable('cache', 'cache_index',
                                  lambda: jnp.array(0, dtype=jnp.uint32))
      if is_initialized:
        i = cache_index.value
        cache_index.value = i + 1
        _, _, df = pos_embedding.shape
        pe = lax.dynamic_slice(pos_embedding,
                               jnp.array((0, i, 0)),
                               (1, 1, df))
    if inputs_positions is None:
      # normal unpacked case:
      return inputs + pe
    else:
      # for packed data we need to use known position indices:
      return inputs + jnp.take(pe[0], inputs_positions, axis=0)


class MlpBlock(nn.Module):
  """Transformer MLP / feed-forward block.

  Args:
    config: TransformerConfig dataclass containing hyperparameters.
    out_dim: optionally specify out dimension.
  """
  config: TransformerConfig
  out_dim: Optional[int] = None

  @nn.compact
  def __call__(self, inputs):
    """Applies Transformer MlpBlock module."""
    config = self.config
    actual_out_dim = (inputs.shape[-1] if self.out_dim is None
                      else self.out_dim)
    x = nn.Dense(
        config.mlp_dim,
        dtype=config.dtype,
        kernel_init=config.kernel_init,
        bias_init=config.bias_init)(
            inputs)
    x = nn.relu(x)
    x = nn.Dropout(rate=config.dropout_rate)(
        x, deterministic=config.deterministic)
    output = nn.Dense(
        actual_out_dim,
        dtype=config.dtype,
        kernel_init=config.kernel_init,
        bias_init=config.bias_init)(
            x)
    output = nn.Dropout(rate=config.dropout_rate)(
        output, deterministic=config.deterministic)
    return output


class EncoderDecoder1DBlock(nn.Module):
  """Transformer encoder-decoder layer.

  Args:
    config: TransformerConfig dataclass containing hyperparameters.
  """
  config: TransformerConfig

  @nn.compact
  def __call__(self,
               inputs,
               decoder_mask=None,
               encoder_decoder_mask=None):
    """Applies EncoderDecoder1DBlock module.

    Args:
      inputs: input data for decoder
      decoder_mask: decoder self-attention mask.
      encoder_decoder_mask: encoder-decoder attention mask.

    Returns:
      output after transformer encoder-decoder block.
    """
    config = self.config

    # Decoder block.
    assert inputs.ndim == 3
    x = nn.LayerNorm(dtype=config.dtype)(inputs)
    x = nn.SelfAttention(
        num_heads=config.num_heads,
        dtype=config.dtype,
        qkv_features=config.qkv_dim,
        kernel_init=config.kernel_init,
        bias_init=config.bias_init,
        use_bias=False,
        broadcast_dropout=False,
        dropout_rate=config.attention_dropout_rate,
        deterministic=config.deterministic,
        decode=config.decode)(x, decoder_mask)
    x = nn.Dropout(rate=config.dropout_rate)(
        x, deterministic=config.deterministic)
    x = x + inputs

    # MLP block.
    z = nn.LayerNorm(dtype=config.dtype)(x)
    z = MlpBlock(config=config)(z)

    return x + z


class Decoder(nn.Module):
  """Transformer Model Decoder for sequence to sequence translation.

  Args:
    config: TransformerConfig dataclass containing hyperparameters.
    shared_embedding: a shared embedding layer to use.
  """
  config: TransformerConfig
  shared_embedding: Any = None

  @nn.compact
  def __call__(self,
               inputs,
               inputs_positions=None,
               inputs_segmentation=None,
               decoder_mask=None,
               encoder_decoder_mask=None):
    """Applies Transformer model on the inputs.

    Args:
      encoded: encoded input data from encoder.
      inputs: input data.
      inputs_positions: input subsequence positions for packed examples.
      inputs_segmentation: input segmentation info for packed examples.
      decoder_mask: decoder self-attention mask.
      encoder_decoder_mask: encoder-decoder attention mask.

    Returns:
      output of a transformer decoder.
    """
    config = self.config
    assert inputs.ndim == 2  # (batch, len)

    # Target Embedding
    if self.shared_embedding is None:
      output_embed = nn.Embed(
          num_embeddings=config.output_vocab_size,
          features=config.emb_dim,
          embedding_init=nn.initializers.normal(stddev=1.0))
    else:
      output_embed = self.shared_embedding

    y = inputs.astype('int32')
    if not config.decode:
      y = shift_inputs(y, segment_ids=inputs_segmentation)
    y = output_embed(y)
    y = AddPositionEmbs(
        config=config, decode=config.decode, name='posembed_output')(
            y, inputs_positions=inputs_positions)
    y = nn.Dropout(rate=config.dropout_rate)(
        y, deterministic=config.deterministic)

    y = y.astype(config.dtype)

    # Target-Input Decoder
    for lyr in range(config.num_layers):
      y = EncoderDecoder1DBlock(
          config=config, name=f'encoderdecoderblock_{lyr}')(
              y,
              decoder_mask=decoder_mask,
              encoder_decoder_mask=encoder_decoder_mask)
    y = nn.LayerNorm(dtype=config.dtype, name='encoderdecoder_norm')(y)

    # Decoded Logits
    if config.logits_via_embedding:
      # Use the transpose of embedding matrix for logit transform.
      logits = output_embed.attend(y.astype(jnp.float32))
      # Correctly normalize pre-softmax logits for this shared case.
      logits = logits / jnp.sqrt(y.shape[-1])
    else:
      logits = nn.Dense(
          config.output_vocab_size,
          dtype=config.dtype,
          kernel_init=config.kernel_init,
          bias_init=config.bias_init,
          name='logitdense')(
              y)
    return logits


class TransformerLM(nn.Module):
  """Transformer pure decoder stack for language modelling.

  Args:
    config: TransformerConfig dataclass containing hyperparameters.
  """
  config: TransformerConfig

  @nn.compact
  def __call__(self,
               inputs,
               inputs_positions=None,
               inputs_segmentation=None):
    """Applies TransformerLM on the inputs.

    Args:
      inputs: target data.
      inputs_positions: input subsequence positions for packed examples.
      inputs_segmentation: input segmentation info for packed examples.

    Returns:
      logits array from transformer decoder.
    """
    config = self.config

    # Make padding attention masks.
    if config.decode:
      # for fast autoregressive decoding we use no decoder mask
      decoder_mask = None
    else:
      decoder_mask = nn.combine_masks(
          nn.make_attention_mask(inputs > 0, inputs > 0, dtype=config.dtype),
          nn.make_causal_mask(inputs, dtype=config.dtype))

    # Add segmentation block-diagonal attention masks if using segmented data.
    if inputs_segmentation is not None:
      decoder_mask = nn.combine_masks(
          decoder_mask,
          nn.make_attention_mask(
              inputs_segmentation,
              inputs_segmentation,
              jnp.equal,
              dtype=config.dtype))

    logits = Decoder(
        config=config, shared_embedding=None, name='decoder')(
            inputs,
            inputs_positions=inputs_positions,
            inputs_segmentation=inputs_segmentation,
            decoder_mask=decoder_mask,
            encoder_decoder_mask=None)
    return logits.astype(self.config.dtype)
