# Copyright 2018 MLBenchmark Group. 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.
# ==============================================================================
"""Input pipeline for the transformer model to read, filter, and batch examples.

Two things to note in the pipeline:

1. Batching scheme

   The examples encoded in the TFRecord files contain data in the format:
     {"inputs": [variable length array of integers],
      "targets": [variable length array of integers]}
   Where integers in the arrays refer to tokens in the English and German vocab
   file (named `vocab.ende.32768`).

   Prior to batching, elements in the dataset are grouped by length (max between
   "inputs" and "targets" length). Each group is then batched such that:
     group_batch_size * length <= batch_size.

   Another way to view batch_size is the maximum number of tokens in each batch.

   Once batched, each element in the dataset will have the shape:
     {"inputs": [group_batch_size, padded_input_length],
      "targets": [group_batch_size, padded_target_length]}
   Lengths are padded to the longest "inputs" or "targets" sequence in the batch
   (padded_input_length and padded_target_length can be different).

   This batching scheme decreases the fraction of padding tokens per training
   batch, thus improving the training speed significantly.

2. Shuffling

   While training, the dataset is shuffled in two places in the code. The first
   is the list of training files. Second, while reading records using
   `parallel_interleave`, the `sloppy` argument is used to generate randomness
   in the order of the examples.
"""

from __future__ import absolute_import
from __future__ import division
from __future__ import print_function

import os

import tensorflow as tf
import horovod.tensorflow as hvd

# Use the number of training files as the shuffle buffer.
_FILE_SHUFFLE_BUFFER = 100
# Buffer size for reading records from a TFRecord file. Each training file is
# 7.2 MB, so 8 MB allows an entire file to be kept in memory.
_READ_RECORD_BUFFER = 8 * 1000 * 1000

# Example grouping constants. Defines length boundaries for each group.
# These values are the defaults used in Tensor2Tensor.
_MIN_BOUNDARY = 8
_BOUNDARY_SCALE = 1.1


def _load_records(filename):
  """Read file and return a dataset of tf.Examples."""
  return tf.data.TFRecordDataset(filename, buffer_size=_READ_RECORD_BUFFER)


def _parse_example(serialized_example):
  """Return inputs and targets Tensors from a serialized tf.Example."""
  data_fields = {
      "inputs": tf.VarLenFeature(tf.int64),
      "targets": tf.VarLenFeature(tf.int64)
  }
  parsed = tf.parse_single_example(serialized_example, data_fields)
  inputs = tf.sparse_tensor_to_dense(parsed["inputs"])
  targets = tf.sparse_tensor_to_dense(parsed["targets"])
  return inputs, targets


def _filter_max_length(example, max_length=256):
  """Indicates whether the example's length is lower than the maximum length."""
  return tf.logical_and(tf.size(example[0]) <= max_length,
                        tf.size(example[1]) <= max_length)


def _get_example_length(example):
  """Returns the maximum length between the example inputs and targets."""
  length = tf.maximum(tf.shape(example[0])[0], tf.shape(example[1])[0])
  return length


def _create_min_max_boundaries(
    max_length, min_boundary=_MIN_BOUNDARY, boundary_scale=_BOUNDARY_SCALE):
  """Create min and max boundary lists up to max_length.

  For example, when max_length=24, min_boundary=4 and boundary_scale=2, the
  returned values will be:
    buckets_min = [0, 4, 8, 16, 24]
    buckets_max = [4, 8, 16, 24, 25]

  Args:
    max_length: The maximum length of example in dataset.
    min_boundary: Minimum length in boundary.
    boundary_scale: Amount to scale consecutive boundaries in the list.

  Returns:
    min and max boundary lists

  """
  # Create bucket boundaries list by scaling the previous boundary or adding 1
  # (to ensure increasing boundary sizes).
  bucket_boundaries = []
  x = min_boundary
  while x < max_length:
    bucket_boundaries.append(x)
    x = max(x + 1, int(x * boundary_scale))

  # Create min and max boundary lists from the initial list.
  buckets_min = [0] + bucket_boundaries
  buckets_max = bucket_boundaries + [max_length + 1]
  return buckets_min, buckets_max


def _batch_examples(dataset, batch_size, max_length):
  """Group examples by similar lengths, and return batched dataset.

  Each batch of similar-length examples are padded to the same length, and may
  have different number of elements in each batch, such that:
    group_batch_size * padded_length <= batch_size.

  This decreases the number of padding tokens per batch, which improves the
  training speed.

  Args:
    dataset: Dataset of unbatched examples.
    batch_size: Max number of tokens per batch of examples.
    max_length: Max number of tokens in an example input or target sequence.

  Returns:
    Dataset of batched examples with similar lengths.
  """
  # Get min and max boundary lists for each example. These are used to calculate
  # the `bucket_id`, which is the index at which:
  # buckets_min[bucket_id] <= len(example) < buckets_max[bucket_id]
  # Note that using both min and max lists improves the performance.
  buckets_min, buckets_max = _create_min_max_boundaries(max_length)

  # Create list of batch sizes for each bucket_id, so that
  # bucket_batch_size[bucket_id] * buckets_max[bucket_id] <= batch_size
  bucket_batch_sizes = [(batch_size // x) // 8 * 8 for x in buckets_max]  
  # bucket_id will be a tensor, so convert this list to a tensor as well.
  bucket_batch_sizes = tf.constant(bucket_batch_sizes, dtype=tf.int64)

  def example_to_bucket_id(example_input, example_target):
    """Return int64 bucket id for this example, calculated based on length."""
    seq_length = _get_example_length((example_input, example_target))

    # TODO: investigate whether removing code branching improves performance.
    conditions_c = tf.logical_and(
        tf.less_equal(buckets_min, seq_length),
        tf.less(seq_length, buckets_max))
    bucket_id = tf.reduce_min(tf.where(conditions_c))
    return bucket_id

  def window_size_fn(bucket_id):
    """Return number of examples to be grouped when given a bucket id."""
    return bucket_batch_sizes[bucket_id]

  def batching_fn(bucket_id, grouped_dataset):
    """Batch and add padding to a dataset of elements with similar lengths."""
    bucket_batch_size = window_size_fn(bucket_id)

    # Batch the dataset and add padding so that all input sequences in the
    # examples have the same length, and all target sequences have the same
    # lengths as well. Resulting lengths of inputs and targets can differ.
    return grouped_dataset.padded_batch(bucket_batch_size, ([None], [None]))

  return dataset.apply(tf.contrib.data.group_by_window(
      key_func=example_to_bucket_id,
      reduce_func=batching_fn,
      window_size=None,
      window_size_func=window_size_fn))


def _read_and_batch_from_files(
    file_pattern, batch_size, max_length, num_cpu_cores, shuffle, repeat, distributed):
  """Create dataset where each item is a dict of "inputs" and "targets".

  Args:
    file_pattern: String used to match the input TFRecord files.
    batch_size: Maximum number of tokens per batch of examples
    max_length: Maximum number of tokens per example
    num_cpu_cores: Number of cpu cores for parallel input processing.
    shuffle: If true, randomizes order of elements.
    repeat: Number of times to repeat the dataset. If None, the dataset is
      repeated forever.

  Returns:
    tf.data.Dataset object containing examples loaded from the files.
  """
  dataset = tf.data.Dataset.list_files(file_pattern)

  if shuffle:
    # Shuffle filenames
    dataset = dataset.shuffle(buffer_size=_FILE_SHUFFLE_BUFFER)

  # Read files and interleave results. When training, the order of the examples
  # will be non-deterministic.
  if distributed:
      dataset= dataset.shard(hvd.size(), hvd.rank())
  dataset = dataset.apply(
      tf.contrib.data.parallel_interleave(
          _load_records, sloppy=shuffle, cycle_length=num_cpu_cores))

  # Parse each tf.Example into a dictionary
  # TODO: Look into prefetch_input_elements for performance optimization.
  dataset = dataset.map(_parse_example,
                        num_parallel_calls=num_cpu_cores)

  # Remove examples where the input or target length exceeds the maximum length,
  dataset = dataset.filter(lambda x, y: _filter_max_length((x, y), max_length))

  # Batch such that each batch has examples of similar length.
  dataset = _batch_examples(dataset, batch_size, max_length)
  dataset = dataset.repeat(repeat)

  # Prefetch the next element to improve speed of input pipeline.
  dataset = dataset.prefetch(1)
  return dataset


def train_input_fn(params):
  """Load and return dataset of batched examples for use during training."""
  file_pattern = os.path.join(getattr(params, "data_dir", ""), "*encoded-train*")
  return _read_and_batch_from_files(
      file_pattern, params.batch_size, params.max_length, params.num_cpu_cores,
      shuffle=True, repeat=None, distributed=params.enable_horovod)


def eval_input_fn(params):
  """Load and return dataset of batched examples for use during evaluation."""
  file_pattern = os.path.join(getattr(params, "data_dir", ""), "*encoded-dev*")
  return _read_and_batch_from_files(
      file_pattern, params.batch_size, params.max_length, params.num_cpu_cores,
      shuffle=False, repeat=1, distributed=False)
