# Copyright 2021 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.

"""ImageNet input pipeline.
"""

import jax

import tensorflow as tf
import tensorflow_datasets as tfds


IMAGE_SIZE = 224
CROP_PADDING = 32
MEAN_RGB = [0.485 * 255, 0.456 * 255, 0.406 * 255]
STDDEV_RGB = [0.229 * 255, 0.224 * 255, 0.225 * 255]


def distorted_bounding_box_crop(image_bytes,
                                bbox,
                                min_object_covered=0.1,
                                aspect_ratio_range=(0.75, 1.33),
                                area_range=(0.05, 1.0),
                                max_attempts=100):
  """Generates cropped_image using one of the bboxes randomly distorted.

  See `tf.image.sample_distorted_bounding_box` for more documentation.

  Args:
    image_bytes: `Tensor` of binary image data.
    bbox: `Tensor` of bounding boxes arranged `[1, num_boxes, coords]`
        where each coordinate is [0, 1) and the coordinates are arranged
        as `[ymin, xmin, ymax, xmax]`. If num_boxes is 0 then use the whole
        image.
    min_object_covered: An optional `float`. Defaults to `0.1`. The cropped
        area of the image must contain at least this fraction of any bounding
        box supplied.
    aspect_ratio_range: An optional list of `float`s. The cropped area of the
        image must have an aspect ratio = width / height within this range.
    area_range: An optional list of `float`s. The cropped area of the image
        must contain a fraction of the supplied image within in this range.
    max_attempts: An optional `int`. Number of attempts at generating a cropped
        region of the image of the specified constraints. After `max_attempts`
        failures, return the entire image.
  Returns:
    cropped image `Tensor`
  """
  shape = tf.io.extract_jpeg_shape(image_bytes)
  sample_distorted_bounding_box = tf.image.sample_distorted_bounding_box(
      shape,
      bounding_boxes=bbox,
      min_object_covered=min_object_covered,
      aspect_ratio_range=aspect_ratio_range,
      area_range=area_range,
      max_attempts=max_attempts,
      use_image_if_no_bounding_boxes=True)
  bbox_begin, bbox_size, _ = sample_distorted_bounding_box

  # Crop the image to the specified bounding box.
  offset_y, offset_x, _ = tf.unstack(bbox_begin)
  target_height, target_width, _ = tf.unstack(bbox_size)
  crop_window = tf.stack([offset_y, offset_x, target_height, target_width])
  image = tf.io.decode_and_crop_jpeg(image_bytes, crop_window, channels=3)

  return image


def resize(image, image_size):
  return tf.image.resize([image], [image_size, image_size],
                         method=tf.image.ResizeMethod.BICUBIC)[0]


def at_least_x_are_equal(a, b, x):
  """At least `x` of `a` and `b` `Tensors` are equal."""
  match = tf.equal(a, b)
  match = tf.cast(match, tf.int32)
  return tf.greater_equal(tf.reduce_sum(match), x)


def decode_and_random_crop(image_bytes, image_size):
  """Make a random crop of image_size."""
  bbox = tf.constant([0.0, 0.0, 1.0, 1.0], dtype=tf.float32, shape=[1, 1, 4])
  image = distorted_bounding_box_crop(
      image_bytes,
      bbox,
      min_object_covered=0.1,
      aspect_ratio_range=(3. / 4, 4. / 3.),
      area_range=(0.08, 1.0),
      max_attempts=10)
  original_shape = tf.io.extract_jpeg_shape(image_bytes)
  bad = at_least_x_are_equal(original_shape, tf.shape(image), 3)

  image = tf.cond(
      bad,
      lambda: decode_and_center_crop(image_bytes, image_size),
      lambda: resize(image, image_size))

  return image


def decode_and_center_crop(image_bytes, image_size):
  """Crops to center of image with padding then scales image_size."""
  shape = tf.io.extract_jpeg_shape(image_bytes)
  image_height = shape[0]
  image_width = shape[1]

  padded_center_crop_size = tf.cast(
      ((image_size / (image_size + CROP_PADDING)) *
       tf.cast(tf.minimum(image_height, image_width), tf.float32)),
      tf.int32)

  offset_height = ((image_height - padded_center_crop_size) + 1) // 2
  offset_width = ((image_width - padded_center_crop_size) + 1) // 2
  crop_window = tf.stack([offset_height, offset_width,
                          padded_center_crop_size, padded_center_crop_size])
  image = tf.io.decode_and_crop_jpeg(image_bytes, crop_window)
  image = resize(image, image_size)

  return image


def normalize_image(image):
  image -= tf.constant(MEAN_RGB, shape=[1, 1, 3], dtype=image.dtype)
  image /= tf.constant(STDDEV_RGB, shape=[1, 1, 3], dtype=image.dtype)
  return image


def preprocess_for_train(image_bytes, dtype=tf.float32, image_size=IMAGE_SIZE):
  """Preprocesses the given image for training.

  Args:
    image_bytes: `Tensor` representing an image binary of arbitrary size.
    dtype: data type of the image.
    image_size: image size.

  Returns:
    A preprocessed image `Tensor`.
  """
  image = decode_and_random_crop(image_bytes, image_size)
  image = tf.reshape(image, [image_size, image_size, 3])
  image = tf.image.random_flip_left_right(image)
  image = normalize_image(image)
  image = tf.image.convert_image_dtype(image, dtype=dtype)
  return image


def preprocess_for_eval(image_bytes, dtype=tf.float32, image_size=IMAGE_SIZE):
  """Preprocesses the given image for evaluation.

  Args:
    image_bytes: `Tensor` representing an image binary of arbitrary size.
    dtype: data type of the image.
    image_size: image size.

  Returns:
    A preprocessed image `Tensor`.
  """
  image = decode_and_center_crop(image_bytes, image_size)
  image = tf.reshape(image, [image_size, image_size, 3])
  image = normalize_image(image)
  image = tf.image.convert_image_dtype(image, dtype=dtype)
  return image


def create_split(dataset_builder: tfds.core.DatasetBuilder, batch_size: int,
                 train: bool, dtype: tf.DType = tf.float32,
                 image_size: int = IMAGE_SIZE, cache: bool = False):
  """Creates a split from the ImageNet dataset using TensorFlow Datasets.

  Args:
    dataset_builder: TFDS dataset builder for ImageNet.
    batch_size: the batch size returned by the data pipeline.
    train: Whether to load the train or evaluation split.
    dtype: data type of the image (default: float32).
    image_size: The target size of the images (default: 224).
    cache: Whether to cache the dataset (default: False).
  Returns:
    A `tf.data.Dataset`.
  """
  if train:
    train_size = dataset_builder.info.splits['train'].num_examples
    split_size = train_size // jax.host_count()
    start = jax.host_id() * split_size
    split = 'train[{}:{}]'.format(start, start + split_size)
  else:
    validation_size = dataset_builder.info.splits['validation'].num_examples
    split_size = validation_size // jax.host_count()
    start = jax.host_id() * split_size
    split = 'validation[{}:{}]'.format(start, start + split_size)

  def _decode_example(example):
    if train:
      image = preprocess_for_train(example['image'], dtype, image_size)
    else:
      image = preprocess_for_eval(example['image'], dtype, image_size)
    return {'image': image, 'label': example['label']}

  ds = dataset_builder.as_dataset(
      split=split,
      decoders={
          'image': tfds.decode.SkipDecoding()
      }
  )
  ds.options().experimental_threading.private_threadpool_size = 48
  ds.options().experimental_threading.max_intra_op_parallelism = 1

  if cache:
    ds = ds.cache()

  if train:
    ds = ds.repeat()
    ds = ds.shuffle(16 * batch_size, seed=0)

  ds = ds.map(
      _decode_example, num_parallel_calls=tf.data.experimental.AUTOTUNE)

  ds = ds.batch(batch_size, drop_remainder=True)

  if not train:
    ds = ds.repeat()

  ds = ds.prefetch(10)

  return ds
