# Copyright 2017 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.
# ==============================================================================

"""PNASNet Faster R-CNN implementation.

Based on PNASNet model: https://arxiv.org/abs/1712.00559
"""

import tensorflow as tf

from object_detection.meta_architectures import faster_rcnn_meta_arch
from nets.nasnet import nasnet_utils
from nets.nasnet import pnasnet

arg_scope = tf.contrib.framework.arg_scope
slim = tf.contrib.slim


def pnasnet_large_arg_scope_for_detection(is_batch_norm_training=False):
  """Defines the default arg scope for the PNASNet Large for object detection.

  This provides a small edit to switch batch norm training on and off.

  Args:
    is_batch_norm_training: Boolean indicating whether to train with batch norm.

  Returns:
    An `arg_scope` to use for the PNASNet Large Model.
  """
  imagenet_scope = pnasnet.pnasnet_large_arg_scope()
  with arg_scope(imagenet_scope):
    with arg_scope([slim.batch_norm], is_training=is_batch_norm_training) as sc:
      return sc


def _filter_scaling(reduction_indices, start_cell_num):
  """Compute the expected filter scaling at given PNASNet cell start_cell_num.

  In the pnasnet.py code, filter_scaling starts at 1.0. We instead
  adapt filter scaling to depend on the starting cell.
  At first cells, before any reduction, filter_scalling is 1.0. With passing
  any reduction cell, the filter_scaling is multiplied by 2.

  Args:
    reduction_indices: list of int indices.
    start_cell_num: int.
  Returns:
    filter_scaling: float.
  """
  filter_scaling = 1.0
  for ind in reduction_indices:
    if ind < start_cell_num:
      filter_scaling *= 2.0
  return filter_scaling


# Note: This is largely a copy of _build_pnasnet_base inside pnasnet.py but
# with special edits to remove instantiation of the stem and the special
# ability to receive as input a pair of hidden states. It constructs only
# a sub-network from the original PNASNet model, starting from the
# start_cell_num cell and with modified final layer.
def _build_pnasnet_base(
    hidden_previous, hidden, normal_cell, hparams, true_cell_num,
    start_cell_num):
  """Constructs a PNASNet image model for proposal classifier features."""

  # Find where to place the reduction cells or stride normal cells
  reduction_indices = nasnet_utils.calc_reduction_layers(
      hparams.num_cells, hparams.num_reduction_layers)
  filter_scaling = _filter_scaling(reduction_indices, start_cell_num)

  # Note: The None is prepended to match the behavior of _imagenet_stem()
  cell_outputs = [None, hidden_previous, hidden]
  net = hidden

  # Run the cells
  for cell_num in range(start_cell_num, hparams.num_cells):
    is_reduction = cell_num in reduction_indices
    stride = 2 if is_reduction else 1
    if is_reduction: filter_scaling *= hparams.filter_scaling_rate
    prev_layer = cell_outputs[-2]
    net = normal_cell(
        net,
        scope='cell_{}'.format(cell_num),
        filter_scaling=filter_scaling,
        stride=stride,
        prev_layer=prev_layer,
        cell_num=true_cell_num)
    true_cell_num += 1
    cell_outputs.append(net)

  # Final nonlinearity.
  # Note that we have dropped the final pooling, dropout and softmax layers
  # from the default pnasnet version.
  with tf.variable_scope('final_layer'):
    net = tf.nn.relu(net)
  return net


# TODO(shlens): Only fixed_shape_resizer is currently supported for PNASNet
# featurization. The reason for this is that pnasnet.py only supports
# inputs with fully known shapes. We need to update pnasnet.py to handle
# shapes not known at compile time.
class FasterRCNNPNASFeatureExtractor(
    faster_rcnn_meta_arch.FasterRCNNFeatureExtractor):
  """Faster R-CNN with PNASNet feature extractor implementation."""

  def __init__(self,
               is_training,
               first_stage_features_stride,
               batch_norm_trainable=False,
               reuse_weights=None,
               weight_decay=0.0):
    """Constructor.

    Args:
      is_training: See base class.
      first_stage_features_stride: See base class.
      batch_norm_trainable: See base class.
      reuse_weights: See base class.
      weight_decay: See base class.

    Raises:
      ValueError: If `first_stage_features_stride` is not 16.
    """
    if first_stage_features_stride != 16:
      raise ValueError('`first_stage_features_stride` must be 16.')
    super(FasterRCNNPNASFeatureExtractor, self).__init__(
        is_training, first_stage_features_stride, batch_norm_trainable,
        reuse_weights, weight_decay)

  def preprocess(self, resized_inputs):
    """Faster R-CNN with PNAS preprocessing.

    Maps pixel values to the range [-1, 1].

    Args:
      resized_inputs: A [batch, height_in, width_in, channels] float32 tensor
        representing a batch of images with values between 0 and 255.0.

    Returns:
      preprocessed_inputs: A [batch, height_out, width_out, channels] float32
        tensor representing a batch of images.

    """
    return (2.0 / 255.0) * resized_inputs - 1.0

  def _extract_proposal_features(self, preprocessed_inputs, scope):
    """Extracts first stage RPN features.

    Extracts features using the first half of the PNASNet network.
    We construct the network in `align_feature_maps=True` mode, which means
    that all VALID paddings in the network are changed to SAME padding so that
    the feature maps are aligned.

    Args:
      preprocessed_inputs: A [batch, height, width, channels] float32 tensor
        representing a batch of images.
      scope: A scope name.

    Returns:
      rpn_feature_map: A tensor with shape [batch, height, width, depth]
      end_points: A dictionary mapping feature extractor tensor names to tensors

    Raises:
      ValueError: If the created network is missing the required activation.
    """
    del scope

    if len(preprocessed_inputs.get_shape().as_list()) != 4:
      raise ValueError('`preprocessed_inputs` must be 4 dimensional, got a '
                       'tensor of shape %s' % preprocessed_inputs.get_shape())

    with slim.arg_scope(pnasnet_large_arg_scope_for_detection(
        is_batch_norm_training=self._train_batch_norm)):
      with arg_scope([slim.conv2d,
                      slim.batch_norm,
                      slim.separable_conv2d],
                     reuse=self._reuse_weights):
        _, end_points = pnasnet.build_pnasnet_large(
            preprocessed_inputs, num_classes=None,
            is_training=self._is_training,
            final_endpoint='Cell_7')

    # Note that both 'Cell_6' and 'Cell_7' have equal depth = 2160.
    # Cell_7 is the last cell before second reduction.
    rpn_feature_map = tf.concat([end_points['Cell_6'],
                                 end_points['Cell_7']], 3)

    # pnasnet.py does not maintain the batch size in the first dimension.
    # This work around permits us retaining the batch for below.
    batch = preprocessed_inputs.get_shape().as_list()[0]
    shape_without_batch = rpn_feature_map.get_shape().as_list()[1:]
    rpn_feature_map_shape = [batch] + shape_without_batch
    rpn_feature_map.set_shape(rpn_feature_map_shape)

    return rpn_feature_map, end_points

  def _extract_box_classifier_features(self, proposal_feature_maps, scope):
    """Extracts second stage box classifier features.

    This function reconstructs the "second half" of the PNASNet
    network after the part defined in `_extract_proposal_features`.

    Args:
      proposal_feature_maps: A 4-D float tensor with shape
        [batch_size * self.max_num_proposals, crop_height, crop_width, depth]
        representing the feature map cropped to each proposal.
      scope: A scope name.

    Returns:
      proposal_classifier_features: A 4-D float tensor with shape
        [batch_size * self.max_num_proposals, height, width, depth]
        representing box classifier features for each proposal.
    """
    del scope

    # Number of used stem cells.
    num_stem_cells = 2

    # Note that we always feed into 2 layers of equal depth
    # where the first N channels corresponds to previous hidden layer
    # and the second N channels correspond to the final hidden layer.
    hidden_previous, hidden = tf.split(proposal_feature_maps, 2, axis=3)

    # Note that what follows is largely a copy of build_pnasnet_large() within
    # pnasnet.py. We are copying to minimize code pollution in slim.

    # TODO(shlens,skornblith): Determine the appropriate drop path schedule.
    # For now the schedule is the default (1.0->0.7 over 250,000 train steps).
    hparams = pnasnet.large_imagenet_config()
    if not self._is_training:
      hparams.set_hparam('drop_path_keep_prob', 1.0)

    # Calculate the total number of cells in the network
    total_num_cells = hparams.num_cells + num_stem_cells

    normal_cell = pnasnet.PNasNetNormalCell(
        hparams.num_conv_filters, hparams.drop_path_keep_prob,
        total_num_cells, hparams.total_training_steps)
    with arg_scope([slim.dropout, nasnet_utils.drop_path],
                   is_training=self._is_training):
      with arg_scope([slim.batch_norm], is_training=self._train_batch_norm):
        with arg_scope([slim.avg_pool2d,
                        slim.max_pool2d,
                        slim.conv2d,
                        slim.batch_norm,
                        slim.separable_conv2d,
                        nasnet_utils.factorized_reduction,
                        nasnet_utils.global_avg_pool,
                        nasnet_utils.get_channel_index,
                        nasnet_utils.get_channel_dim],
                       data_format=hparams.data_format):

          # This corresponds to the cell number just past 'Cell_7' used by
          # _extract_proposal_features().
          start_cell_num = 8
          true_cell_num = start_cell_num + num_stem_cells

          with slim.arg_scope(pnasnet.pnasnet_large_arg_scope()):
            net = _build_pnasnet_base(
                hidden_previous,
                hidden,
                normal_cell=normal_cell,
                hparams=hparams,
                true_cell_num=true_cell_num,
                start_cell_num=start_cell_num)

    proposal_classifier_features = net
    return proposal_classifier_features

  def restore_from_classification_checkpoint_fn(
      self,
      first_stage_feature_extractor_scope,
      second_stage_feature_extractor_scope):
    """Returns a map of variables to load from a foreign checkpoint.

    Note that this overrides the default implementation in
    faster_rcnn_meta_arch.FasterRCNNFeatureExtractor which does not work for
    PNASNet checkpoints.

    Args:
      first_stage_feature_extractor_scope: A scope name for the first stage
        feature extractor.
      second_stage_feature_extractor_scope: A scope name for the second stage
        feature extractor.

    Returns:
      A dict mapping variable names (to load from a checkpoint) to variables in
      the model graph.
    """
    variables_to_restore = {}
    for variable in tf.global_variables():
      if variable.op.name.startswith(
          first_stage_feature_extractor_scope):
        var_name = variable.op.name.replace(
            first_stage_feature_extractor_scope + '/', '')
        var_name += '/ExponentialMovingAverage'
        variables_to_restore[var_name] = variable
      if variable.op.name.startswith(
          second_stage_feature_extractor_scope):
        var_name = variable.op.name.replace(
            second_stage_feature_extractor_scope + '/', '')
        var_name += '/ExponentialMovingAverage'
        variables_to_restore[var_name] = variable
    return variables_to_restore
