code: from absl import flags
  from absl.flags import FLAGS
  import numpy as np
  import tensorflow as tf
  from tensorflow.keras import Model
  from tensorflow.keras.layers import (
      Add,
      Concatenate,
      Conv2D,
      Input,
      Lambda,
      LeakyReLU,
      MaxPool2D,
      UpSampling2D,
      ZeroPadding2D,
  )
  from tensorflow.keras.regularizers import l2
  from tensorflow.keras.losses import (
      binary_crossentropy,
      sparse_categorical_crossentropy
  )
  from .batch_norm import BatchNormalization
  from .utils import broadcast_iou
  
  yolo_max_boxes = 100
  yolo_iou_threshold = 0.1
  yolo_score_threshold = 0.1
  # customize your model through the following parameters
  flags.DEFINE_integer('yolo_max_boxes', 100, 'maximum number of detections at one time')
  flags.DEFINE_float('yolo_iou_threshold', 0.5, 'iou threshold')
  flags.DEFINE_float('yolo_score_threshold', 0.5, 'score threshold')
  
  yolo_anchors = np.array([(10, 13), (16, 30), (33, 23), (30, 61), (62, 45),
                           (59, 119), (116, 90), (156, 198), (373, 326)],
                          np.float32) / 416
  yolo_anchor_masks = np.array([[6, 7, 8], [3, 4, 5], [0, 1, 2]])
  
  yolo_tiny_anchors = np.array([(10, 14), (23, 27), (37, 58),
                                (81, 82), (135, 169),  (344, 319)],
                               np.float32) / 416
  yolo_tiny_anchor_masks = np.array([[3, 4, 5], [0, 1, 2]])
  
  
  def DarknetConv(x, filters, size, strides=1, batch_norm=True):
      if strides == 1:
          padding = 'same'
      else:
          x = ZeroPadding2D(((1, 0), (1, 0)))(x)  # top left half-padding
          padding = 'valid'
      x = Conv2D(filters=filters, kernel_size=size,
                 strides=strides, padding=padding,
                 use_bias=not batch_norm, kernel_regularizer=l2(0.0005))(x)
      if batch_norm:
          x = BatchNormalization()(x)
          x = LeakyReLU(alpha=0.1)(x)
      return x
  
  
  def DarknetResidual(x, filters):
      prev = x
      x = DarknetConv(x, filters // 2, 1)
      x = DarknetConv(x, filters, 3)
      x = Add()([prev, x])
      return x
  
  
  def DarknetBlock(x, filters, blocks):
      x = DarknetConv(x, filters, 3, strides=2)
      for _ in range(blocks):
          x = DarknetResidual(x, filters)
      return x
  
  
  def Darknet(name=None):
      x = inputs = Input([None, None, 3])
      x = DarknetConv(x, 32, 3)
      x = DarknetBlock(x, 64, 1)
      x = DarknetBlock(x, 128, 2)  # skip connection
      x = x_36 = DarknetBlock(x, 256, 8)  # skip connection
      x = x_61 = DarknetBlock(x, 512, 8)
      x = DarknetBlock(x, 1024, 4)
      return tf.keras.Model(inputs, (x_36, x_61, x), name=name)
  
  
  def DarknetTiny(name=None):
      x = inputs = Input([None, None, 3])
      x = DarknetConv(x, 16, 3)
      x = MaxPool2D(2, 2, 'same')(x)
      x = DarknetConv(x, 32, 3)
      x = MaxPool2D(2, 2, 'same')(x)
      x = DarknetConv(x, 64, 3)
      x = MaxPool2D(2, 2, 'same')(x)
      x = DarknetConv(x, 128, 3)
      x = MaxPool2D(2, 2, 'same')(x)
      x = x_8 = DarknetConv(x, 256, 3)  # skip connection
      x = MaxPool2D(2, 2, 'same')(x)
      x = DarknetConv(x, 512, 3)
      x = MaxPool2D(2, 1, 'same')(x)
      x = DarknetConv(x, 1024, 3)
      return tf.keras.Model(inputs, (x_8, x), name=name)
  
  
  def YoloConv(filters, name=None):
      def yolo_conv(x_in):
          if isinstance(x_in, tuple):
              inputs = Input(x_in[0].shape[1:]), Input(x_in[1].shape[1:])
              x, x_skip = inputs
  
              # concat with skip connection
              x = DarknetConv(x, filters, 1)
              x = UpSampling2D(2)(x)
              x = Concatenate()([x, x_skip])
          else:
              x = inputs = Input(x_in.shape[1:])
  
          x = DarknetConv(x, filters, 1)
          x = DarknetConv(x, filters * 2, 3)
          x = DarknetConv(x, filters, 1)
          x = DarknetConv(x, filters * 2, 3)
          x = DarknetConv(x, filters, 1)
          return Model(inputs, x, name=name)(x_in)
      return yolo_conv
  
  
  def YoloConvTiny(filters, name=None):
      def yolo_conv(x_in):
          if isinstance(x_in, tuple):
              inputs = Input(x_in[0].shape[1:]), Input(x_in[1].shape[1:])
              x, x_skip = inputs
  
              # concat with skip connection
              x = DarknetConv(x, filters, 1)
              x = UpSampling2D(2)(x)
              x = Concatenate()([x, x_skip])
          else:
              x = inputs = Input(x_in.shape[1:])
              x = DarknetConv(x, filters, 1)
  
          return Model(inputs, x, name=name)(x_in)
      return yolo_conv
  
  
  def YoloOutput(filters, anchors, classes, name=None):
      def yolo_output(x_in):
          x = inputs = Input(x_in.shape[1:])
          x = DarknetConv(x, filters * 2, 3)
          x = DarknetConv(x, anchors * (classes + 5), 1, batch_norm=False)
          x = Lambda(lambda x: tf.reshape(x, (-1, tf.shape(x)[1], tf.shape(x)[2],
                                              anchors, classes + 5)))(x)
          return tf.keras.Model(inputs, x, name=name)(x_in)
      return yolo_output
  
  
  def yolo_boxes(pred, anchors, classes):
      # pred: (batch_size, grid, grid, anchors, (x, y, w, h, obj, ...classes))
      grid_size = tf.shape(pred)[1]
      box_xy, box_wh, objectness, class_probs = tf.split(
          pred, (2, 2, 1, classes), axis=-1)
  
      box_xy = tf.sigmoid(box_xy)
      objectness = tf.sigmoid(objectness)
      class_probs = tf.sigmoid(class_probs)
      pred_box = tf.concat((box_xy, box_wh), axis=-1)  # original xywh for loss
  
      # !!! grid[x][y] == (y, x)
      grid = tf.meshgrid(tf.range(grid_size), tf.range(grid_size))
      grid = tf.expand_dims(tf.stack(grid, axis=-1), axis=2)  # [gx, gy, 1, 2]
  
      box_xy = (box_xy + tf.cast(grid, tf.float32)) / \
          tf.cast(grid_size, tf.float32)
      box_wh = tf.exp(box_wh) * anchors
  
      box_x1y1 = box_xy - box_wh / 2
      box_x2y2 = box_xy + box_wh / 2
      bbox = tf.concat([box_x1y1, box_x2y2], axis=-1)
  
      return bbox, objectness, class_probs, pred_box
  
  
  def yolo_nms(outputs, anchors, masks, classes):
      # boxes, conf, type
      b, c, t = [], [], []
  
      for o in outputs:
          b.append(tf.reshape(o[0], (tf.shape(o[0])[0], -1, tf.shape(o[0])[-1])))
          c.append(tf.reshape(o[1], (tf.shape(o[1])[0], -1, tf.shape(o[1])[-1])))
          t.append(tf.reshape(o[2], (tf.shape(o[2])[0], -1, tf.shape(o[2])[-1])))
  
      bbox = tf.concat(b, axis=1)
      confidence = tf.concat(c, axis=1)
      class_probs = tf.concat(t, axis=1)
  
      scores = confidence * class_probs
      boxes, scores, classes, valid_detections = tf.image.combined_non_max_suppression(
          boxes=tf.reshape(bbox, (tf.shape(bbox)[0], -1, 1, 4)),
          scores=tf.reshape(
              scores, (tf.shape(scores)[0], -1, tf.shape(scores)[-1])),
          max_output_size_per_class=yolo_max_boxes,
          max_total_size=yolo_max_boxes,
          iou_threshold=yolo_iou_threshold,
          score_threshold=yolo_score_threshold
      )
  
      return boxes, scores, classes, valid_detections
  
  
  def YoloV3(size=None, channels=3, anchors=yolo_anchors,
             masks=yolo_anchor_masks, classes=80, training=False):
      physical_devices = tf.config.experimental.list_physical_devices('GPU')
      if len(physical_devices) > 0:
          tf.config.experimental.set_memory_growth(physical_devices[0], True)
      x = inputs = Input([size, size, channels], name='input')
  
      x_36, x_61, x = Darknet(name='yolo_darknet')(x)
  
      x = YoloConv(512, name='yolo_conv_0')(x)
      output_0 = YoloOutput(512, len(masks[0]), classes, name='yolo_output_0')(x)
  
      x = YoloConv(256, name='yolo_conv_1')((x, x_61))
      output_1 = YoloOutput(256, len(masks[1]), classes, name='yolo_output_1')(x)
  
      x = YoloConv(128, name='yolo_conv_2')((x, x_36))
      output_2 = YoloOutput(128, len(masks[2]), classes, name='yolo_output_2')(x)
  
      if training:
          return Model(inputs, (output_0, output_1, output_2), name='yolov3')
  
      boxes_0 = Lambda(lambda x: yolo_boxes(x, anchors[masks[0]], classes),
                       name='yolo_boxes_0')(output_0)
      boxes_1 = Lambda(lambda x: yolo_boxes(x, anchors[masks[1]], classes),
                       name='yolo_boxes_1')(output_1)
      boxes_2 = Lambda(lambda x: yolo_boxes(x, anchors[masks[2]], classes),
                       name='yolo_boxes_2')(output_2)
  
      outputs = Lambda(lambda x: yolo_nms(x, anchors, masks, classes),
                       name='yolo_nms')((boxes_0[:3], boxes_1[:3], boxes_2[:3]))
  
      return Model(inputs, outputs, name='yolov3')
  
  
  def YoloV3Tiny(size=None, channels=3, anchors=yolo_tiny_anchors,
                 masks=yolo_tiny_anchor_masks, classes=80, training=False):
      physical_devices = tf.config.experimental.list_physical_devices('GPU')
      if len(physical_devices) > 0:
          tf.config.experimental.set_memory_growth(physical_devices[0], True)
      x = inputs = Input([size, size, channels], name='input')
  
      x_8, x = DarknetTiny(name='yolo_darknet')(x)
  
      x = YoloConvTiny(256, name='yolo_conv_0')(x)
      output_0 = YoloOutput(256, len(masks[0]), classes, name='yolo_output_0')(x)
  
      x = YoloConvTiny(128, name='yolo_conv_1')((x, x_8))
      output_1 = YoloOutput(128, len(masks[1]), classes, name='yolo_output_1')(x)
  
      if training:
          return Model(inputs, (output_0, output_1), name='yolov3')
  
      boxes_0 = Lambda(lambda x: yolo_boxes(x, anchors[masks[0]], classes),
                       name='yolo_boxes_0')(output_0)
      boxes_1 = Lambda(lambda x: yolo_boxes(x, anchors[masks[1]], classes),
                       name='yolo_boxes_1')(output_1)
      outputs = Lambda(lambda x: yolo_nms(x, anchors, masks, classes),
                       name='yolo_nms')((boxes_0[:3], boxes_1[:3]))
      return Model(inputs, outputs, name='yolov3_tiny')
  
  
  def YoloLoss(anchors, classes=80, ignore_thresh=0.5):
      def yolo_loss(y_true, y_pred):
          # 1. transform all pred outputs
          # y_pred: (batch_size, grid, grid, anchors, (x, y, w, h, obj, ...cls))
          pred_box, pred_obj, pred_class, pred_xywh = yolo_boxes(
              y_pred, anchors, classes)
          pred_xy = pred_xywh[..., 0:2]
          pred_wh = pred_xywh[..., 2:4]
  
          # 2. transform all true outputs
          # y_true: (batch_size, grid, grid, anchors, (x1, y1, x2, y2, obj, cls))
          true_box, true_obj, true_class_idx = tf.split(
              y_true, (4, 1, 1), axis=-1)
          true_xy = (true_box[..., 0:2] + true_box[..., 2:4]) / 2
          true_wh = true_box[..., 2:4] - true_box[..., 0:2]
  
          # give higher weights to small boxes
          box_loss_scale = 2 - true_wh[..., 0] * true_wh[..., 1]
  
          # 3. inverting the pred box equations
          grid_size = tf.shape(y_true)[1]
          grid = tf.meshgrid(tf.range(grid_size), tf.range(grid_size))
          grid = tf.expand_dims(tf.stack(grid, axis=-1), axis=2)
          true_xy = true_xy * tf.cast(grid_size, tf.float32) - \
              tf.cast(grid, tf.float32)
          true_wh = tf.math.log(true_wh / anchors)
          true_wh = tf.where(tf.math.is_inf(true_wh),
                             tf.zeros_like(true_wh), true_wh)
  
          # 4. calculate all masks
          obj_mask = tf.squeeze(true_obj, -1)
          # ignore false positive when iou is over threshold
          best_iou = tf.map_fn(
              lambda x: tf.reduce_max(broadcast_iou(x[0], tf.boolean_mask(
                  x[1], tf.cast(x[2], tf.bool))), axis=-1),
              (pred_box, true_box, obj_mask),
              tf.float32)
          ignore_mask = tf.cast(best_iou < ignore_thresh, tf.float32)
  
          # 5. calculate all losses
          xy_loss = obj_mask * box_loss_scale * \
              tf.reduce_sum(tf.square(true_xy - pred_xy), axis=-1)
          wh_loss = obj_mask * box_loss_scale * \
              tf.reduce_sum(tf.square(true_wh - pred_wh), axis=-1)
          obj_loss = binary_crossentropy(true_obj, pred_obj)
          obj_loss = obj_mask * obj_loss + \
              (1 - obj_mask) * ignore_mask * obj_loss
          # TODO: use binary_crossentropy instead
          class_loss = obj_mask * sparse_categorical_crossentropy(
              true_class_idx, pred_class)
  
          # 6. sum over (batch, gridx, gridy, anchors) => (batch, 1)
          xy_loss = tf.reduce_sum(xy_loss, axis=(1, 2, 3))
          wh_loss = tf.reduce_sum(wh_loss, axis=(1, 2, 3))
          obj_loss = tf.reduce_sum(obj_loss, axis=(1, 2, 3))
          class_loss = tf.reduce_sum(class_loss, axis=(1, 2, 3))
  
          return xy_loss + wh_loss + obj_loss + class_loss
      return yolo_loss
  
apis: tensorflow.keras.layers.Concatenate tensorflow.sigmoid tensorflow.squeeze tensorflow.keras.layers.ZeroPadding2D tensorflow.keras.layers.Add tensorflow.math.is_inf tensorflow.concat tensorflow.keras.losses.sparse_categorical_crossentropy tensorflow.config.experimental.list_physical_devices tensorflow.reduce_sum tensorflow.split tensorflow.math.log tensorflow.config.experimental.set_memory_growth tensorflow.stack tensorflow.shape tensorflow.keras.losses.binary_crossentropy tensorflow.keras.regularizers.l2 tensorflow.zeros_like tensorflow.keras.layers.LeakyReLU tensorflow.cast tensorflow.keras.layers.UpSampling2D tensorflow.range tensorflow.keras.Model tensorflow.exp tensorflow.square numpy.array tensorflow.keras.layers.MaxPool2D tensorflow.keras.layers.Input