# coding=utf-8
# Copyright (c) 2019 Alibaba PAI team.
#
# 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.

import tensorflow as tf


def sequence_labeling_loss(logits, labels, num_labels):
    one_hot_labels = tf.one_hot(
        labels, depth=num_labels, dtype=tf.float32)
    log_probs = tf.nn.log_softmax(logits, axis=-1)
    loss = -tf.reduce_mean(
        tf.reduce_sum(one_hot_labels * log_probs, axis=-1))
    return loss