# Copyright (c) 2019 NVIDIA CORPORATION. All rights reserved.
# Copyright 2018 The Google AI Language Team Authors and The HugginFace Inc. 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.

#the code here are adapted from https://github.com/NVIDIA/DeepLearningExamples/blob/180382499f791d38eb9e91c105d75764cd2f1cd7/PyTorch/LanguageModeling/BERT/schedulers.py

import math
import torch
from torch.optim.optimizer import Optimizer
from torch.optim.lr_scheduler import _LRScheduler


class LRScheduler(_LRScheduler):
    def __init__(self, optimizer, last_epoch=-1):
        # Check if using mixed precision training
        self.mixed_training = False
        base_optimizer = optimizer

        # Check that optimizer param is valid
        if not isinstance(optimizer, Optimizer):
            raise TypeError('{} is not an Optimizer'.format(
                type(optimizer).__name__))

        super(LRScheduler, self).__init__(base_optimizer, last_epoch)

    def step(self, epoch=None):
        # Set the current training step
        # ('epoch' is used to be consistent with _LRScheduler)
        if self.mixed_training:
            # The assumption is that the step will be constant
            state_dict = self.optimizer.state[self.optimizer.param_groups[0]['params'][0]]
            if 'step' in state_dict:
                self.last_epoch = state_dict['step'] + 1
            else:
                self.last_epoch = 1
        else:
            self.last_epoch = epoch if epoch is not None else self.last_epoch + 1

        for param_group, lr in zip(self.optimizer.param_groups, self.get_lr()):
            param_group['lr'] = lr


class CosineWarmUpScheduler(LRScheduler):
    """
    Applies a warm up period to the learning rate.
    """

    def __init__(self, optimizer, warmup, total_steps, last_epoch=-1):
        self.warmup = warmup
        self.total_steps = total_steps
        super(CosineWarmUpScheduler, self).__init__(optimizer, last_epoch)

    def get_lr(self):
        progress = self.last_epoch / self.total_steps
        if progress < self.warmup:
            return [base_lr * progress / self.warmup for base_lr in self.base_lrs]
        else:
            return [base_lr * (0.5 * (1.0 + torch.cos(math.pi + progress))) for base_lr in self.base_lrs]


class ConstantWarmUpScheduler(LRScheduler):
    """
    Applies a warm up period to the learning rate.
    """

    def __init__(self, optimizer, warmup, total_steps, last_epoch=-1):
        self.warmup = warmup
        self.total_steps = total_steps
        super(ConstantWarmUpScheduler, self).__init__(optimizer, last_epoch)

    def get_lr(self):
        progress = self.last_epoch / self.total_steps
        if progress < self.warmup:
            return [base_lr * progress / self.warmup for base_lr in self.base_lrs]
        else:
            return self.base_lrs


class LinearWarmUpScheduler(LRScheduler):
    """
    Applies a warm up period to the learning rate.
    """

    def __init__(self, optimizer, warmup, total_steps, last_epoch=-1):
        self.warmup = warmup
        self.total_steps = total_steps
        super(LinearWarmUpScheduler, self).__init__(optimizer, last_epoch)

    def get_lr(self):
        progress = self.last_epoch / self.total_steps
        if progress < self.warmup:
            return [base_lr * progress / self.warmup for base_lr in self.base_lrs]
        else:
            return [base_lr * max(( progress - 1.0)/(self.warmup - 1.0), 0.) for base_lr in self.base_lrs]


class PolyWarmUpScheduler(LRScheduler):
    """
    Applies a warm up period to the learning rate.
    """

    def __init__(self, optimizer, warmup, total_steps, degree=0.5, last_epoch=-1):
        self.warmup = warmup
        self.total_steps = total_steps
        self.degree = degree
        super(PolyWarmUpScheduler, self).__init__(optimizer, last_epoch)

    def step(self, epoch=None):
        param_group = self.optimizer.param_groups[0]
        if 'step' in param_group:
            self.last_epoch = param_group['step'] + 1
        else:
            self.last_epoch = 1

        for param_group, lr in zip(self.optimizer.param_groups, self.get_lr()):
            param_group['lr'] = lr

        self._last_lr = [group['lr'] for group in self.optimizer.param_groups]

    def get_lr(self):
        progress = self.last_epoch / self.total_steps
        if progress < self.warmup:
            return [base_lr * progress / self.warmup for base_lr in self.base_lrs]
        else:
            return [base_lr * ((1.0 - progress) ** self.degree) for base_lr in self.base_lrs]
