# Copyright 2020 - 2021 MONAI Consortium
# 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 math
import warnings
from typing import List

from torch.optim import Adam, Optimizer
from torch.optim.lr_scheduler import _LRScheduler


class LinearWarmupCosineAnnealingLR(_LRScheduler):
    def __init__(
        self,
        optimizer: Optimizer,
        warmup_epochs: int,
        max_epochs: int,
        warmup_start_lr: float = 0.0,
        eta_min: float = 0.0,
        last_epoch: int = -1,
    ) -> None:
        """
        Args:
            optimizer (Optimizer): Wrapped optimizer.
            warmup_epochs (int): Maximum number of iterations for linear warmup
            max_epochs (int): Maximum number of iterations
            warmup_start_lr (float): Learning rate to start the linear warmup. Default: 0.
            eta_min (float): Minimum learning rate. Default: 0.
            last_epoch (int): The index of last epoch. Default: -1.
        """
        self.warmup_epochs = warmup_epochs
        self.max_epochs = max_epochs
        self.warmup_start_lr = warmup_start_lr
        self.eta_min = eta_min

        super(LinearWarmupCosineAnnealingLR, self).__init__(optimizer, last_epoch)

    def get_lr(self) -> List[float]:
        """
        Compute learning rate using chainable form of the scheduler
        """
        if not self._get_lr_called_within_step:
            warnings.warn(
                "To get the last learning rate computed by the scheduler, " "please use `get_last_lr()`.", UserWarning
            )

        if self.last_epoch == 0:
            return [self.warmup_start_lr] * len(self.base_lrs)
        elif self.last_epoch < self.warmup_epochs:
            return [
                group["lr"] + (base_lr - self.warmup_start_lr) / (self.warmup_epochs - 1)
                for base_lr, group in zip(self.base_lrs, self.optimizer.param_groups)
            ]
        elif self.last_epoch == self.warmup_epochs:
            return self.base_lrs
        elif (self.last_epoch - 1 - self.max_epochs) % (2 * (self.max_epochs - self.warmup_epochs)) == 0:
            return [
                group["lr"]
                + (base_lr - self.eta_min) * (1 - math.cos(math.pi / (self.max_epochs - self.warmup_epochs))) / 2
                for base_lr, group in zip(self.base_lrs, self.optimizer.param_groups)
            ]

        return [
            (1 + math.cos(math.pi * (self.last_epoch - self.warmup_epochs) / (self.max_epochs - self.warmup_epochs)))
            / (
                1
                + math.cos(
                    math.pi * (self.last_epoch - self.warmup_epochs - 1) / (self.max_epochs - self.warmup_epochs)
                )
            )
            * (group["lr"] - self.eta_min)
            + self.eta_min
            for group in self.optimizer.param_groups
        ]

    def _get_closed_form_lr(self) -> List[float]:
        """
        Called when epoch is passed as a param to the `step` function of the scheduler.
        """
        if self.last_epoch < self.warmup_epochs:
            return [
                self.warmup_start_lr + self.last_epoch * (base_lr - self.warmup_start_lr) / (self.warmup_epochs - 1)
                for base_lr in self.base_lrs
            ]

        return [
            self.eta_min
            + 0.5
            * (base_lr - self.eta_min)
            * (1 + math.cos(math.pi * (self.last_epoch - self.warmup_epochs) / (self.max_epochs - self.warmup_epochs)))
            for base_lr in self.base_lrs
        ]