"""Copyright [2019] [Liyuan Liu]

   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 torch
from torch.optim import Optimizer

class RAdam(Optimizer):
    """ Implements RAdam algorithm.

    Parameters
    ----------
    params : iterable
        iterable of parameters to optimize or dicts defining parameter groups
    lr : float, optional
        learning rate
        (default: 1e-3)
    betas : Tuple[float, float], optional
        coefficients used for computing running averages of gradient and its square
        (default: (0.9, 0.999))
    eps : float, optional
        term added to the denominator to improve numerical stability
        (default: 1e-8)
    weight_decay : float, optional
        weight decay (L2 penalty)
        (default: 0)
    degenerated_to_sgd : bool, optional
        defines whether to use SGD in first steps or just update parameters
        (default: True)
    """

    def __init__(self, params, lr=1e-3, betas=(0.9, 0.999), eps=1e-8, weight_decay=0, degenerated_to_sgd=True):
        if lr < 0:
            raise ValueError("Invalid learning rate: {}".format(lr))
        if eps < 0:
            raise ValueError("Invalid epsilon value: {}".format(eps))
        if not 0.0 <= betas[0] < 1.0:
            raise ValueError("Invalid beta parameter at index 0: {}".format(betas[0]))
        if not 0.0 <= betas[1] < 1.0:
            raise ValueError("Invalid beta parameter at index 1: {}".format(betas[1]))

        self.degenerated_to_sgd = degenerated_to_sgd
        if isinstance(params, (list, tuple)) and len(params) > 0 and isinstance(params[0], dict):
            for param in params:
                if 'betas' in param and (param['betas'][0] != betas[0] or param['betas'][1] != betas[1]):
                    param['buffer'] = [[None, None, None] for _ in range(10)]
        defaults = dict(lr=lr, betas=betas, eps=eps, weight_decay=weight_decay,
                        buffer=[[None, None, None] for _ in range(10)])
        super(RAdam, self).__init__(params, defaults)

    def step(self, closure=None):
        """Performs a single optimization step.

        Parameters
        ----------
        closure : callable, optional
            A closure that reevaluates the model and returns the loss.
        """
        loss = None
        if closure is not None:
            loss = closure()

        for group in self.param_groups:

            for p in group['params']:
                if p.grad is None:
                    continue
                grad = p.grad.data.float()
                if grad.is_sparse:
                    raise RuntimeError('RAdam does not support sparse gradients')

                p_data_fp32 = p.data.float()

                state = self.state[p]

                if len(state) == 0:
                    state['step'] = 0
                    state['exp_avg'] = torch.zeros_like(p_data_fp32)
                    state['exp_avg_sq'] = torch.zeros_like(p_data_fp32)
                else:
                    state['exp_avg'] = state['exp_avg'].type_as(p_data_fp32)
                    state['exp_avg_sq'] = state['exp_avg_sq'].type_as(p_data_fp32)

                exp_avg, exp_avg_sq = state['exp_avg'], state['exp_avg_sq']
                beta1, beta2 = group['betas']

                exp_avg_sq.mul_(beta2).addcmul_(1 - beta2, grad, grad)
                exp_avg.mul_(beta1).add_(1 - beta1, grad)

                state['step'] += 1
                buffered = group['buffer'][int(state['step'] % 10)]
                if state['step'] == buffered[0]:
                    n_sma, step_size = buffered[1], buffered[2]
                else:
                    buffered[0] = state['step']
                    beta2_t = beta2 ** state['step']
                    n_sma_max = 2 / (1 - beta2) - 1
                    n_sma = n_sma_max - 2 * state['step'] * beta2_t / (1 - beta2_t)
                    buffered[1] = n_sma

                    # more conservative since it's an approximated value
                    if n_sma >= 5:
                        step_size = math.sqrt((1 - beta2_t) * (n_sma - 4) / (n_sma_max - 4) *
                                              (n_sma - 2) / n_sma * n_sma_max / (n_sma_max - 2))
                        step_size = step_size / (1 - beta1 ** state['step'])
                    elif self.degenerated_to_sgd:
                        step_size = 1.0 / (1 - beta1 ** state['step'])
                    else:
                        step_size = -1
                    buffered[2] = step_size

                # more conservative since it's an approximated value
                if n_sma >= 5:
                    if group['weight_decay'] != 0:
                        p_data_fp32.add_(-group['weight_decay'] * group['lr'], p_data_fp32)
                    denom = exp_avg_sq.sqrt().add_(group['eps'])
                    p_data_fp32.addcdiv_(-step_size * group['lr'], exp_avg, denom)
                    p.data.copy_(p_data_fp32)
                elif step_size > 0:
                    if group['weight_decay'] != 0:
                        p_data_fp32.add_(-group['weight_decay'] * group['lr'], p_data_fp32)
                    p_data_fp32.add_(-step_size * group['lr'], exp_avg)
                    p.data.copy_(p_data_fp32)

        return loss
