import torch
from . import _functional as F
from ..optimizer import Optimizer

class NAdam(Optimizer):
    r"""Implements NAdam algorithm with multi tensor APIs.

    It has been proposed in `Incorporating Nesterov Momentum into Adam`_.

    Args:
        params (iterable): iterable of parameters to optimize or dicts defining
            parameter groups
        lr (float, optional): learning rate (default: 2e-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)
        momentum_decay (float, optional): momentum momentum_decay (default: 4e-3)

    .. _Incorporating Nesterov Momentum into Adam:
        https://openreview.net/forum?id=OM0jvwB8jIp57ZJjtNEZ
    """

    def __init__(self, params, lr=2e-3, betas=(0.9, 0.999), eps=1e-8,
                 weight_decay=0, momentum_decay=4e-3):
        if not 0.0 <= lr:
            raise ValueError("Invalid learning rate: {}".format(lr))
        if not 0.0 <= eps:
            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]))
        if not 0.0 <= weight_decay:
            raise ValueError("Invalid weight_decay value: {}".format(weight_decay))
        if not 0.0 <= momentum_decay:
            raise ValueError("Invalid momentum_decay value: {}".format(momentum_decay))
        defaults = dict(lr=lr, betas=betas, eps=eps,
                        weight_decay=weight_decay, momentum_decay=momentum_decay, foreach=True)
        super(NAdam, self).__init__(params, defaults)

    @torch.no_grad()
    def step(self, closure=None):
        """Performs a single optimization step.
        Args:
            closure (callable, optional): A closure that reevaluates the model
                and returns the loss.
        """
        loss = None
        if closure is not None:
            with torch.enable_grad():
                loss = closure()

        for group in self.param_groups:
            params_with_grad = []
            grads = []
            exp_avg = []
            exp_avg_sq = []
            mu_products = []
            states = []
            beta1, beta2 = group['betas']

            for p in group['params']:
                if p.grad is not None:
                    if p.grad.is_sparse:
                        raise RuntimeError('NAdam does not support sparse gradients')
                    params_with_grad.append(p)
                    grads.append(p.grad)

            for p in params_with_grad:
                state = self.state[p]

                # Lazy state initialization
                if len(state) == 0:
                    state['step'] = 0
                    state['mu_product'] = 1.
                    # Exponential moving average of gradient values
                    state['exp_avg'] = torch.zeros_like(p, memory_format=torch.preserve_format)
                    # Exponential moving average of squared gradient values
                    state['exp_avg_sq'] = torch.zeros_like(p, memory_format=torch.preserve_format)

                exp_avg.append(state['exp_avg'])
                exp_avg_sq.append(state['exp_avg_sq'])

                state['step'] += 1
                states.append(state)

                mu = beta1 * (1. - 0.5 * (0.96 ** (state['step'] * group['momentum_decay'])))
                state['mu_product'] *= mu
                mu_products.append(state['mu_product'])

            F.nadam(params_with_grad,
                    grads,
                    exp_avg,
                    exp_avg_sq,
                    mu_products,
                    states,
                    beta1=beta1,
                    beta2=beta2,
                    lr=group['lr'],
                    weight_decay=group['weight_decay'],
                    momentum_decay=group['momentum_decay'],
                    eps=group['eps'])

            return loss
