# Source code for archai.algos.darts.bilevel_optimizer_slow

    # Copyright (c) Microsoft Corporation.
    # Licensed under the MIT license.

    from typing import Mapping, Optional, Union, Iterator
    import copy

    import torch
    from torch import Tensor, nn, autograd
    from torch.nn.modules.loss import _Loss
    from torch.optim.optimizer import Optimizer

    from archai.common.config import Config
    from archai.common import utils, ml_utils
    from archai.nas.model import Model
    from archai.common.common import logger
    from archai.common.utils import zip_eq

    def _flatten_concate(xs):
        """
        flatten all tensor from [d1,d2,...dn] to [d]
        and then concat all [d_1] to [d_1+d_2+d_3+...]
        :param xs:
        :return:
        """
        return torch.cat([x.view(-1) for x in xs])

    def _get_alphas(model:Model)->Iterator[nn.Parameter]:
        return model.all_owned().param_by_kind('alphas')

    def _get_loss(model:Model, lossfn, x, y):
        logits, *_ = model(x) # might also return aux tower logits
        return lossfn(logits, y)

    class BilevelOptimizer:
        def __init__(self, conf_alpha_optim:Config, w_momentum: float, w_decay: float,
                     model: Model, lossfn: _Loss) -> None:
            self._w_momentum = w_momentum  # momentum for w
            self._w_weight_decay = w_decay  # weight decay for w
            self._lossfn = lossfn
            self._model = model  # main model with respect to w and alpha

            self._alphas = list(_get_alphas(self._model))

            # this is the optimizer to optimize alphas parameter
            self._alpha_optim = ml_utils.create_optimizer(conf_alpha_optim, self._alphas)

        def state_dict(self)->dict:
            return {
                'alpha_optim': self._alpha_optim.state_dict()
            }

        def load_state_dict(self, state_dict)->None:
            self._alpha_optim.load_state_dict(state_dict['alpha_optim'])

        def _unrolled_model(self, x, y, lr: float, main_optim: Optimizer)->Model:
            # TODO: should this loss be stored for later use?
            loss = _get_loss(self._model, self._lossfn, x, y)
            params = _flatten_concate(self._model.parameters()).detach()

            try:
                moment = _flatten_concate(main_optim.state[v]['momentum_buffer'] for v in self._model.parameters())
                moment.mul_(self._w_momentum)
            except:
                moment = torch.zeros_like(params)

            # flatten all gradients
            grads = _flatten_concate(autograd.grad(loss, self._model.parameters())).data
            # indeed, here we implement a simple SGD with momentum and weight decay
            # theta = theta - eta * (moment + weight decay + dtheta)
            params = params.sub(lr, moment + grads + self._w_weight_decay*params)
            # construct a new model
            return self._params2model(params)

        def _params2model(self, params)->Model:
            """
            construct a new model with initialized weight from params
            it use .state_dict() and load_state_dict() instead of
            .parameters() + fill_()
            :params: flatten weights, need to reshape to original shape
            :return:
            """

            params_d, offset = {}, 0
            for k, v in self._model.named_parameters():
                v_length = v.numel()
                # restore params[] value to original shape
                params_d[k] = params[offset: offset + v_length].view(v.size())
                offset += v_length

            assert offset == len(params)

            model_new = copy.deepcopy(self._model)
            model_dict = self._model.state_dict()
            model_dict.update(params_d)
            model_new.load_state_dict(model_dict)

            return model_new.cuda()

        def step(self, x_train: Tensor, y_train: Tensor, x_valid: Tensor, y_valid: Tensor,
                 main_optim: Optimizer) -> None:
            # TODO: unlike darts paper, we get lr from optimizer insead of scheduler
            lr = main_optim.param_groups[0]['lr']
            self._alpha_optim.zero_grad()

            # compute the gradient and write it into tensor.grad
            # instead of generated by loss.backward()
            self._backward_bilevel(x_train, y_train, x_valid, y_valid,
                                   lr, main_optim)

            # at this point we should have model with updated gradients for w and alpha
            self._alpha_optim.step()

        def _backward_bilevel(self, x_train, y_train, x_valid, y_valid, lr, main_optim):
            """ Compute unrolled loss and backward its gradients """

            # update vmodel with w', but leave alphas as-is
            # w' = w - lr * grad
            unrolled_model = self._unrolled_model(x_train, y_train, lr, main_optim)

            # compute loss on validation set for model with w'
            # wrt alphas. The autograd.grad is used instead of backward()
            # to avoid having to loop through params
            vloss = _get_loss(unrolled_model, self._lossfn, x_valid, y_valid)
            vloss.backward()
            dalpha = [v.grad for v in _get_alphas(unrolled_model)]
            dparams = [v.grad.data for v in unrolled_model.parameters()]

            hessian = self._hessian_vector_product(dparams, x_train, y_train)

            # dalpha we have is from the unrolled model so we need to
            # transfer those grades back to our main model
            # update final gradient = dalpha - xi*hessian
            # TODO: currently alphas lr is same as w lr
            with torch.no_grad():
                for alpha, da, h in zip_eq(self._alphas, dalpha, hessian):
                    alpha.grad = da - lr*h
            # now that model has both w and alpha grads,
            # we can run main_optim.step() to update the param values

        def _hessian_vector_product(self, dw, x, y, epsilon_unit=1e-2):
            """
            Implements equation 8

            dw = dw` {L_val(w`, alpha)}
            w+ = w + eps * dw
            w- = w - eps * dw
            hessian = (dalpha {L_trn(w+, alpha)} -dalpha {L_trn(w-, alpha)})/(2*eps)
            eps = 0.01 / ||dw||
            """

            """scale epsilon with grad magnitude. The dw
            is a multiplier on RHS of eq 8. So this scalling is essential
            in making sure that finite differences approximation is not way off
            Below, we flatten each w, concate all and then take norm"""
            # TODO: is cat along dim 0 correct?
            dw_norm = torch.cat([w.view(-1) for w in dw]).norm()
            epsilon = epsilon_unit / dw_norm

            # w+ = w + epsilon * grad(w')
            with torch.no_grad():
                for p, v in zip_eq(self._model.parameters(), dw):
                    p += epsilon * v

            # Now that we have model with w+, we need to compute grads wrt alphas
            # This loss needs to be on train set, not validation set
            loss = _get_loss(self._model, self._lossfn, x, y)
            dalpha_plus = autograd.grad(
                loss, self._alphas)  # dalpha{L_trn(w+)}

            # get model with w- and then compute grads wrt alphas
            # w- = w - eps*dw`
            with torch.no_grad():
                for p, v in zip_eq(self._model.parameters(), dw):
                    # we had already added dw above so sutracting twice gives w-
                    p -= 2. * epsilon * v

            # similarly get dalpha_minus
            loss = _get_loss(self._model, self._lossfn, x, y)
            dalpha_minus = autograd.grad(loss, self._alphas)

            # reset back params to original values by adding dw
            with torch.no_grad():
                for p, v in zip_eq(self._model.parameters(), dw):
                    p += epsilon * v

            # apply eq 8, final difference to compute hessian
            h = [(p - m) / (2. * epsilon)
                 for p, m in zip_eq(dalpha_plus, dalpha_minus)]
            return h