import torch.nn as nn
import torch.nn.functional as F


class GANLoss(nn.Module):
    def __init__(self, loss_type, real_label_val=1.0, fake_label_val=0.0):
        super().__init__()
        assert loss_type in ["vanilla", "lsgan", "hinge", "wgan"]
        self.real_label_val = real_label_val
        self.fake_label_val = fake_label_val
        self.loss_type = loss_type

    def forward(self, prediction, target_is_real: bool, is_discriminator=False):
        """
        gan loss forward
        :param prediction: network prediction
        :param target_is_real: whether the target is real or fake
        :param is_discriminator: whether the loss for is_discriminator or not. default False
        :return: Tensor, GAN loss value
        """
        target_val = self.real_label_val if target_is_real else self.fake_label_val
        target = prediction.new_ones(prediction.size()) * target_val

        if self.loss_type == "vanilla":
            return F.binary_cross_entropy_with_logits(prediction, target)
        elif self.loss_type == "lsgan":
            return F.mse_loss(prediction, target)
        elif self.loss_type == "hinge":
            if is_discriminator:
                prediction = -prediction if target_is_real else prediction
                loss = F.relu(1 + prediction).mean()
            else:
                loss = -prediction.mean()
            return loss
        elif self.loss_type == "wgan":
            loss = -prediction.mean() if target_is_real else prediction.mean()
            return loss
        else:
            raise NotImplementedError(f'GAN type {self.loss_type} is not implemented.')