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

class FocalLoss(nn.Module):
    def __init__(self, gamma=2, alpha=None, reduction='mean'):
        super(FocalLoss, self).__init__()
        self.gamma = gamma
        self.alpha = alpha
        if isinstance(alpha,(float,int)):
            self.alpha = torch.Tensor([alpha,1-alpha])
        if isinstance(alpha,list):
            self.alpha = torch.Tensor(alpha)
        self.reduction = reduction

    def forward(self, input, target):
        if input.dim()>2:
            input = input.view(input.size(0),input.size(1),-1)  # N,C,H,W => N,C,H*W
            input = input.transpose(1,2)    # N,C,H*W => N,H*W,C
            input = input.contiguous().view(-1,input.size(2))   # N,H*W,C => N*H*W,C
        target = target.view(-1,1)

        logpt = F.log_softmax(input,dim=1)
        logpt = logpt.gather(1,target)
        logpt = logpt.view(-1)
        pt = logpt.exp()

        if self.alpha is not None:
            if self.alpha.type()!=input.data.type():
                self.alpha = self.alpha.type_as(input.data)
            at = self.alpha.gather(0,target.data.view(-1))
            logpt = logpt * at

        loss = -1 * (1-pt)**self.gamma * logpt

        if self.reduction=='sum':
            loss = loss.sum()
        elif self.reduction=='mean':
            loss = loss.mean()

        return loss