import torch
import torch_stablesort_cpp


class StableSort(torch.autograd.Function):
    @staticmethod
    def forward(ctx, input, dim=-1, descending=False, out=None):
        values, indices = \
            torch_stablesort_cpp.stable_sort(input, dim, descending, out)
        ctx.save_for_backward(input, indices, torch.tensor(dim))
        return values, indices.detach()

    @staticmethod
    def backward(ctx, grad_values, grad_indices):
        input, indices, dim = ctx.saved_variables
        # print('backward(), grad_indices:', grad_indices, 'indices:', indices,
        #    'grad_values:', grad_values)

        res = torch.gather(grad_values, dim, indices)

        # res = torch.empty_like(grad_values)
        # print('here')
        # res = res.view(-1, res.size(-1))
        # indices = indices.view(-1, indices.size(-1))
        # torch.repeat_interleave(torch.arange(indices.size(0))
        # res[indices] = grad_values # + grad_indices
        # print('here 2')
        return res