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)
        return values, indices

    @staticmethod
    def backward(ctx, grad_values, grad_indices):
        input, indices = ctx.saved_variables
        res = torch.empty_like(grad_values)
        res[indices] = grad_values + grad_indices
        return res