import torch
from torch import nn
from torch.autograd import Function

from .voxel_layer import (dynamic_point_to_voxel_backward,
                          dynamic_point_to_voxel_forward)


class _dynamic_scatter(Function):

    @staticmethod
    def forward(ctx, feats, coors, reduce_type='max'):
        """convert kitti points(N, >=3) to voxels.

        Args:
            feats: [N, C] float tensor. points features to be reduced
                into voxels.
            coors: [N, ndim] int tensor. corresponding voxel coordinates
                (specifically multi-dim voxel index) of each points.
            reduce_type: str. reduce op. support 'max', 'sum' and 'mean'
        Returns:
            tuple
            voxel_feats: [M, C] float tensor. reduced features. input features
                that shares the same voxel coordinates are reduced to one row
            coordinates: [M, ndim] int tensor, voxel coordinates.
        """
        results = dynamic_point_to_voxel_forward(feats, coors, reduce_type)
        (voxel_feats, voxel_coors, point2voxel_map,
         voxel_points_count) = results
        ctx.reduce_type = reduce_type
        ctx.save_for_backward(feats, voxel_feats, point2voxel_map,
                              voxel_points_count)
        ctx.mark_non_differentiable(voxel_coors)
        return voxel_feats, voxel_coors

    @staticmethod
    def backward(ctx, grad_voxel_feats, grad_voxel_coors=None):
        (feats, voxel_feats, point2voxel_map,
         voxel_points_count) = ctx.saved_tensors
        grad_feats = torch.zeros_like(feats)
        # TODO: whether to use index put or use cuda_backward
        # To use index put, need point to voxel index
        dynamic_point_to_voxel_backward(grad_feats,
                                        grad_voxel_feats.contiguous(), feats,
                                        voxel_feats, point2voxel_map,
                                        voxel_points_count, ctx.reduce_type)
        return grad_feats, None, None


dynamic_scatter = _dynamic_scatter.apply


class DynamicScatter(nn.Module):

    def __init__(self, voxel_size, point_cloud_range, average_points: bool):
        super(DynamicScatter, self).__init__()
        """Scatters points into voxels, used in the voxel encoder with
           dynamic voxelization

        **Note**: The CPU and GPU implementation get the same output, but
        have numerical difference after summation and division (e.g., 5e-7).

        Args:
            average_points (bool): whether to use avg pooling to scatter
                points into voxel voxel_size (list): list [x, y, z] size
                of three dimension
            point_cloud_range (list):
                [x_min, y_min, z_min, x_max, y_max, z_max]
        """
        self.voxel_size = voxel_size
        self.point_cloud_range = point_cloud_range
        self.average_points = average_points

    def forward_single(self, points, coors):
        reduce = 'mean' if self.average_points else 'max'
        return dynamic_scatter(points.contiguous(), coors.contiguous(), reduce)

    def forward(self, points, coors):
        """
        Args:
            input: NC points
        """
        if coors.size(-1) == 3:
            return self.forward_single(points, coors)
        else:
            batch_size = coors[-1, 0] + 1
            voxels, voxel_coors = [], []
            for i in range(batch_size):
                inds = torch.where(coors[:, 0] == i)
                voxel, voxel_coor = self.forward_single(
                    points[inds], coors[inds][:, 1:])
                coor_pad = nn.functional.pad(
                    voxel_coor, (1, 0), mode='constant', value=i)
                voxel_coors.append(coor_pad)
                voxels.append(voxel)
            features = torch.cat(voxels, dim=0)
            feature_coors = torch.cat(voxel_coors, dim=0)

            return features, feature_coors

    def __repr__(self):
        tmpstr = self.__class__.__name__ + '('
        tmpstr += 'voxel_size=' + str(self.voxel_size)
        tmpstr += ', point_cloud_range=' + str(self.point_cloud_range)
        tmpstr += ', average_points=' + str(self.average_points)
        tmpstr += ')'
        return tmpstr
