# Copyright (c) Meta Platforms, Inc. and affiliates.
# All rights reserved.
#
# This source code is licensed under the BSD-style license found in the
# LICENSE file in the root directory of this source tree.

from typing import Any, Callable, Dict, Tuple

import torch
from pytorch3d.implicitron.models.renderer.base import RendererOutput
from pytorch3d.implicitron.tools.config import registry, ReplaceableBase
from pytorch3d.renderer.implicit.raymarching import _check_raymarcher_inputs


_TTensor = torch.Tensor


class RaymarcherBase(ReplaceableBase):
    """
    Defines a base class for raymarchers. Specifically, a raymarcher is responsible
    for taking a set of features and density descriptors along rendering rays
    and marching along them in order to generate a feature render.
    """

    def forward(
        self,
        rays_densities: torch.Tensor,
        rays_features: torch.Tensor,
        aux: Dict[str, Any],
    ) -> RendererOutput:
        """
        Args:
            rays_densities: Per-ray density values represented with a tensor
                of shape `(..., n_points_per_ray, 1)`.
            rays_features: Per-ray feature values represented with a tensor
                of shape `(..., n_points_per_ray, feature_dim)`.
            aux: a dictionary with extra information.
        """
        raise NotImplementedError()


class AccumulativeRaymarcherBase(RaymarcherBase, torch.nn.Module):
    """
    This generalizes the `pytorch3d.renderer.EmissionAbsorptionRaymarcher`
    and NeuralVolumes' cumsum ray marcher. It additionally returns
    the rendering weights that can be used in the NVS pipeline to carry out
    the importance ray-sampling in the refining pass.
    Different from `pytorch3d.renderer.EmissionAbsorptionRaymarcher`, it takes raw
    (non-exponentiated) densities.

    Args:
        surface_thickness: The thickness of the raymarched surface.
        bg_color: The background color. A tuple of either 1 element or of D elements,
            where D matches the feature dimensionality; it is broadcast when necessary.
        replicate_last_interval: If True, the ray length assigned to the last interval
            for the opacity delta calculation is copied from the penultimate interval.
        background_opacity: The length over which the last raw opacity value
            (i.e. before exponentiation) is considered to apply, for the delta
            calculation. Ignored if replicate_last_interval=True.
        density_relu: If `True`, passes the input density through ReLU before
            raymarching.
        blend_output: If `True`, alpha-blends the output renders with the
            background color using the rendered opacity mask.

        capping_function: The capping function of the raymarcher.
            Options:
                - "exponential" (`cap_fn(x) = 1 - exp(-x)`)
                - "cap1" (`cap_fn(x) = min(x, 1)`)
            Set to "exponential" for the standard Emission Absorption raymarching.
        weight_function: The weighting function of the raymarcher.
            Options:
                - "product" (`weight_fn(w, x) = w * x`)
                - "minimum" (`weight_fn(w, x) = min(w, x)`)
            Set to "product" for the standard Emission Absorption raymarching.
    """

    surface_thickness: int = 1
    bg_color: Tuple[float, ...] = (0.0,)
    replicate_last_interval: bool = False
    background_opacity: float = 0.0
    density_relu: bool = True
    blend_output: bool = False

    @property
    def capping_function_type(self) -> str:
        raise NotImplementedError()

    @property
    def weight_function_type(self) -> str:
        raise NotImplementedError()

    def __post_init__(self):
        """
        Args:
            surface_thickness: Denotes the overlap between the absorption
                function and the density function.
        """
        bg_color = torch.tensor(self.bg_color)
        if bg_color.ndim != 1:
            raise ValueError(f"bg_color (shape {bg_color.shape}) should be a 1D tensor")

        self.register_buffer("_bg_color", bg_color, persistent=False)

        self._capping_function: Callable[[_TTensor], _TTensor] = {
            "exponential": lambda x: 1.0 - torch.exp(-x),
            "cap1": lambda x: x.clamp(max=1.0),
        }[self.capping_function_type]

        self._weight_function: Callable[[_TTensor, _TTensor], _TTensor] = {
            "product": lambda curr, acc: curr * acc,
            "minimum": lambda curr, acc: torch.minimum(curr, acc),
        }[self.weight_function_type]

    def forward(
        self,
        rays_densities: torch.Tensor,
        rays_features: torch.Tensor,
        aux: Dict[str, Any],
        ray_lengths: torch.Tensor,
        density_noise_std: float = 0.0,
        **kwargs,
    ) -> RendererOutput:
        """
        Args:
            rays_densities: Per-ray density values represented with a tensor
                of shape `(..., n_points_per_ray, 1)`.
            rays_features: Per-ray feature values represented with a tensor
                of shape `(..., n_points_per_ray, feature_dim)`.
            aux: a dictionary with extra information.
            ray_lengths: Per-ray depth values represented with a tensor
                of shape `(..., n_points_per_ray, feature_dim)`.
            density_noise_std: the magnitude of the noise added to densities.

        Returns:
            features: A tensor of shape `(..., feature_dim)` containing
                the rendered features for each ray.
            depth: A tensor of shape `(..., 1)` containing estimated depth.
            opacities: A tensor of shape `(..., 1)` containing rendered opacities.
            weights: A tensor of shape `(..., n_points_per_ray)` containing
                the ray-specific non-negative opacity weights. In general, they
                don't sum to 1 but do not overcome it, i.e.
                `(weights.sum(dim=-1) <= 1.0).all()` holds.
        """
        _check_raymarcher_inputs(
            rays_densities,
            rays_features,
            ray_lengths,
            z_can_be_none=True,
            features_can_be_none=False,
            density_1d=True,
        )

        ray_lengths_diffs = ray_lengths[..., 1:] - ray_lengths[..., :-1]
        if self.replicate_last_interval:
            last_interval = ray_lengths_diffs[..., -1:]
        else:
            last_interval = torch.full_like(
                ray_lengths[..., :1], self.background_opacity
            )
        deltas = torch.cat((ray_lengths_diffs, last_interval), dim=-1)

        rays_densities = rays_densities[..., 0]

        if density_noise_std > 0.0:
            noise: _TTensor = torch.randn_like(rays_densities).mul(density_noise_std)
            rays_densities = rays_densities + noise
        if self.density_relu:
            rays_densities = torch.relu(rays_densities)

        weighted_densities = deltas * rays_densities
        capped_densities = self._capping_function(weighted_densities)  # pyre-ignore: 29

        rays_opacities = self._capping_function(  # pyre-ignore: 29
            torch.cumsum(weighted_densities, dim=-1)
        )
        opacities = rays_opacities[..., -1:]
        absorption_shifted = (-rays_opacities + 1.0).roll(
            self.surface_thickness, dims=-1
        )
        absorption_shifted[..., : self.surface_thickness] = 1.0

        weights = self._weight_function(  # pyre-ignore: 29
            capped_densities, absorption_shifted
        )
        features = (weights[..., None] * rays_features).sum(dim=-2)
        depth = (weights * ray_lengths)[..., None].sum(dim=-2)

        alpha = opacities if self.blend_output else 1
        if self._bg_color.shape[-1] not in [1, features.shape[-1]]:
            raise ValueError("Wrong number of background color channels.")
        features = alpha * features + (1 - opacities) * self._bg_color

        return RendererOutput(
            features=features,
            depths=depth,
            masks=opacities,
            weights=weights,
            aux=aux,
        )


@registry.register
class EmissionAbsorptionRaymarcher(AccumulativeRaymarcherBase):
    """
    Implements the EmissionAbsorption raymarcher.
    """

    background_opacity: float = 1e10

    @property
    def capping_function_type(self) -> str:
        return "exponential"

    @property
    def weight_function_type(self) -> str:
        return "product"


@registry.register
class CumsumRaymarcher(AccumulativeRaymarcherBase):
    """
    Implements the NeuralVolumes' cumulative-sum raymarcher.
    """

    @property
    def capping_function_type(self) -> str:
        return "cap1"

    @property
    def weight_function_type(self) -> str:
        return "minimum"
