# Copyright 2022 The Nerfstudio Team. All rights reserved.
#
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
#
#     http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.

"""
Multi Layer Perceptron
"""
from typing import Optional, Set, Tuple

import torch
from torch import nn
from torchtyping import TensorType

from nerfstudio.field_components.base_field_component import FieldComponent


class MLP(FieldComponent):
    """Multilayer perceptron

    Args:
        in_dim: Input layer dimension
        num_layers: Number of network layers
        layer_width: Width of each MLP layer
        out_dim: Ouput layer dimension. Uses layer_width if None.
        activation: intermediate layer activation function.
        out_activation: output activation function.
    """

    def __init__(
        self,
        in_dim: int,
        num_layers: int,
        layer_width: int,
        out_dim: Optional[int] = None,
        skip_connections: Optional[Tuple[int]] = None,
        activation: Optional[nn.Module] = nn.ReLU(),
        out_activation: Optional[nn.Module] = None,
    ) -> None:

        super().__init__()
        self.in_dim = in_dim
        assert self.in_dim > 0
        self.out_dim = out_dim if out_dim is not None else layer_width
        self.num_layers = num_layers
        self.layer_width = layer_width
        self.skip_connections = skip_connections
        self._skip_connections: Set[int] = set(skip_connections) if skip_connections else set()
        self.activation = activation
        self.out_activation = out_activation
        self.net = None
        self.build_nn_modules()

    def build_nn_modules(self) -> None:
        """Initialize multi-layer perceptron."""
        layers = []
        if self.num_layers == 1:
            layers.append(nn.Linear(self.in_dim, self.out_dim))
        else:
            for i in range(self.num_layers - 1):
                if i == 0:
                    assert i not in self._skip_connections, "Skip connection at layer 0 doesn't make sense."
                    layers.append(nn.Linear(self.in_dim, self.layer_width))
                elif i in self._skip_connections:
                    layers.append(nn.Linear(self.layer_width + self.in_dim, self.layer_width))
                else:
                    layers.append(nn.Linear(self.layer_width, self.layer_width))
            layers.append(nn.Linear(self.layer_width, self.out_dim))
        self.layers = nn.ModuleList(layers)

    def forward(self, in_tensor: TensorType["bs":..., "in_dim"]) -> TensorType["bs":..., "out_dim"]:
        """Process input with a multilayer perceptron.

        Args:
            in_tensor: Network input

        Returns:
            MLP network output
        """
        x = in_tensor
        for i, layer in enumerate(self.layers):
            # as checked in `build_nn_modules`, 0 should not be in `_skip_connections`
            if i in self._skip_connections:
                x = torch.cat([in_tensor, x], -1)
            x = layer(x)
            if self.activation is not None and i < len(self.layers) - 1:
                x = self.activation(x)
        if self.out_activation is not None:
            x = self.out_activation(x)
        return x