# Copyright (c) 2021, NVIDIA CORPORATION.  All rights reserved.
#
# NVIDIA CORPORATION and its licensors retain all intellectual property
# and proprietary rights in and to this software, related documentation
# and any modifications thereto.  Any use, reproduction, disclosure or
# distribution of this software and related documentation without an express
# license agreement from NVIDIA CORPORATION is strictly prohibited.
"""Custom replacement for `torch.nn.functional.conv2d` that supports
arbitrarily high order gradients with zero performance penalty."""

import contextlib
import warnings

import torch

# pylint: disable=redefined-builtin
# pylint: disable=arguments-differ
# pylint: disable=protected-access

#----------------------------------------------------------------------------

enabled = False    # Enable the custom op by setting this to true.
weight_gradients_disabled = False    # Forcefully disable computation of gradients with respect to the weights.


@contextlib.contextmanager
def no_weight_gradients():
    global weight_gradients_disabled
    old = weight_gradients_disabled
    weight_gradients_disabled = True
    yield
    weight_gradients_disabled = old


#----------------------------------------------------------------------------


def conv2d(input, weight, bias=None, stride=1, padding=0, dilation=1, groups=1):
    if _should_use_custom_op(input):
        return _conv2d_gradfix(
            transpose=False,
            weight_shape=weight.shape,
            stride=stride,
            padding=padding,
            output_padding=0,
            dilation=dilation,
            groups=groups
        ).apply(input, weight, bias)
    return torch.nn.functional.conv2d(
        input=input,
        weight=weight,
        bias=bias,
        stride=stride,
        padding=padding,
        dilation=dilation,
        groups=groups
    )


def conv_transpose2d(
    input, weight, bias=None, stride=1, padding=0, output_padding=0, groups=1, dilation=1
):
    if _should_use_custom_op(input):
        return _conv2d_gradfix(
            transpose=True,
            weight_shape=weight.shape,
            stride=stride,
            padding=padding,
            output_padding=output_padding,
            groups=groups,
            dilation=dilation
        ).apply(input, weight, bias)
    return torch.nn.functional.conv_transpose2d(
        input=input,
        weight=weight,
        bias=bias,
        stride=stride,
        padding=padding,
        output_padding=output_padding,
        groups=groups,
        dilation=dilation
    )


#----------------------------------------------------------------------------


def _should_use_custom_op(input):
    assert isinstance(input, torch.Tensor)
    if (not enabled) or (not torch.backends.cudnn.enabled):
        return False
    if input.device.type != 'cuda':
        return False
    if any(torch.__version__.startswith(x) for x in ['1.7.', '1.8.', '1.9']):
        return True
    warnings.warn(
        f'conv2d_gradfix not supported on PyTorch {torch.__version__}. Falling back to torch.nn.functional.conv2d().'
    )
    return False


def _tuple_of_ints(xs, ndim):
    xs = tuple(xs) if isinstance(xs, (tuple, list)) else (xs, ) * ndim
    assert len(xs) == ndim
    assert all(isinstance(x, int) for x in xs)
    return xs


#----------------------------------------------------------------------------

_conv2d_gradfix_cache = dict()


def _conv2d_gradfix(transpose, weight_shape, stride, padding, output_padding, dilation, groups):
    # Parse arguments.
    ndim = 2
    weight_shape = tuple(weight_shape)
    stride = _tuple_of_ints(stride, ndim)
    padding = _tuple_of_ints(padding, ndim)
    output_padding = _tuple_of_ints(output_padding, ndim)
    dilation = _tuple_of_ints(dilation, ndim)

    # Lookup from cache.
    key = (transpose, weight_shape, stride, padding, output_padding, dilation, groups)
    if key in _conv2d_gradfix_cache:
        return _conv2d_gradfix_cache[key]

    # Validate arguments.
    assert groups >= 1
    assert len(weight_shape) == ndim + 2
    assert all(stride[i] >= 1 for i in range(ndim))
    assert all(padding[i] >= 0 for i in range(ndim))
    assert all(dilation[i] >= 0 for i in range(ndim))
    if not transpose:
        assert all(output_padding[i] == 0 for i in range(ndim))
    else:    # transpose
        assert all(0 <= output_padding[i] < max(stride[i], dilation[i]) for i in range(ndim))

    # Helpers.
    common_kwargs = dict(stride=stride, padding=padding, dilation=dilation, groups=groups)

    def calc_output_padding(input_shape, output_shape):
        if transpose:
            return [0, 0]
        return [
            input_shape[i + 2] - (output_shape[i + 2] - 1) * stride[i] - (1 - 2 * padding[i]) -
            dilation[i] * (weight_shape[i + 2] - 1) for i in range(ndim)
        ]

    # Forward & backward.
    class Conv2d(torch.autograd.Function):
        @staticmethod
        def forward(ctx, input, weight, bias):
            assert weight.shape == weight_shape
            if not transpose:
                output = torch.nn.functional.conv2d(
                    input=input, weight=weight, bias=bias, **common_kwargs
                )
            else:    # transpose
                output = torch.nn.functional.conv_transpose2d(
                    input=input,
                    weight=weight,
                    bias=bias,
                    output_padding=output_padding,
                    **common_kwargs
                )
            ctx.save_for_backward(input, weight)
            return output

        @staticmethod
        def backward(ctx, grad_output):
            input, weight = ctx.saved_tensors
            grad_input = None
            grad_weight = None
            grad_bias = None

            if ctx.needs_input_grad[0]:
                p = calc_output_padding(input_shape=input.shape, output_shape=grad_output.shape)
                grad_input = _conv2d_gradfix(
                    transpose=(not transpose),
                    weight_shape=weight_shape,
                    output_padding=p,
                    **common_kwargs
                ).apply(grad_output, weight, None)
                assert grad_input.shape == input.shape

            if ctx.needs_input_grad[1] and not weight_gradients_disabled:
                grad_weight = Conv2dGradWeight.apply(grad_output, input)
                assert grad_weight.shape == weight_shape

            if ctx.needs_input_grad[2]:
                grad_bias = grad_output.sum([0, 2, 3])

            return grad_input, grad_weight, grad_bias

    # Gradient with respect to the weights.
    class Conv2dGradWeight(torch.autograd.Function):
        @staticmethod
        def forward(ctx, grad_output, input):
            op = torch._C._jit_get_operation(
                'aten::cudnn_convolution_backward_weight'
                if not transpose else 'aten::cudnn_convolution_transpose_backward_weight'
            )
            flags = [
                torch.backends.cudnn.benchmark, torch.backends.cudnn.deterministic,
                torch.backends.cudnn.allow_tf32
            ]
            grad_weight = op(
                weight_shape, grad_output, input, padding, stride, dilation, groups, *flags
            )
            assert grad_weight.shape == weight_shape
            ctx.save_for_backward(grad_output, input)
            return grad_weight

        @staticmethod
        def backward(ctx, grad2_grad_weight):
            grad_output, input = ctx.saved_tensors
            grad2_grad_output = None
            grad2_input = None

            if ctx.needs_input_grad[0]:
                grad2_grad_output = Conv2d.apply(input, grad2_grad_weight, None)
                assert grad2_grad_output.shape == grad_output.shape

            if ctx.needs_input_grad[1]:
                p = calc_output_padding(input_shape=input.shape, output_shape=grad_output.shape)
                grad2_input = _conv2d_gradfix(
                    transpose=(not transpose),
                    weight_shape=weight_shape,
                    output_padding=p,
                    **common_kwargs
                ).apply(grad_output, grad2_grad_weight, None)
                assert grad2_input.shape == input.shape

            return grad2_grad_output, grad2_input

    _conv2d_gradfix_cache[key] = Conv2d
    return Conv2d


#----------------------------------------------------------------------------