import torch
import torch.nn as nn
#import torch.autograd.function as func

class ReversibleBlock(nn.Module):
    '''
    Elementary building block for building (partially) reversible architectures

    Implementation of the Reversible block described in the RevNet paper
    (https://arxiv.org/abs/1707.04585). Must be used inside a :class:`revtorch.ReversibleSequence`
    for autograd support.

    Arguments:
        f_block (nn.Module): arbitrary subnetwork whos output shape is equal to its input shape
        g_block (nn.Module): arbitrary subnetwork whos output shape is equal to its input shape
    '''

    def __init__(self, f_block, g_block):
        super(ReversibleBlock, self).__init__()
        self.f_block = f_block
        self.g_block = g_block

    def forward(self, x):
        """
        Performs the forward pass of the reversible block. Does not record any gradients.
        :param x: Input tensor. Must be splittable along dimension 1.
        :return: Output tensor of the same shape as the input tensor
        """
        x1, x2 = torch.chunk(x, 2, dim=1)
        y1, y2 = None, None
        with torch.no_grad():
            y1 = x1 + self.f_block(x2)
            y2 = x2 + self.g_block(y1)

        return torch.cat([y1, y2], dim=1)

    def backward_pass(self, y, dy):
        """
        Performs the backward pass of the reversible block.

        Calculates the derivatives of the block's parameters in f_block and g_block, as well as the inputs of the
        forward pass and its gradients.

        :param y: Outputs of the reversible block
        :param dy: Derivatives of the outputs
        :return: A tuple of (block input, block input derivatives). The block inputs are the same shape as the block outptus.
        """
        
        # Split the arguments channel-wise
        y1, y2 = torch.chunk(y, 2, dim=1)
        del y
        assert (not y1.requires_grad), "y1 must already be detached"
        assert (not y2.requires_grad), "y2 must already be detached"
        dy1, dy2 = torch.chunk(dy, 2, dim=1)
        del dy
        assert (not dy1.requires_grad), "dy1 must not require grad"
        assert (not dy2.requires_grad), "dy2 must not require grad"

        # Enable autograd for y1 and y2. This ensures that PyTorch
        # keeps track of ops. that use y1 and y2 as inputs in a DAG
        y1.requires_grad = True
        y2.requires_grad = True

        # Ensures that PyTorch tracks the operations in a DAG
        with torch.enable_grad():
            gy1 = self.g_block(y1)

            # Use autograd framework to differentiate the calculation. The
            # derivatives of the parameters of G are set as a side effect
            gy1.backward(dy2)

        with torch.no_grad():
            x2 = y2 - gy1 # Restore first input of forward()
            del y2, gy1

            # The gradient of x1 is the sum of the gradient of the output
            # y1 as well as the gradient that flows back through G
            # (The gradient that flows back through G is stored in y1.grad)
            dx1 = dy1 + y1.grad
            del dy1
            y1.grad = None

        with torch.enable_grad():
            x2.requires_grad = True
            fx2 = self.f_block(x2)

            # Use autograd framework to differentiate the calculation. The
            # derivatives of the parameters of F are set as a side effec
            fx2.backward(dx1)

        with torch.no_grad():
            x1 = y1 - fx2 # Restore second input of forward()
            del y1, fx2

            # The gradient of x2 is the sum of the gradient of the output
            # y2 as well as the gradient that flows back through F
            # (The gradient that flows back through F is stored in x2.grad)
            dx2 = dy2 + x2.grad
            del dy2
            x2.grad = None

            # Undo the channelwise split
            x = torch.cat([x1, x2.detach()], dim=1)
            dx = torch.cat([dx1, dx2], dim=1)

        return x, dx

class _ReversibleModuleFunction(torch.autograd.function.Function):
    '''
    Integrates the reversible sequence into the autograd framework
    '''

    @staticmethod
    def forward(ctx, x, reversible_blocks):
        '''
        Performs the forward pass of a reversible sequence within the autograd framework
        :param ctx: autograd context
        :param x: input tensor
        :param reversible_blocks: nn.Modulelist of reversible blocks
        :return: output tensor
        '''
        assert (isinstance(reversible_blocks, nn.ModuleList))
        for block in reversible_blocks:
            assert (isinstance(block, ReversibleBlock))
            x = block(x)
        ctx.y = x #not using ctx.save_for_backward(x) saves us memory by beeing able to free ctx.y earlier in the backward pass
        ctx.reversible_blocks = reversible_blocks
        return x

    @staticmethod
    def backward(ctx, dy):
        '''
        Performs the backward pass of a reversible sequence within the autograd framework
        :param ctx: autograd context
        :param dy: derivatives of the outputs
        :return: derivatives of the inputs
        '''
        y = ctx.y
        del ctx.y
        for i in range(len(ctx.reversible_blocks) - 1, -1, -1):
            y, dy = ctx.reversible_blocks[i].backward_pass(y, dy)
        del ctx.reversible_blocks
        return dy, None

class ReversibleSequence(nn.Module):
    '''
    Basic building element for (partially) reversible networks

    A reversible sequence is a sequence of arbitrarly many reversible blocks. The entire sequence is reversible.
    The activations are only saved at the end of the sequence. Backpropagation leverages the reversible nature of
    the reversible sequece to save memory.

    Arguments:
        reversible_blocks (nn.ModuleList): A ModuleList that exclusivly contains instances of ReversibleBlock whic
        which are to be used in the reversible sequence.
    '''

    def __init__(self, reversible_blocks):
        super(ReversibleSequence, self).__init__()
        assert (isinstance(reversible_blocks, nn.ModuleList))
        for block in reversible_blocks:
            assert(isinstance(block, ReversibleBlock))

        self.reversible_blocks = reversible_blocks

    def forward(self, x):
        '''
        Forward pass of a reversible sequence
        :param x: Input tensor
        :return: Output tensor
        '''
        x = _ReversibleModuleFunction.apply(x, self.reversible_blocks)
        return x