import torch
import torch.distributed as dist

from torch.autograd.function import Function

class SyncBatchNorm(Function):

    @staticmethod
    def forward(self, input, weight, bias, running_mean, running_var, eps, momentum, process_group, world_size):
        if not (
            input.is_contiguous(memory_format=torch.channels_last) or
            input.is_contiguous(memory_format=torch.channels_last_3d)
        ):
            input = input.contiguous()
        if weight is not None:
            weight = weight.contiguous()

        size = int(input.numel() // input.size(1))
        if size == 1 and world_size < 2:
            raise ValueError('Expected more than 1 value per channel when training, got input size {}'.format(size))

        num_channels = input.shape[1]
        if input.numel() > 0:
            # calculate mean/invstd for input.
            mean, invstd = torch.batch_norm_stats(input, eps)

            count = torch.full(
                (1,),
                input.numel() // input.size(1),
                dtype=mean.dtype,
                device=mean.device
            )

            # C, C, 1 -> (2C + 1)
            combined = torch.cat([mean, invstd, count], dim=0)
        else:
            # for empty input, set stats and the count to zero. The stats with
            # zero count will be filtered out later when computing global mean
            # & invstd, but they still needs to participate the all_gather
            # collective communication to unblock other peer processes.
            combined = torch.zeros(
                2 * num_channels + 1,
                dtype=input.dtype,
                device=input.device
            )

        # Use allgather instead of allreduce because count could be different across
        # ranks, simple all reduce op can not give correct results.
        # batch_norm_gather_stats_with_counts calculates global mean & invstd based on
        # all gathered mean, invstd and count.
        # for nccl backend, use the optimized version of all gather.
        # The Gloo backend does not support `all_gather_into_tensor`.
        if process_group._get_backend_name() != "gloo":
            # world_size * (2C + 1)
            combined_size = combined.numel()
            combined_flat = torch.empty(1,
                                        combined_size * world_size,
                                        dtype=combined.dtype,
                                        device=combined.device)
            dist.all_gather_into_tensor(combined_flat, combined, process_group, async_op=False)
            combined = torch.reshape(combined_flat, (world_size, combined_size))
            # world_size * (2C + 1) -> world_size * C, world_size * C, world_size * 1
            mean_all, invstd_all, count_all = torch.split(combined, num_channels, dim=1)
        else:
            # world_size * (2C + 1)
            combined_list = [
                torch.empty_like(combined) for _ in range(world_size)
            ]
            dist.all_gather(combined_list, combined, process_group, async_op=False)
            combined = torch.stack(combined_list, dim=0)
            # world_size * (2C + 1) -> world_size * C, world_size * C, world_size * 1
            mean_all, invstd_all, count_all = torch.split(combined, num_channels, dim=1)

        if not (torch.cuda.is_available() and torch.cuda.is_current_stream_capturing()):
            # The lines below force a synchronization between CUDA and CPU, because
            # the shape of the result count_all depends on the values in mask tensor.
            # Such synchronizations break CUDA Graph capturing.
            # See https://github.com/pytorch/pytorch/issues/78549
            # FIXME: https://github.com/pytorch/pytorch/issues/78656 describes
            # a better longer-term solution.

            # remove stats from empty inputs
            mask = count_all.squeeze(-1) >= 1
            count_all = count_all[mask]
            mean_all = mean_all[mask]
            invstd_all = invstd_all[mask]

        # calculate global mean & invstd
        counts = count_all.view(-1)
        if running_mean is not None and counts.dtype != running_mean.dtype:
            counts = counts.to(running_mean.dtype)
        mean, invstd = torch.batch_norm_gather_stats_with_counts(
            input,
            mean_all,
            invstd_all,
            running_mean,
            running_var,
            momentum,
            eps,
            counts,
        )

        self.save_for_backward(input, weight, mean, invstd, count_all.to(torch.int32))
        self.process_group = process_group

        # apply element-wise normalization
        if input.numel() > 0:
            return torch.batch_norm_elemt(input, weight, bias, mean, invstd, eps)
        else:
            return torch.empty_like(input)

    @staticmethod
    def backward(self, grad_output):
        if not (
            grad_output.is_contiguous(memory_format=torch.channels_last) or
            grad_output.is_contiguous(memory_format=torch.channels_last_3d)
        ):
            grad_output = grad_output.contiguous()
        saved_input, weight, mean, invstd, count_tensor = self.saved_tensors
        grad_input = grad_weight = grad_bias = None
        process_group = self.process_group

        if saved_input.numel() > 0:
            # calculate local stats as well as grad_weight / grad_bias
            sum_dy, sum_dy_xmu, grad_weight, grad_bias = torch.batch_norm_backward_reduce(
                grad_output,
                saved_input,
                mean,
                invstd,
                weight,
                self.needs_input_grad[0],
                self.needs_input_grad[1],
                self.needs_input_grad[2]
            )

            if self.needs_input_grad[0]:
                # synchronizing stats used to calculate input gradient.
                num_channels = sum_dy.shape[0]
                combined = torch.cat([sum_dy, sum_dy_xmu], dim=0)
                torch.distributed.all_reduce(
                    combined, torch.distributed.ReduceOp.SUM, process_group, async_op=False)
                sum_dy, sum_dy_xmu = torch.split(combined, num_channels)

                # backward pass for gradient calculation
                if weight is not None and weight.dtype != mean.dtype:
                    weight = weight.to(mean.dtype)
                grad_input = torch.batch_norm_backward_elemt(
                    grad_output,
                    saved_input,
                    mean,
                    invstd,
                    weight,
                    sum_dy,
                    sum_dy_xmu,
                    count_tensor
                )
            # synchronizing of grad_weight / grad_bias is not needed as distributed
            # training would handle all reduce.
            if weight is None or not self.needs_input_grad[1]:
                grad_weight = None

            if weight is None or not self.needs_input_grad[2]:
                grad_bias = None
        else:
            # This process got an empty input tensor in the forward pass.
            # Although this process can directly set grad_input as an empty
            # tensor of zeros, it still needs to participate in the collective
            # communication to unblock its peers, as other peer processes might
            # have received non-empty inputs.
            num_channels = saved_input.shape[1]
            if self.needs_input_grad[0]:
                # launch all_reduce to unblock other peer processes
                combined = torch.zeros(
                    2 * num_channels,
                    dtype=saved_input.dtype,
                    device=saved_input.device
                )
                torch.distributed.all_reduce(
                    combined, torch.distributed.ReduceOp.SUM, process_group, async_op=False)

            # Leave grad_input, grad_weight and grad_bias as None, which will be
            # interpreted by the autograd engine as Tensors full of zeros.

        return grad_input, grad_weight, grad_bias, None, None, None, None, None, None

class CrossMapLRN2d(Function):

    @staticmethod
    def forward(ctx, input, size, alpha=1e-4, beta=0.75, k=1):
        ctx.size = size
        ctx.alpha = alpha
        ctx.beta = beta
        ctx.k = k
        ctx.scale = None

        if input.dim() != 4:
            raise ValueError("CrossMapLRN2d: Expected input to be 4D, got {}D instead.".format(input.dim()))

        ctx.scale = ctx.scale or input.new()
        output = input.new()

        batch_size = input.size(0)
        channels = input.size(1)
        input_height = input.size(2)
        input_width = input.size(3)

        output.resize_as_(input)
        ctx.scale.resize_as_(input)

        # use output storage as temporary buffer
        input_square = output
        torch.pow(input, 2, out=input_square)

        pre_pad = int((ctx.size - 1) / 2 + 1)
        pre_pad_crop = channels if pre_pad > channels else pre_pad

        scale_first = ctx.scale.select(1, 0)
        scale_first.zero_()
        # compute first feature map normalization
        for c in range(pre_pad_crop):
            scale_first.add_(input_square.select(1, c))

        # reuse computations for next feature maps normalization
        # by adding the next feature map and removing the previous
        for c in range(1, channels):
            scale_previous = ctx.scale.select(1, c - 1)
            scale_current = ctx.scale.select(1, c)
            scale_current.copy_(scale_previous)
            if c < channels - pre_pad + 1:
                square_next = input_square.select(1, c + pre_pad - 1)
                scale_current.add_(square_next, alpha=1)

            if c > pre_pad:
                square_previous = input_square.select(1, c - pre_pad)
                scale_current.add_(square_previous, alpha=-1)

        ctx.scale.mul_(ctx.alpha / ctx.size).add_(ctx.k)

        torch.pow(ctx.scale, -ctx.beta, out=output)
        output.mul_(input)

        ctx.save_for_backward(input, output)
        return output

    @staticmethod
    def backward(ctx, grad_output):
        input, output = ctx.saved_tensors
        grad_input = grad_output.new()

        batch_size = input.size(0)
        channels = input.size(1)
        input_height = input.size(2)
        input_width = input.size(3)

        paddded_ratio = input.new(channels + ctx.size - 1, input_height,
                                  input_width)
        accum_ratio = input.new(input_height, input_width)

        cache_ratio_value = 2 * ctx.alpha * ctx.beta / ctx.size
        inversePrePad = int(ctx.size - (ctx.size - 1) / 2)

        grad_input.resize_as_(input)
        torch.pow(ctx.scale, -ctx.beta, out=grad_input).mul_(grad_output)

        paddded_ratio.zero_()
        padded_ratio_center = paddded_ratio.narrow(0, inversePrePad,
                                                   channels)
        for n in range(batch_size):
            torch.mul(grad_output[n], output[n], out=padded_ratio_center)
            padded_ratio_center.div_(ctx.scale[n])
            torch.sum(
                paddded_ratio.narrow(0, 0, ctx.size - 1), 0, keepdim=False, out=accum_ratio)
            for c in range(channels):
                accum_ratio.add_(paddded_ratio[c + ctx.size - 1])
                grad_input[n][c].addcmul_(input[n][c], accum_ratio, value=-cache_ratio_value)
                accum_ratio.add_(paddded_ratio[c], alpha=-1)

        return grad_input, None, None, None, None

class BackwardHookFunction(torch.autograd.Function):
    @staticmethod
    def forward(ctx, *args):
        ctx.mark_non_differentiable(*[arg for arg in args if not arg.requires_grad])
        return args

    @staticmethod
    def backward(ctx, *args):
        return args
