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):
        input = input.contiguous()

        count = torch.empty(1,
                            dtype=running_mean.dtype,
                            device=input.device).fill_(input.numel() // input.size(1))

        # calculate mean/invstd for input.
        mean, invstd = torch.batch_norm_stats(input, eps)

        num_channels = input.shape[1]
        # C, C, 1 -> (2C + 1)
        combined = torch.cat([mean, invstd, count], dim=0)
        # world_size * (2C + 1)
        combined_list = [
            torch.empty_like(combined) for k in range(world_size)
        ]
        # Use allgather instead of allreduce since I don't trust in-place operations ..
        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)

        size = count_all.view(-1).long().sum()
        if size == 1:
            raise ValueError('Expected more than 1 value per channel when training, got input size {}'.format(size))

        # calculate global mean & invstd
        mean, invstd = torch.batch_norm_gather_stats_with_counts(
            input,
            mean_all,
            invstd_all,
            running_mean,
            running_var,
            momentum,
            eps,
            count_all.view(-1)
        )

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

        # apply element-wise normalization
        out = torch.batch_norm_elemt(input, weight, bias, mean, invstd, eps)
        return out

    @staticmethod
    def backward(self, grad_output):
        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

        # 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.
            # TODO: move div_ into batch_norm_backward_elemt kernel
            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)

            divisor = count_tensor.sum()
            mean_dy = sum_dy / divisor
            mean_dy_xmu = sum_dy_xmu / divisor
            # backward pass for gradient calculation
            grad_input = torch.batch_norm_backward_elemt(
                grad_output,
                saved_input,
                mean,
                invstd,
                weight,
                mean_dy,
                mean_dy_xmu
            )

        # 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

        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

        assert input.dim() == 4

        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
