import torch
import torch.nn as nn


class RMSNorm(nn.Module):
    def __init__(
        self,
        dim: int,
        elementwise_affine=True,
        eps: float = 1e-6,
        device=None,
        dtype=None,
    ):
        """
        Initialize the RMSNorm normalization layer.

        Args:
            dim (int): The dimension of the input tensor.
            eps (float, optional): A small value added to the denominator for numerical stability. Default is 1e-6.

        Attributes:
            eps (float): A small value added to the denominator for numerical stability.
            weight (nn.Parameter): Learnable scaling parameter.

        """
        factory_kwargs = {"device": device, "dtype": dtype}
        super().__init__()
        self.eps = eps
        if elementwise_affine:
            self.weight = nn.Parameter(torch.ones(dim, **factory_kwargs))

    def _norm(self, x):
        """
        Apply the RMSNorm normalization to the input tensor.

        Args:
            x (torch.Tensor): The input tensor.

        Returns:
            torch.Tensor: The normalized tensor.

        """
        return x * torch.rsqrt(x.pow(2).mean(-1, keepdim=True) + self.eps)
    
    def forward(self, x):
        """
        Forward pass through the RMSNorm layer.

        Args:
            x (torch.Tensor): The input tensor.

        Returns:
            torch.Tensor: The output tensor after applying RMSNorm.

        """
        output = self._norm(x.float()).type_as(x)
        if hasattr(self, "weight"):
            # output = output * self.weight
            # support fp8
            output = output * self.weight.to(output.dtype)
        return output


def get_norm_layer(norm_layer):
    """
    Get the normalization layer.

    Args:
        norm_layer (str): The type of normalization layer.

    Returns:
        norm_layer (nn.Module): The normalization layer.
    """
    if norm_layer == "layer":
        return nn.LayerNorm
    elif norm_layer == "rms":
        return RMSNorm
    else:
        raise NotImplementedError(f"Norm layer {norm_layer} is not implemented")
