# Copyright (c) 2022-2024, NVIDIA CORPORATION & AFFILIATES. All rights reserved.
#
# See LICENSE for license information.

"""LayerNorm API"""
import os
from typing import Union, Tuple, Optional

import torch
from torch.nn.parameter import Parameter
from torch.nn import init

import transformer_engine_extensions as tex
from .base import TransformerEngineBaseModule
from ..cpp_extensions import (
    layernorm_fwd_inf,
 )
from ..jit import no_torch_dynamo
from ..utils import cast_if_needed

__all__ = ["LayerNorm"]


class _LayerNorm(torch.autograd.Function):
    """functional LayerNorm"""

    @staticmethod
    def forward(
        ctx,
        inp: torch.Tensor,
        ln_weight: torch.Tensor,
        ln_bias: torch.Tensor,
        eps: float,
        fwd_ln_sm_margin: int,
        bwd_ln_sm_margin: int,
        zero_centered_gamma: bool,
        is_grad_enabled: bool,
        activation_dtype: torch.dtype,
    ) -> torch.Tensor:
        # Make sure input dimensions are compatible
        in_features = ln_weight.numel()
        assert inp.is_cuda, "TransformerEngine needs CUDA."
        assert inp.shape[-1] == in_features, "LayerNorm not possible"
        inputmat = inp.view((-1, in_features))

        # Cast for native AMP
        inputmat = cast_if_needed(inputmat, activation_dtype)
        ln_weight = cast_if_needed(ln_weight, activation_dtype)
        ln_bias = cast_if_needed(ln_bias, activation_dtype)

        if is_grad_enabled:
            ln_out, mu, rsigma = tex.layernorm_fwd(inputmat, ln_weight,
                ln_bias, eps, fwd_ln_sm_margin, zero_centered_gamma)
            ctx.save_for_backward(inputmat, ln_weight, mu, rsigma)
            ctx.inp_shape = inp.shape
            ctx.bwd_ln_sm_margin = bwd_ln_sm_margin
            ctx.zero_centered_gamma = zero_centered_gamma
        else:
            ln_out, mu, rsigma = layernorm_fwd_inf(inputmat, ln_weight,
                ln_bias, eps, zero_centered_gamma), None, None
        return ln_out.view_as(inp)

    @staticmethod
    def backward(
        ctx, grad_output: torch.Tensor
    ) -> Tuple[Union[torch.Tensor, None], ...]:
        inputmat, ln_weight, mu, rsigma = ctx.saved_tensors
        grad_output = grad_output.contiguous()
        d_ln_out = grad_output.view(inputmat.shape)
        dxmat, dgamma, dbeta = tex.layernorm_bwd(
            d_ln_out, inputmat, mu, rsigma, ln_weight,
            ctx.bwd_ln_sm_margin, ctx.zero_centered_gamma
        )
        return dxmat.view(ctx.inp_shape), dgamma, dbeta, None, None, None, None, None, None


class LayerNorm(torch.nn.Module):
    r"""
    Applies Layer Normalization over a mini-batch of inputs as described in
    the paper `Layer Normalization <https://arxiv.org/abs/1607.06450>`__

    .. math::
        y = \frac{x - \mathrm{E}[x]}{ \sqrt{\mathrm{Var}[x] + \varepsilon}} * \gamma + \beta

    :math:`\gamma` and :math:`\beta` are learnable affine transform parameters of
    size :attr:`hidden_size`

    Parameters
    ----------
    hidden_size : int
                size of each input sample.
    eps : float, default = 1e-5
        a value added to the denominator of layer normalization for numerical stability.
    sequence_parallel : bool, default = `False`
                        if set to `True`, uses sequence parallelism.
    params_dtype : torch.dtype, default = `torch.get_default_dtype()`
                    it controls the type used to allocate the initial parameters. Useful when
                    the model is trained with lower precision and the original FP32 parameters
                    would not fit in GPU memory.
    zero_centered_gamma : bool, default = 'False'
                         if set to 'True', gamma parameter in LayerNorm is initialized to 0 and
                         the LayerNorm formula changes to

                         .. math::
                            y = \frac{x - \mathrm{E}[x]}{ \sqrt{\mathrm{Var}[x] + \varepsilon}} *
                            (1 + \gamma) + \beta
    device : Union[torch.device, str], default = "cuda"
          The device on which the parameters of the model will allocated. It is the user's
          responsibility to ensure all parameters are moved to the GPU before running the
          forward pass.
    """

    def __init__(
        self,
        hidden_size: int,
        eps: float = 1e-5,
        sequence_parallel: bool = False,
        params_dtype: Optional[torch.dtype] = None,
        zero_centered_gamma: bool = False,
        device: Union[torch.device, str] = "cuda",
    ) -> None:
        super().__init__()
        params_dtype = torch.get_default_dtype() if params_dtype is None else params_dtype
        self.eps = eps
        self.zero_centered_gamma = zero_centered_gamma
        self.weight = Parameter(
            torch.empty(
                hidden_size,
                device=device,
                dtype=params_dtype,
            )
        )
        self.bias = Parameter(
            torch.empty(
                hidden_size,
                device=device,
                dtype=params_dtype,
            )
        )
        setattr(self.weight, "sequence_parallel", sequence_parallel)
        setattr(self.bias, "sequence_parallel", sequence_parallel)
        self.reset_layer_norm_parameters()

        # These many SMs are subtracted from the total SM count when calling forward
        # and backward LayerNorm C APIs. These envvars can be used to prevent the LN
        # kernels from using all SMs in the device. This is useful for cases such as
        # communication overlap with LN.
        self.fwd_ln_sm_margin = int(os.getenv("NVTE_FWD_LAYERNORM_SM_MARGIN", "0"))
        self.bwd_ln_sm_margin = int(os.getenv("NVTE_BWD_LAYERNORM_SM_MARGIN", "0"))

    def reset_layer_norm_parameters(self) -> None:
        """Init LN params"""
        if not self.zero_centered_gamma:
            init.ones_(self.weight)
        else:
            init.zeros_(self.weight)
        init.zeros_(self.bias)

    @no_torch_dynamo()
    def forward(self, inp: torch.Tensor) -> torch.Tensor:
        """LayerNorm FWD"""
        # Set the activation type for AMP.
        TransformerEngineBaseModule.set_activation_dtype(self, inp)

        if torch.is_grad_enabled():
            fwd_fn = _LayerNorm.apply
            args = []
        else:
            fwd_fn = _LayerNorm.forward
            args = [None]

        args += (
            inp,
            self.weight,
            self.bias,
            self.eps,
            self.fwd_ln_sm_margin,
            self.bwd_ln_sm_margin,
            self.zero_centered_gamma,
            torch.is_grad_enabled(),
            self.activation_dtype,
        )

        return fwd_fn(*args)
