# Adapt from https://github.com/fla-org/flash-linear-attention/blob/main/fla/modules/layernorm_gated.py
# Copyright (c) 2024, Tri Dao.
# Based on the Triton LayerNorm tutorial: https://triton-lang.org/main/getting-started/tutorials/05-layer-norm.html
# For the backward pass, we keep weight_grad and bias_grad in registers and accumulate.
# This backward pass is faster for dimensions up to 8k, but after that it's much slower due to register spilling.
# The models we train have hidden dim up to 8k anyway (e.g. Llama 70B), so this is fine.

import math

import torch
import torch.nn.functional as F
import triton
import triton.language as tl
from einops import rearrange


def rms_norm_ref(
    x,
    weight,
    bias,
    z=None,
    eps=1e-6,
    group_size=None,
    norm_before_gate=True,
    upcast=True,
):
    dtype = x.dtype
    N = x.shape[-1]
    weight = weight.float()
    bias = bias.float() if bias is not None else None
    if upcast:
        x = x.float()
        z = z.float() if z is not None else z
    if z is not None and not norm_before_gate:
        x = x * F.silu(z)
    if group_size is None:
        rstd = 1 / torch.sqrt((x.square()).mean(dim=-1, keepdim=True) + eps)
        out = (x * rstd * weight) + bias if bias is not None else (x * rstd * weight)
    else:
        x_group = rearrange(x, "... (g d) -> ... g d", d=group_size)
        rstd = 1 / torch.sqrt((x_group.square()).mean(dim=-1, keepdim=True) + eps)
        out = rearrange(x_group * rstd, "... g d -> ... (g d)") * weight
        if bias is not None:
            out = out + bias
    if z is not None and norm_before_gate:
        out *= F.silu(z)
    return out.to(dtype)


@triton.heuristics({"HAS_BIAS": lambda args: args["B"] is not None})
@triton.heuristics({"HAS_Z": lambda args: args["Z"] is not None})
@triton.jit
def _layer_norm_fwd_1pass_kernel(
    X,  # pointer to the input
    Y,  # pointer to the output
    W,  # pointer to the weights
    B,  # pointer to the biases
    Z,  # pointer to the other branch
    Mean,  # pointer to the mean
    Rstd,  # pointer to the 1/std
    stride_x_row,  # how much to increase the pointer when moving by 1 row
    stride_y_row,
    stride_z_row,
    M,  # number of rows in X
    N,  # number of columns in X
    eps,  # epsilon to avoid division by zero
    BLOCK_N: tl.constexpr,
    HAS_BIAS: tl.constexpr,
    HAS_Z: tl.constexpr,
    NORM_BEFORE_GATE: tl.constexpr,
    IS_RMS_NORM: tl.constexpr,
):
    # Map the program id to the row of X and Y it should compute.
    row = tl.program_id(0)
    group = tl.program_id(1)
    X += row * stride_x_row + group * N
    Y += row * stride_y_row + group * N
    if HAS_Z:
        Z += row * stride_z_row + group * N
    if not IS_RMS_NORM:
        Mean += group * M
    Rstd += group * M
    W += group * N
    if HAS_BIAS:
        B += group * N
    # Compute mean and variance
    cols = tl.arange(0, BLOCK_N)
    x = tl.load(X + cols, mask=cols < N, other=0.0).to(tl.float32)
    if HAS_Z and not NORM_BEFORE_GATE:
        z = tl.load(Z + cols, mask=cols < N).to(tl.float32)
        x *= z * tl.sigmoid(z)
    if not IS_RMS_NORM:
        mean = tl.sum(x, axis=0) / N
        tl.store(Mean + row, mean)
        xbar = tl.where(cols < N, x - mean, 0.0)
        var = tl.sum(xbar * xbar, axis=0) / N
    else:
        xbar = tl.where(cols < N, x, 0.0)
        var = tl.sum(xbar * xbar, axis=0) / N
    rstd = 1 / tl.sqrt(var + eps)
    tl.store(Rstd + row, rstd)
    # Normalize and apply linear transformation
    mask = cols < N
    w = tl.load(W + cols, mask=mask).to(tl.float32)
    if HAS_BIAS:
        b = tl.load(B + cols, mask=mask).to(tl.float32)
    x_hat = (x - mean) * rstd if not IS_RMS_NORM else x * rstd
    y = x_hat * w + b if HAS_BIAS else x_hat * w
    if HAS_Z and NORM_BEFORE_GATE:
        z = tl.load(Z + cols, mask=mask).to(tl.float32)
        y *= z * tl.sigmoid(z)
    # Write output
    tl.store(Y + cols, y, mask=mask)


def _layer_norm_fwd(
    x,
    weight,
    bias,
    eps,
    z=None,
    out=None,
    group_size=None,
    norm_before_gate=True,
    is_rms_norm=False,
):
    M, N = x.shape
    if group_size is None:
        group_size = N
    assert N % group_size == 0
    ngroups = N // group_size
    assert x.stride(-1) == 1
    if z is not None:
        assert z.stride(-1) == 1
        assert z.shape == (M, N)
    assert weight.shape == (N,)
    assert weight.stride(-1) == 1
    if bias is not None:
        assert bias.stride(-1) == 1
        assert bias.shape == (N,)
    # allocate output
    if out is not None:
        assert out.shape == x.shape
    else:
        out = torch.empty_like(x)
    assert out.stride(-1) == 1
    mean = (
        torch.empty((ngroups * M,), dtype=torch.float32, device=x.device)
        if not is_rms_norm
        else None
    )
    rstd = torch.empty((ngroups * M,), dtype=torch.float32, device=x.device)
    # Less than 64KB per feature: enqueue fused kernel
    MAX_FUSED_SIZE = 65536 // x.element_size()
    BLOCK_N = min(MAX_FUSED_SIZE, triton.next_power_of_2(group_size))
    if group_size > BLOCK_N:
        raise RuntimeError("This layer norm doesn't support feature dim >= 64KB.")
    # heuristics for number of warps
    num_warps = min(max(BLOCK_N // 256, 1), 8)
    grid = (M, ngroups)
    with torch.get_device_module(x.device).device(x.device.index):
        _layer_norm_fwd_1pass_kernel[grid](
            x,
            out,
            weight,
            bias,
            z,
            mean,
            rstd,
            x.stride(0),
            out.stride(0),
            z.stride(0) if z is not None else 0,
            M,
            group_size,
            eps,
            BLOCK_N=BLOCK_N,
            NORM_BEFORE_GATE=norm_before_gate,
            IS_RMS_NORM=is_rms_norm,
            num_warps=num_warps,
        )
    return out, mean, rstd


def rms_norm_gated(
    *,
    x,
    weight,
    bias,
    z=None,
    eps=1e-6,
    group_size=None,
    norm_before_gate=True,
    is_rms_norm=False,
):
    """If z is not None, we do norm(x) * silu(z) if norm_before_gate, else norm(x * silu(z))"""

    x_shape_og = x.shape
    # reshape input data into 2D tensor
    x = x.reshape(-1, x.shape[-1])
    if x.stride(-1) != 1:
        x = x.contiguous()
    if z is not None:
        assert z.shape == x_shape_og
        z = z.reshape(-1, z.shape[-1])
        if z.stride(-1) != 1:
            z = z.contiguous()
    weight = weight.contiguous()
    if bias is not None:
        bias = bias.contiguous()
    y, mean, rstd = _layer_norm_fwd(
        x,
        weight,
        bias,
        eps,
        z=z,
        group_size=group_size,
        norm_before_gate=norm_before_gate,
        is_rms_norm=is_rms_norm,
    )
    return y.reshape(x_shape_og)


class LayerNormFn(torch.autograd.Function):

    @staticmethod
    def forward(
        ctx,
        x,
        weight,
        bias,
        z=None,
        eps=1e-6,
        group_size=None,
        norm_before_gate=True,
        is_rms_norm=False,
    ):
        return rms_norm_gated(
            x=x,
            weight=weight,
            bias=bias,
            eps=eps,
            z=z,
            group_size=group_size,
            norm_before_gate=norm_before_gate,
            is_rms_norm=is_rms_norm,
        )


def layernorm_fn(
    x,
    weight,
    bias,
    z=None,
    eps=1e-6,
    group_size=None,
    norm_before_gate=True,
    is_rms_norm=False,
):
    return LayerNormFn.apply(
        x, weight, bias, z, eps, group_size, norm_before_gate, is_rms_norm
    )


class LayerNorm(torch.nn.Module):

    def __init__(
        self,
        hidden_size,
        eps=1e-5,
        group_size=None,
        norm_before_gate=True,
        device=None,
        dtype=None,
    ):
        """If group_size is not None, we do GroupNorm with each group having group_size elements.
        group_size=None is equivalent to group_size=hidden_size (i.e. there's only 1 group).
        """

        factory_kwargs = {"device": device, "dtype": dtype}
        super().__init__()
        self.eps = eps
        self.weight = torch.nn.Parameter(torch.empty(hidden_size, **factory_kwargs))
        self.bias = torch.nn.Parameter(torch.empty(hidden_size, **factory_kwargs))
        self.group_size = group_size
        self.norm_before_gate = norm_before_gate
        self.reset_parameters()

    def reset_parameters(self):
        torch.nn.init.ones_(self.weight)
        torch.nn.init.zeros_(self.bias)

    def forward(self, x, z=None):
        """If z is not None, we do norm(x) * silu(z) if norm_before_gate, else norm(x * silu(z))"""
        return layernorm_fn(
            x,
            self.weight,
            self.bias,
            z=z,
            group_size=self.group_size,
            eps=self.eps,
            norm_before_gate=self.norm_before_gate,
            is_rms_norm=False,
        )


class RMSNorm(torch.nn.Module):

    def __init__(
        self,
        hidden_size,
        eps=1e-5,
        group_size=None,
        norm_before_gate=True,
        device=None,
        dtype=None,
    ):
        """If group_size is not None, we do GroupNorm with each group having group_size elements.
        group_size=None is equivalent to group_size=hidden_size (i.e. there's only 1 group).
        """
        factory_kwargs = {"device": device, "dtype": dtype}
        super().__init__()
        self.eps = eps
        self.weight = torch.nn.Parameter(torch.empty(hidden_size, **factory_kwargs))
        self.register_parameter("bias", None)
        self.group_size = group_size
        self.norm_before_gate = norm_before_gate
        self.reset_parameters()

    def reset_parameters(self):
        torch.nn.init.ones_(self.weight)

    def forward(self, x, z=None):
        """If z is not None, we do norm(x) * silu(z) if norm_before_gate, else norm(x * silu(z))"""
        return layernorm_fn(
            x,
            self.weight,
            self.bias,
            z=z,
            eps=self.eps,
            group_size=self.group_size,
            norm_before_gate=self.norm_before_gate,
            is_rms_norm=True,
        )
