"""Stable LatentMoE (K3): shared 全宽 + routed 半宽专家 + SiTU-GLU + Top-k.

对照 learning/kimi-k3-notes §Stable LatentMoE:
    z = W_down(x)                       [B, T, ℓ]   ℓ = d/2  latent 接口宽
    u = Σ_{i∈Top-k(x)} p_i E_i^rt(z)    [B, T, ℓ]   routed 专家只在 ℓ 上算
    y = Σ_j E_j^sh(x) + W_up RMSNorm(u) [B, T, d]   shared 全宽

SiTU-GLU:  gate = β1·tanh(W_g x/β1)⊙σ(W_g x);  up = β2·tanh(W_u x/β2)
    ||SiTU-GLU||_∞ ≤ β1·β2 (=100), 原点附近≈SwiGLU, 远端软饱和防低精度溢出.
    E: R^in → R^in (内部中间维 d_ff).

Router: Top-k logits 基于全宽 x (笔记 Topk(x)); 归一化权重取 softmax(topk).
"""
from __future__ import annotations

import torch
import torch.nn.functional as F
from torch import nn

from .rmsnorm import RMSNorm


class SiTU(nn.Module):
    """SiTU-GLU expert: gate 支软上限 β1, up 支软上限 β2, 输出回到输入维."""

    def __init__(self, dim_in: int, dim_ff: int, beta1: float = 4.0, beta2: float = 25.0):
        super().__init__()
        self.beta1, self.beta2 = beta1, beta2
        self.w_g = nn.Linear(dim_in, dim_ff, bias=False)
        self.w_u = nn.Linear(dim_in, dim_ff, bias=False)
        self.w_o = nn.Linear(dim_ff, dim_in, bias=False)

    def forward(self, x: torch.Tensor):
        wg = self.w_g(x)
        g = self.beta1 * torch.tanh(wg / self.beta1) * torch.sigmoid(wg)
        u = self.beta2 * torch.tanh(self.w_u(x) / self.beta2)
        return self.w_o(g * u)


class LatentMoE(nn.Module):
    def __init__(
        self,
        hidden_size: int,
        latent_size: int,
        n_routed: int,
        top_k: int,
        n_shared: int,
        d_ff: int,
        beta1: float = 4.0,
        beta2: float = 25.0,
    ):
        super().__init__()
        self.latent_size = latent_size
        self.n_routed = n_routed
        self.top_k = top_k

        self.down = nn.Linear(hidden_size, latent_size, bias=False)   # W↓
        self.router = nn.Linear(hidden_size, n_routed, bias=False)    # Top-k logits
        self.shared = nn.ModuleList(
            [SiTU(hidden_size, d_ff, beta1, beta2) for _ in range(n_shared)]
        )
        self.experts = nn.ModuleList(
            [SiTU(latent_size, d_ff, beta1, beta2) for _ in range(n_routed)]
        )
        self.norm = RMSNorm(latent_size)
        self.up = nn.Linear(latent_size, hidden_size, bias=False)     # W↑
        self.last_route_ids: torch.Tensor | None = None

    @classmethod
    def from_config(cls, config) -> LatentMoE:
        return cls(
            config.hidden_size,
            config.moe_latent_size,
            config.n_routed,
            config.top_k,
            config.n_shared,
            config.moe_d_ff,
            config.situ_beta1,
            config.situ_beta2,
        )

    def forward(self, x: torch.Tensor):
        B, T, _ = x.shape
        z = self.down(x)                                    # [B, T, ℓ]

        logits = self.router(x)                             # [B, T, n_routed]
        topk = torch.topk(logits, self.top_k, dim=-1)
        ids = topk.indices                                  # [B, T, k]
        self.last_route_ids = ids.detach()
        probs = F.softmax(topk.values, dim=-1)              # [B, T, k]

        # 向量化 routed: 预计算全部专家输出, 按 token 的 Top-k id 取
        all_out = torch.stack([e(z) for e in self.experts])     # [R, B, T, ℓ]
        all_out = all_out.permute(1, 2, 0, 3).reshape(B * T, self.n_routed, self.latent_size)
        u = torch.zeros(B, T, self.latent_size, device=x.device, dtype=x.dtype)
        for i in range(self.top_k):
            idx = ids[:, :, i].reshape(B * T)               # [B*T]
            sel = all_out[torch.arange(B * T, device=x.device), idx]  # [B*T, ℓ]
            u += probs[:, :, i : i + 1] * sel.reshape(B, T, self.latent_size)

        shared_out = torch.stack([e(x) for e in self.shared]).sum(0)   # [B, T, d]
        return shared_out + self.up(self.norm(u))


def moe_route_frac(model: nn.Module) -> torch.Tensor | None:
    """Mean expert occupancy over LatentMoE layers from the last forward."""
    hists: list[torch.Tensor] = []
    n_routed: int | None = None
    for module in model.modules():
        if not isinstance(module, LatentMoE) or module.last_route_ids is None:
            continue
        n_routed = module.n_routed
        ids = module.last_route_ids.reshape(-1)
        hists.append(torch.bincount(ids, minlength=n_routed).float())
    if not hists or n_routed is None:
        return None
    stacked = torch.stack(hists).sum(0)
    return stacked / stacked.sum().clamp_min(1.0)