"""Cross-loop Transition Adapter (CTA) from LoopQ Equation (7)."""

from __future__ import annotations

import math
from collections.abc import Mapping
from typing import Any

import torch
from torch import nn


DEFAULT_CTA_RANK = 8
DEFAULT_RMSNORM_EPSILON = 1e-6
OURO_TRANSITION_COUNT = 3
HUGINN_TRANSITION_COUNT = 31


class CrossLoopTransitionAdapter(nn.Module):
    """Apply LoopQ CTA only at a true cross-loop transition.

    ``U`` and ``V`` are shared across all transitions.  The affine vectors
    ``a_t``, ``b_t`` and low-rank gate ``eta_t`` are transition-dependent.
    Identity initialization is exact because ``a=1``, ``b=0`` and ``eta=0``.
    """

    FORMAT_VERSION = 1

    def __init__(
        self,
        hidden_size: int,
        transition_count: int,
        *,
        rank: int = DEFAULT_CTA_RANK,
        rmsnorm_epsilon: float = DEFAULT_RMSNORM_EPSILON,
        dtype: torch.dtype = torch.float32,
    ) -> None:
        super().__init__()
        if hidden_size <= 0:
            raise ValueError("hidden_size must be positive")
        if transition_count <= 0:
            raise ValueError("transition_count must be positive")
        if rank <= 0 or rank > hidden_size:
            raise ValueError("rank must be positive and no larger than hidden_size")
        if not math.isfinite(rmsnorm_epsilon) or rmsnorm_epsilon <= 0:
            raise ValueError("rmsnorm_epsilon must be finite and positive")
        self.enabled = True
        self.hidden_size = int(hidden_size)
        self.transition_count = int(transition_count)
        self.rank = int(rank)
        self.rmsnorm_epsilon = float(rmsnorm_epsilon)

        # Deterministic full-rank column initialization. eta=0 keeps the CTA
        # exactly identity while leaving a nonzero gradient path to its gates.
        basis = torch.eye(hidden_size, dtype=dtype)[:, :rank]
        self.U = nn.Parameter(basis.clone())
        self.V = nn.Parameter(basis.clone())
        self.a = nn.Parameter(torch.ones(transition_count, hidden_size, dtype=dtype))
        self.b = nn.Parameter(torch.zeros(transition_count, hidden_size, dtype=dtype))
        self.eta = nn.Parameter(torch.zeros(transition_count, rank, dtype=dtype))

    @classmethod
    def for_ouro(
        cls,
        hidden_size: int,
        *,
        rank: int = DEFAULT_CTA_RANK,
        rmsnorm_epsilon: float = DEFAULT_RMSNORM_EPSILON,
    ) -> "CrossLoopTransitionAdapter":
        return cls(
            hidden_size,
            OURO_TRANSITION_COUNT,
            rank=rank,
            rmsnorm_epsilon=rmsnorm_epsilon,
        )

    @classmethod
    def for_huginn(
        cls,
        hidden_size: int,
        *,
        rank: int = DEFAULT_CTA_RANK,
        rmsnorm_epsilon: float = DEFAULT_RMSNORM_EPSILON,
    ) -> "CrossLoopTransitionAdapter":
        return cls(
            hidden_size,
            HUGINN_TRANSITION_COUNT,
            rank=rank,
            rmsnorm_epsilon=rmsnorm_epsilon,
        )

    def _rmsnorm(self, hidden_state: torch.Tensor) -> torch.Tensor:
        work = hidden_state.to(torch.float32)
        normalized = work * torch.rsqrt(work.square().mean(dim=-1, keepdim=True) + self.rmsnorm_epsilon)
        return normalized.to(hidden_state.dtype)

    def forward(self, hidden_state: torch.Tensor, transition_index: int) -> torch.Tensor:
        if hidden_state.ndim == 0 or hidden_state.shape[-1] != self.hidden_size:
            raise ValueError(
                f"hidden_state last dimension must be {self.hidden_size}, "
                f"got {tuple(hidden_state.shape)}"
            )
        if not 0 <= transition_index < self.transition_count:
            raise IndexError(
                f"transition_index must be in [0, {self.transition_count}), "
                f"got {transition_index}"
            )
        if not self.enabled:
            return hidden_state
        normalized = self._rmsnorm(hidden_state)
        a = self.a[transition_index].to(hidden_state)
        b = self.b[transition_index].to(hidden_state)
        eta = self.eta[transition_index].to(hidden_state)
        U = self.U.to(hidden_state)
        V = self.V.to(hidden_state)
        affine = (a - 1) * normalized + b
        low_rank = ((normalized @ V) * eta) @ U.T
        return hidden_state + affine + low_rank

    def export_state(self) -> dict[str, Any]:
        return {
            "format": "loopq_cta",
            "format_version": self.FORMAT_VERSION,
            "hidden_size": self.hidden_size,
            "transition_count": self.transition_count,
            "rank": self.rank,
            "enabled": self.enabled,
            "rmsnorm_epsilon": self.rmsnorm_epsilon,
            "state_dict": {name: value.detach().cpu() for name, value in self.state_dict().items()},
        }

    @classmethod
    def from_export_state(cls, state: Mapping[str, Any]) -> "CrossLoopTransitionAdapter":
        required = {
            "format",
            "format_version",
            "hidden_size",
            "transition_count",
            "rank",
            "rmsnorm_epsilon",
            "state_dict",
        }
        missing = required.difference(state)
        if missing:
            raise ValueError(f"CTA export is missing fields: {sorted(missing)}")
        if state["format"] != "loopq_cta" or state["format_version"] != cls.FORMAT_VERSION:
            raise ValueError("unsupported CTA export format or version")
        adapter = cls(
            int(state["hidden_size"]),
            int(state["transition_count"]),
            rank=int(state["rank"]),
            rmsnorm_epsilon=float(state["rmsnorm_epsilon"]),
            dtype=state["state_dict"]["U"].dtype,
        )
        adapter.load_state_dict(state["state_dict"])
        adapter.enabled = bool(state.get("enabled", True))
        if not adapter.enabled:
            adapter.requires_grad_(False)
        return adapter