"""Initial velocity fitting and cache management for ClimODE training."""

from __future__ import annotations

import sys
from pathlib import Path

import numpy as np
import torch
import torch.nn as nn
import torch.optim as optim

PROJECT_ROOT = Path(__file__).resolve().parents[1]
if str(PROJECT_ROOT) not in sys.path:
    sys.path.insert(0, str(PROJECT_ROOT))

from model.climode import OptimVelocity

try:
    from torchcubicspline import NaturalCubicSpline, natural_cubic_spline_coeffs
except ImportError:  # pragma: no cover - dependency is optional for tiny smoke tests
    NaturalCubicSpline = None
    natural_cubic_spline_coeffs = None


def _time_derivative(history: torch.Tensor, interval_hours: float = 6.0) -> torch.Tensor:
    """Estimate the derivative at the final history point.

    The cubic-spline path is identical to the official implementation.  The
    finite-difference fallback is only for environments without the optional
    package and is explicitly reported to the caller.
    """

    if history.ndim != 5:
        raise ValueError(f"Expected history [N,3,K,H,W], got {tuple(history.shape)}")
    if natural_cubic_spline_coeffs is not None:
        times = torch.arange(3, device=history.device, dtype=history.dtype) * interval_hours
        values = history.permute(1, 0, 2, 3, 4)
        coeffs = natural_cubic_spline_coeffs(times, values)
        spline = NaturalCubicSpline(coeffs)
        return spline.derivative(times[-1])
    return (3.0 * history[:, 2] - 4.0 * history[:, 1] + history[:, 0]) / (2.0 * interval_hours)


def build_rbf_kernel(
    lat2d: torch.Tensor,
    lon2d: torch.Tensor,
    sigma: float = 1.0,
) -> torch.Tensor:
    coords = torch.stack([lat2d.reshape(-1), lon2d.reshape(-1)], dim=1).float()
    distances = torch.cdist(coords, coords).square()
    kernel = torch.exp(-distances / (2.0 * sigma * sigma))
    return torch.linalg.inv(kernel)


def optimize_velocity(
    history: torch.Tensor,
    current: torch.Tensor,
    kernel_inv: torch.Tensor,
    epochs: int = 200,
    learning_rate: float = 2.0,
    smoothing_alpha: float = 1.0e-7,
) -> torch.Tensor:
    """Fit [N,2K,H,W] velocities using the official penalized objective."""

    if current.ndim != 4:
        raise ValueError(f"Expected current [N,K,H,W], got {tuple(current.shape)}")
    num_years, channels, height, width = current.shape
    model = OptimVelocity(num_years, height, width, channels).to(current.device)
    optimizer = optim.Adam(model.parameters(), lr=learning_rate)
    delta_u = _time_derivative(history)
    best_loss = float("inf")
    best_velocity = None
    for _ in range(max(int(epochs), 1)):
        optimizer.zero_grad(set_to_none=True)
        out, vx, vy = model(current.unsqueeze(1))
        vx_flat = vx.view(num_years, channels, -1, 1)
        vy_flat = vy.view(num_years, channels, -1, 1)
        kernel = kernel_inv.to(current.device).expand(num_years, channels, -1, -1)
        smooth_x = torch.matmul(torch.matmul(vx_flat.transpose(2, 3), kernel), vx_flat).mean()
        smooth_y = torch.matmul(torch.matmul(vy_flat.transpose(2, 3), kernel), vy_flat).mean()
        loss = nn.functional.mse_loss(out.squeeze(1), delta_u) + smoothing_alpha * (smooth_x + smooth_y)
        loss.backward()
        optimizer.step()
        if float(loss.detach()) < best_loss:
            best_loss = float(loss.detach())
            best_velocity = torch.cat([vx.detach(), vy.detach()], dim=2).squeeze(1).clone()
    if best_velocity is None:
        raise RuntimeError("Velocity optimization produced no result")
    return best_velocity


def fit_velocity_cache(
    dataset,
    constants: torch.Tensor,
    lat2d: torch.Tensor,
    lon2d: torch.Tensor,
    output_path: str | Path,
    epochs: int = 200,
    learning_rate: float = 2.0,
    smoothing_alpha: float = 1.0e-7,
    kernel_sigma: float = 1.0,
) -> torch.Tensor:
    del constants  # Kept in the signature to make the training handoff explicit.
    kernel_inv = build_rbf_kernel(lat2d, lon2d, kernel_sigma)
    velocities = []
    for index in range(len(dataset)):
        item = dataset[index]
        history = item["history"].float()
        current = item["observations"][0].float()
        velocities.append(
            optimize_velocity(
                history,
                current,
                kernel_inv,
                epochs=epochs,
                learning_rate=learning_rate,
                smoothing_alpha=smoothing_alpha,
            )
        )
    result = torch.stack(velocities)
    path = Path(output_path)
    path.parent.mkdir(parents=True, exist_ok=True)
    torch.save({"velocity": result, "starts": dataset.starts, "years": dataset.years}, path)
    return result


def load_velocity_cache(path: str | Path, expected_length: int | None = None) -> torch.Tensor:
    try:
        checkpoint = torch.load(path, map_location="cpu", weights_only=True)
    except TypeError:  # PyTorch before weights_only support.
        checkpoint = torch.load(path, map_location="cpu")
    velocity = checkpoint["velocity"] if isinstance(checkpoint, dict) else checkpoint
    if expected_length is not None and len(velocity) != expected_length:
        raise ValueError(f"Velocity cache length {len(velocity)} != dataset length {expected_length}")
    return velocity.float()