"""ClimODE evaluation metrics and output serialization helpers."""

from __future__ import annotations

import json
import math
from pathlib import Path
from typing import Sequence

import numpy as np

try:
    from scipy.special import erf as _erf
except ImportError:  # pragma: no cover - only used in minimal environments
    _erf = np.vectorize(math.erf)


VARIABLES = ("z", "t", "t2m", "u10", "v10")


def latitude_weights(lat2d: np.ndarray) -> np.ndarray:
    lat = np.asarray(lat2d, dtype=np.float64)
    if lat.ndim == 2:
        lat = lat[:, 0]
    weights = np.cos(np.deg2rad(lat))
    weights = weights / np.mean(weights)
    return weights[:, None]


def _check_arrays(
    predictions: np.ndarray,
    targets: np.ndarray,
    std: np.ndarray | None,
    valid_lengths: Sequence[int] | None = None,
) -> None:
    if predictions.shape != targets.shape:
        raise ValueError(f"predictions {predictions.shape} != targets {targets.shape}")
    if predictions.ndim != 6:
        raise ValueError("Expected [samples, lead, years, channels, height, width]")
    if predictions.shape[3] != len(VARIABLES):
        raise ValueError(f"Expected {len(VARIABLES)} channels, got {predictions.shape[3]}")
    if std is not None and std.shape != predictions.shape:
        raise ValueError(f"std {std.shape} != predictions {predictions.shape}")
    if valid_lengths is not None:
        lengths = np.asarray(valid_lengths, dtype=np.int64)
        if lengths.shape != (predictions.shape[0],):
            raise ValueError(f"valid_lengths {lengths.shape} != ({predictions.shape[0]},)")
        if np.any(lengths < 1) or np.any(lengths > predictions.shape[1]):
            raise ValueError("valid_lengths must be within the lead dimension")


def _lead_mask(
    predictions: np.ndarray,
    valid_lengths: Sequence[int] | None,
) -> np.ndarray:
    lengths = (
        np.full(predictions.shape[0], predictions.shape[1], dtype=np.int64)
        if valid_lengths is None
        else np.asarray(valid_lengths, dtype=np.int64)
    )
    return (np.arange(predictions.shape[1])[None, :] < lengths[:, None]).reshape(
        predictions.shape[0], predictions.shape[1], 1, 1, 1, 1
    )


def _weighted_mean(values: np.ndarray, weights: np.ndarray) -> np.ndarray:
    # values: [N,L,Y,K,H,W], weights: [H,1]
    weighted = values * weights[None, None, None, None, :, :]
    return weighted.mean(axis=(-1, -2))


def latitude_weighted_rmse(
    predictions: np.ndarray,
    targets: np.ndarray,
    lat2d: np.ndarray,
    valid_lengths: Sequence[int] | None = None,
) -> np.ndarray:
    weights = latitude_weights(lat2d)
    error = np.square(np.nan_to_num(predictions - targets, nan=0.0))
    per_field = np.sqrt(_weighted_mean(error, weights))
    valid = _lead_mask(predictions, valid_lengths)[..., 0, 0, 0, 0]
    valid_fields = np.broadcast_to(valid[:, :, None, None], per_field.shape)
    return (per_field * valid_fields).sum(axis=(0, 2)) / np.maximum(
        valid_fields.sum(axis=(0, 2)), 1.0
    )


def anomaly_correlation(
    predictions: np.ndarray,
    targets: np.ndarray,
    lat2d: np.ndarray,
    valid_lengths: Sequence[int] | None = None,
) -> np.ndarray:
    weights = latitude_weights(lat2d)
    valid = _lead_mask(predictions, valid_lengths)
    valid_broadcast = np.broadcast_to(valid, targets.shape)
    target_clean = np.nan_to_num(targets, nan=0.0)
    valid_count = valid_broadcast.sum(axis=(0, 1))
    # Official evaluation uses one test-set climatology for each year/channel/grid.
    climatology = (target_clean * valid_broadcast).sum(axis=(0, 1)) / np.maximum(
        valid_count, 1.0
    )
    pred_anomaly = np.nan_to_num(predictions, nan=0.0) - climatology[None, None]
    target_anomaly = target_clean - climatology[None, None]
    pred_anomaly -= pred_anomaly.mean(axis=(-1, -2), keepdims=True)
    target_anomaly -= target_anomaly.mean(axis=(-1, -2), keepdims=True)
    weighted_mask = weights[None, None, None, None] * valid
    numerator = (pred_anomaly * target_anomaly * weighted_mask).sum(axis=(-1, -2))
    pred_norm = np.sqrt((np.square(pred_anomaly) * weighted_mask).sum(axis=(-1, -2)))
    target_norm = np.sqrt((np.square(target_anomaly) * weighted_mask).sum(axis=(-1, -2)))
    per_field = numerator / np.maximum(pred_norm * target_norm, 1.0e-12)
    valid_fields = np.broadcast_to(valid[..., 0, 0], per_field.shape)
    return (per_field * valid_fields).sum(axis=(0, 2)) / np.maximum(
        valid_fields.sum(axis=(0, 2)), 1.0
    )


def _normal_crps(
    observations: np.ndarray,
    means: np.ndarray,
    scales: np.ndarray,
) -> np.ndarray:
    """Closed-form CRPS for a Gaussian predictive distribution."""

    scales = np.maximum(np.asarray(scales, dtype=np.float64), 1.0e-6)
    z = (np.asarray(observations, dtype=np.float64) - means) / scales
    phi = np.exp(-0.5 * np.square(z)) / math.sqrt(2.0 * math.pi)
    cdf = 0.5 * (1.0 + _erf(z / math.sqrt(2.0)))
    return scales * (z * (2.0 * cdf - 1.0) + 2.0 * phi - 1.0 / math.sqrt(math.pi))


def gaussian_crps(
    predictions: np.ndarray,
    targets: np.ndarray,
    std: np.ndarray,
    valid_lengths: Sequence[int] | None = None,
) -> np.ndarray:
    values = np.nan_to_num(_normal_crps(targets, predictions, std), nan=0.0)
    mask = np.broadcast_to(_lead_mask(predictions, valid_lengths), values.shape)
    return (values * mask).sum(axis=(0, 2, 4, 5)) / np.maximum(
        mask.sum(axis=(0, 2, 4, 5)), 1.0
    )


def evaluate(
    predictions: np.ndarray,
    targets: np.ndarray,
    lat2d: np.ndarray,
    std: np.ndarray | None = None,
    crps_predictions: np.ndarray | None = None,
    crps_targets: np.ndarray | None = None,
    crps_std: np.ndarray | None = None,
    valid_lengths: Sequence[int] | None = None,
) -> dict:
    _check_arrays(predictions, targets, std, valid_lengths)
    result = {
        "variables": list(VARIABLES),
        "lead_times_hours": [6 * (index + 1) for index in range(predictions.shape[1])],
        "rmse": latitude_weighted_rmse(predictions, targets, lat2d, valid_lengths).tolist(),
        "acc": anomaly_correlation(predictions, targets, lat2d, valid_lengths).tolist(),
        "rmse_space": "physical",
        "acc_space": "physical",
    }
    if std is not None:
        result["crps"] = gaussian_crps(
            crps_predictions if crps_predictions is not None else predictions,
            crps_targets if crps_targets is not None else targets,
            crps_std if crps_std is not None else std,
            valid_lengths,
        ).tolist()
        result["crps_space"] = "normalized" if crps_predictions is not None else "physical"
        result["crps_implementation"] = "closed_form_gaussian"
    return result


def save_metrics(metrics: dict, path: str | Path) -> None:
    output = Path(path)
    output.parent.mkdir(parents=True, exist_ok=True)
    output.write_text(json.dumps(metrics, indent=2), encoding="utf-8")