"""Shared offline-aligned preprocessing helpers for 3D brain MRI volumes."""

from __future__ import annotations

import math
from pathlib import Path
from typing import Any, Mapping

import nibabel as nib
import numpy as np
import torch
import torch.nn.functional as F

try:
    from scipy import ndimage as scipy_ndimage
except Exception:  # pragma: no cover - optional import surface
    scipy_ndimage = None


TARGET_SHAPE = (128, 192, 192)
TARGET_SPACING = (1.25, 1.0, 1.0)
CROP_MARGIN_MM = 5.0
FOREGROUND_THRESHOLD = 1e-3
BACKGROUND_VALUE = -1.0
FOREGROUND_STRATEGY = "largest_component_nonzero"
GENERIC_RECIPE_ID = "generic_foreground_128x192x192_fp16_v1"
GENERIC_CACHE_VERSION = 1


def load_canonical_nifti(path: str | Path):
    return nib.as_closest_canonical(nib.load(str(path)))


def load_image_spacing(image) -> tuple[float, float, float]:
    zooms = image.header.get_zooms()[:3]
    if len(zooms) != 3:
        raise ValueError(f"Expected a 3D image spacing tuple, got {zooms}.")
    return tuple(float(value) for value in zooms)


def coerce_volume_to_3d(volume: np.ndarray) -> np.ndarray:
    if volume.ndim == 3:
        return volume.astype(np.float32, copy=False)
    if volume.ndim != 4:
        raise ValueError(f"Expected a 3D or 4D volume, got shape {volume.shape}.")

    if volume.shape[0] <= 4 and volume.shape[-1] > 4:
        selected = volume[0]
    else:
        selected = volume[..., 0]
    return np.asarray(selected, dtype=np.float32)


def largest_connected_component(mask: np.ndarray) -> np.ndarray:
    if not mask.any() or scipy_ndimage is None:
        return mask
    structure = scipy_ndimage.generate_binary_structure(mask.ndim, 1)
    labels, num_labels = scipy_ndimage.label(mask, structure=structure)
    if num_labels <= 1:
        return mask
    counts = np.bincount(labels.reshape(-1))
    if counts.size <= 1:
        return mask
    counts[0] = 0
    winning_label = int(counts.argmax())
    if winning_label <= 0 or counts[winning_label] <= 0:
        return mask
    return labels == winning_label


def build_foreground_mask(volume: np.ndarray, threshold: float = FOREGROUND_THRESHOLD) -> np.ndarray:
    sanitized = np.nan_to_num(volume, nan=0.0, posinf=0.0, neginf=0.0)
    raw_mask = np.abs(sanitized) > float(threshold)
    if not raw_mask.any():
        return np.ones_like(sanitized, dtype=bool)

    component_mask = largest_connected_component(raw_mask)
    component_count = int(component_mask.sum())
    raw_count = int(raw_mask.sum())
    if component_count <= 0:
        return raw_mask
    if component_count < 512 and raw_count > component_count:
        return raw_mask
    return component_mask


def compute_crop_bbox(
    mask: np.ndarray,
    spacing: tuple[float, float, float],
    margin_mm: float = CROP_MARGIN_MM,
) -> tuple[tuple[int, int], ...]:
    coords = np.where(mask)
    if coords[0].size == 0:
        raise ValueError("Foreground mask contains no positive voxels after selection.")

    bbox = []
    for axis, values in enumerate(coords):
        margin_voxels = int(math.ceil(float(margin_mm) / float(spacing[axis])))
        start = max(0, int(values.min()) - margin_voxels)
        stop = min(mask.shape[axis], int(values.max()) + margin_voxels + 1)
        bbox.append((start, stop))
    return tuple(bbox)


def crop_volume_and_mask(
    volume: np.ndarray,
    mask: np.ndarray,
    spacing: tuple[float, float, float],
    margin_mm: float = CROP_MARGIN_MM,
) -> tuple[np.ndarray, np.ndarray, tuple[tuple[int, int], ...]]:
    bbox = compute_crop_bbox(mask, spacing, margin_mm=margin_mm)
    slices = tuple(slice(start, stop) for start, stop in bbox)
    return volume[slices], mask[slices], bbox


def normalize_foreground_only(volume: np.ndarray, mask: np.ndarray) -> np.ndarray:
    sanitized = np.nan_to_num(volume, nan=0.0, posinf=0.0, neginf=0.0).astype(np.float32, copy=False)
    foreground_values = sanitized[mask]
    if foreground_values.size == 0:
        raise ValueError("Cannot normalize volume because the foreground mask is empty.")

    if foreground_values.size > 1_000_000:
        step = max(1, foreground_values.size // 1_000_000)
        foreground_values = foreground_values[::step]

    low, high = np.percentile(foreground_values, [0.5, 99.5])
    if not np.isfinite(low) or not np.isfinite(high) or high <= low:
        normalized = np.zeros_like(sanitized, dtype=np.float32)
    else:
        normalized = np.clip(sanitized, float(low), float(high))
        normalized = np.clip((normalized - float(low)) / float(high - low), 0.0, 1.0)
        normalized = normalized * 2.0 - 1.0
    return normalized.astype(np.float32, copy=False)


def resize_volume(volume: np.ndarray, size: tuple[int, int, int], mode: str) -> np.ndarray:
    tensor = torch.from_numpy(volume).unsqueeze(0).unsqueeze(0)
    kwargs = {}
    if mode in {"linear", "bilinear", "bicubic", "trilinear"}:
        kwargs["align_corners"] = False
    tensor = F.interpolate(tensor, size=size, mode=mode, **kwargs)
    return tensor.squeeze(0).squeeze(0).cpu().numpy().astype(np.float32, copy=False)


def resize_mask(mask: np.ndarray, size: tuple[int, int, int]) -> np.ndarray:
    tensor = torch.from_numpy(mask.astype(np.float32, copy=False)).unsqueeze(0).unsqueeze(0)
    tensor = F.interpolate(tensor, size=size, mode="nearest")
    return tensor.squeeze(0).squeeze(0).cpu().numpy() > 0.5


def resample_to_target_spacing(
    volume: np.ndarray,
    mask: np.ndarray,
    source_spacing: tuple[float, float, float],
    target_spacing: tuple[float, float, float] = TARGET_SPACING,
) -> tuple[np.ndarray, np.ndarray]:
    target_shape = []
    for current_size, src, dst in zip(volume.shape, source_spacing, target_spacing):
        target_shape.append(max(1, int(round(float(current_size) * float(src) / float(dst)))))
    target_shape_tuple = tuple(target_shape)
    if target_shape_tuple == tuple(int(v) for v in volume.shape):
        return volume.astype(np.float32, copy=False), mask
    return (
        resize_volume(volume, target_shape_tuple, mode="trilinear"),
        resize_mask(mask, target_shape_tuple),
    )


def downscale_to_fit(
    volume: np.ndarray,
    mask: np.ndarray,
    target_shape: tuple[int, int, int] = TARGET_SHAPE,
) -> tuple[np.ndarray, np.ndarray]:
    current_shape = tuple(int(v) for v in volume.shape)
    if all(current <= target for current, target in zip(current_shape, target_shape)):
        return volume, mask

    scale = min(float(target) / float(current) for current, target in zip(current_shape, target_shape))
    if scale >= 1.0:
        return volume, mask

    new_shape = tuple(
        min(target, max(1, int(math.floor(float(current) * scale))))
        for current, target in zip(current_shape, target_shape)
    )
    return (
        resize_volume(volume, new_shape, mode="trilinear"),
        resize_mask(mask, new_shape),
    )


def center_pad(
    array: np.ndarray,
    target_shape: tuple[int, int, int] = TARGET_SHAPE,
    fill_value: float = BACKGROUND_VALUE,
) -> np.ndarray:
    if any(current > target for current, target in zip(array.shape, target_shape)):
        raise ValueError(f"Cannot center-pad shape {array.shape} into smaller target {target_shape}.")
    pad_width = []
    for current, target in zip(array.shape, target_shape):
        delta = target - current
        before = delta // 2
        after = delta - before
        pad_width.append((before, after))
    return np.pad(array, pad_width=tuple(pad_width), mode="constant", constant_values=fill_value)


def preprocess_image_with_foreground_mask(
    image_path: str | Path,
    *,
    target_shape: tuple[int, int, int] = TARGET_SHAPE,
    target_spacing: tuple[float, float, float] = TARGET_SPACING,
    crop_margin_mm: float = CROP_MARGIN_MM,
    foreground_threshold: float = FOREGROUND_THRESHOLD,
    background_value: float = BACKGROUND_VALUE,
    foreground_strategy: str = FOREGROUND_STRATEGY,
    recipe_id: str = GENERIC_RECIPE_ID,
    cache_version: int = GENERIC_CACHE_VERSION,
) -> dict[str, object]:
    image_path = Path(image_path)
    image = load_canonical_nifti(image_path)
    source_shape = tuple(int(value) for value in image.shape)
    source_spacing = load_image_spacing(image)
    volume = np.asarray(image.get_fdata(dtype=np.float32), dtype=np.float32)
    volume = coerce_volume_to_3d(volume)

    foreground_mask = build_foreground_mask(volume, threshold=foreground_threshold)
    cropped_volume, cropped_mask, crop_bbox = crop_volume_and_mask(
        volume,
        foreground_mask,
        source_spacing,
        margin_mm=crop_margin_mm,
    )
    normalized_volume = normalize_foreground_only(cropped_volume, cropped_mask)
    resampled_volume, resampled_mask = resample_to_target_spacing(
        normalized_volume,
        cropped_mask,
        source_spacing=source_spacing,
        target_spacing=target_spacing,
    )
    fitted_volume, fitted_mask = downscale_to_fit(
        resampled_volume,
        resampled_mask,
        target_shape=target_shape,
    )
    fitted_volume = np.clip(fitted_volume, -1.0, 1.0).astype(np.float32, copy=False)
    fitted_volume[~fitted_mask] = float(background_value)

    padded_volume = center_pad(
        fitted_volume,
        target_shape=target_shape,
        fill_value=float(background_value),
    ).astype(np.float32, copy=False)
    pixel_values = torch.from_numpy(padded_volume).unsqueeze(0).to(dtype=torch.float16).contiguous()

    return {
        "pixel_values": pixel_values,
        "source_image": str(image_path),
        "source_shape": list(source_shape),
        "source_spacing": list(source_spacing),
        "crop_bbox": [[int(start), int(stop)] for start, stop in crop_bbox],
        "foreground_strategy": foreground_strategy,
        "recipe_id": recipe_id,
        "cache_version": int(cache_version),
    }


def validate_fixed_payload(
    payload: Mapping[str, Any],
    *,
    target_shape: tuple[int, int, int] = TARGET_SHAPE,
) -> None:
    pixel_values = payload.get("pixel_values")
    if not isinstance(pixel_values, torch.Tensor):
        raise TypeError("`pixel_values` must be a torch.Tensor.")
    expected_shape = (1,) + tuple(target_shape)
    if tuple(pixel_values.shape) != expected_shape:
        raise ValueError(f"Expected tensor shape {expected_shape}, got {tuple(pixel_values.shape)}.")
    if pixel_values.dtype != torch.float16:
        raise ValueError(f"Expected tensor dtype torch.float16, got {pixel_values.dtype}.")
    if not torch.isfinite(pixel_values).all():
        raise ValueError("Tensor contains non-finite values.")
    min_value = float(pixel_values.min().item())
    max_value = float(pixel_values.max().item())
    if min_value < -1.01 or max_value > 1.01:
        raise ValueError(f"Expected tensor values in [-1, 1]. Got min={min_value}, max={max_value}.")