import json
import logging
import os
from typing import Dict, List, Tuple

import torch


def _label_to_name(label: int, label2cat_id: Dict[int, int], cats: Dict[int, dict]) -> str:
    """Return human readable category name for a dataset label."""
    cat_id = label2cat_id.get(label)
    if cat_id is None:
        return f"label_{label}"
    cat_info = cats.get(cat_id, {})
    return cat_info.get("name", f"cat_{cat_id}")


def _top_mismatch_pairs(
    preds: torch.Tensor,
    labels: torch.Tensor,
    num_classes: int,
    label2cat_id: Dict[int, int],
    cats: Dict[int, dict],
    top_k: int = 30,
) -> Tuple[float, List[dict]]:
    """Compute mismatch rate and the most frequent gt->pred pairs."""
    if labels.numel() == 0:
        return 0.0, []
    mismatch_mask = preds != labels
    total_mismatch = mismatch_mask.sum().item()
    mismatch_rate = float(total_mismatch) / float(labels.numel())
    if total_mismatch == 0:
        return mismatch_rate, []

    pair_labels = torch.stack([labels[mismatch_mask], preds[mismatch_mask]], dim=1)
    # flatten pair (gt, pred) to single index for counting
    flat = pair_labels[:, 0] * num_classes + pair_labels[:, 1]
    counts = torch.bincount(flat, minlength=num_classes * num_classes)
    # guard in case bincount returns empty
    if counts.numel() == 0:
        return mismatch_rate, []

    values, indices = torch.topk(counts, k=min(top_k, counts.numel()))
    results = []
    for idx, cnt in zip(indices.tolist(), values.tolist()):
        if cnt == 0:
            continue
        gt_label = idx // num_classes
        pred_label = idx % num_classes
        results.append(
            {
                "gt_label": int(gt_label),
                "pred_label": int(pred_label),
                "gt_name": _label_to_name(gt_label, label2cat_id, cats),
                "pred_name": _label_to_name(pred_label, label2cat_id, cats),
                "count": int(cnt),
                "ratio_within_mismatch": float(cnt) / float(total_mismatch),
            }
        )
    return mismatch_rate, results


def save_mismatch_reports(
    preds_dict: Dict[str, torch.Tensor],
    labels: torch.Tensor,
    dataset,
    save_dir: str,
    epoch: int,
    top_k: int = 30,
) -> None:
    """
    Save mismatch statistics to disk.

    Args:
        preds_dict: mapping from head name to top1 predictions.
        labels: ground-truth labels (long tensor).
        dataset: dataset object providing label2cat_id and coco.cats metadata.
        save_dir: root directory to write reports.
        epoch: current epoch number (used in filenames).
        top_k: number of most frequent mismatch pairs to keep.
    """
    if not hasattr(dataset, "label2cat_id") or not hasattr(dataset, "coco") or not hasattr(dataset.coco, "cats"):
        logging.warning("Dataset missing category metadata, skip mismatch report.")
        return

    os.makedirs(save_dir, exist_ok=True)
    label2cat_id = dataset.label2cat_id
    cats = dataset.coco.cats
    num_classes = len(label2cat_id)

    summary = {}
    for head, preds in preds_dict.items():
        if preds.numel() != labels.numel():
            logging.warning("Preds and labels length mismatch for head %s, skip.", head)
            continue
        mismatch_rate, top_pairs = _top_mismatch_pairs(
            preds=preds,
            labels=labels,
            num_classes=num_classes,
            label2cat_id=label2cat_id,
            cats=cats,
            top_k=top_k,
        )
        report = {
            "epoch": int(epoch),
            "total_samples": int(labels.numel()),
            "total_mismatch": int((preds != labels).sum().item()),
            "mismatch_rate": mismatch_rate,
            "top_pairs": top_pairs,
        }
        summary[head] = report
        filename = os.path.join(save_dir, f"{head}_mismatch_epoch{epoch}.json")
        try:
            with open(filename, "w") as f:
                json.dump(report, f, indent=2)
        except OSError as e:
            logging.error("Failed to write mismatch report for %s: %s", head, e)

    # save a combined summary for quick inspection
    combined_path = os.path.join(save_dir, f"mismatch_summary_epoch{epoch}.json")
    try:
        with open(combined_path, "w") as f:
            json.dump(summary, f, indent=2)
    except OSError as e:
        logging.error("Failed to write combined mismatch summary: %s", e)