"""Read back what ros2 bag already knows about a recording — duration,
message counts, per-topic counts — instead of just listing directory names.
Also a plain on-disk size sum (metadata.yaml doesn't carry a byte size in
the ros2 bag version installed here)."""
from __future__ import annotations

import os
from pathlib import Path
from typing import Optional

import yaml


def dir_size_bytes(path: Path) -> int:
    total = 0
    try:
        for entry in path.iterdir():
            if entry.is_file():
                total += entry.stat().st_size
    except OSError:
        pass
    return total


def read_bag_metadata(bag_dir: Path) -> Optional[dict]:
    """Returns None if metadata.yaml is missing (e.g. bag was killed mid-write
    before rosbag2 flushed it) or unparseable — a real, if incomplete, state
    the operator should be able to see rather than a crash."""
    meta_path = bag_dir / "metadata.yaml"
    if not meta_path.exists():
        return None
    try:
        with open(meta_path, encoding="utf-8") as f:
            raw = yaml.safe_load(f)
        info = raw["rosbag2_bagfile_information"]
    except Exception:
        return None

    duration_s = info.get("duration", {}).get("nanoseconds", 0) / 1e9
    message_count = info.get("message_count", 0)
    topics = []
    for entry in info.get("topics_with_message_count", []):
        tm = entry.get("topic_metadata", {})
        topics.append({
            "name": tm.get("name", "?"),
            "type": tm.get("type", "?"),
            "message_count": entry.get("message_count", 0),
        })

    return {
        "duration_s": round(duration_s, 1),
        "message_count": message_count,
        "topics": topics,
        "size_bytes": dir_size_bytes(bag_dir),
    }