"""File locking and atomic JSON helpers."""
import fcntl
import json
import os

__all__ = [
    "locked_file",
    "atomic_write_json",
    "load_json_locked",
    "save_json_locked",
    "read_modify_write_json",
]
from contextlib import contextmanager
from pathlib import Path


@contextmanager
def locked_file(path: Path):
    path.parent.mkdir(parents=True, exist_ok=True)
    fd = None
    try:
        fd = os.open(path, os.O_RDWR | os.O_CREAT)
        fcntl.flock(fd, fcntl.LOCK_EX)
        yield fd
    finally:
        if fd is not None:
            try:
                os.fsync(fd)
            except Exception:
                pass
            try:
                fcntl.flock(fd, fcntl.LOCK_UN)
            except Exception:
                pass
            os.close(fd)


def atomic_write_json(path: Path, data: dict):
    path.parent.mkdir(parents=True, exist_ok=True)
    tmp = path.with_suffix(path.suffix + ".tmp")
    tmp.write_text(json.dumps(data, indent=2, ensure_ascii=False), encoding="utf-8")
    os.replace(tmp, path)


def load_json_locked(path: Path, lock_path: Path, default):
    with locked_file(lock_path):
        if not path.exists():
            return default
        try:
            return json.loads(path.read_text(encoding="utf-8"))
        except Exception:
            return default


def save_json_locked(path: Path, lock_path: Path, data: dict):
    with locked_file(lock_path):
        atomic_write_json(path, data)


def read_modify_write_json(path: Path, lock_path: Path, default, modifier):
    """Atomic read-modify-write under a single file lock.

    Loads JSON from *path* (or uses *default* if missing/invalid),
    calls ``modifier(data)``, then atomically writes the result back.
    If *modifier* returns None, the mutated *data* is written.
    """
    with locked_file(lock_path):
        if path.exists():
            try:
                data = json.loads(path.read_text(encoding="utf-8"))
            except Exception:
                data = default
        else:
            data = default
        result = modifier(data)
        atomic_write_json(path, result if result is not None else data)