"""Small JSON read/write helpers for the state files under DATA_DIR.

Every file is written via a temp-file + os.replace so a crash mid-write
never leaves a half-written JSON file behind for the next read.
"""
import json
import os
import threading

DATA_DIR = os.environ.get("DATA_DIR", "/data")

_lock = threading.Lock()


def _path(name):
    return os.path.join(DATA_DIR, name)


def load(name, default):
    path = _path(name)
    with _lock:
        if not os.path.exists(path):
            return default
        with open(path) as f:
            return json.load(f)


def save(name, data):
    path = _path(name)
    with _lock:
        os.makedirs(DATA_DIR, exist_ok=True)
        tmp = path + ".tmp"
        with open(tmp, "w") as f:
            json.dump(data, f, indent=2)
        os.replace(tmp, path)