import json
import os
import threading


class FileStore:
    def __init__(self, data_dir):
        self.path = os.path.join(data_dir, "measures.json")
        self._lock = threading.Lock()
        os.makedirs(data_dir, exist_ok=True)
        if not os.path.exists(self.path):
            self._write([])

    def _read(self):
        with open(self.path, "r", encoding="utf-8") as f:
            return json.load(f)

    def _write(self, data):
        with open(self.path, "w", encoding="utf-8") as f:
            json.dump(data, f, ensure_ascii=False, indent=2)

    def add_measure(self, measure):
        with self._lock:
            data = self._read()
            data.append(measure)
            self._write(data)

    def list_measures(self, device_id, limit=50):
        with self._lock:
            data = self._read()
        return [m for m in data if m["device_id"] == device_id][-limit:]