"""Low-frequency read-only observation emits transitions, never frame writes."""

import json

from pigway_frame_flow.storage.database import encode


class DeviceObserver:
    def __init__(self, service):
        self.service = service

    def tick(self):
        for id, driver in self.service.registry.drivers.items():
            try:
                device = driver.device()
            except Exception:
                # Cloud query failure is not evidence that the physical device is offline.
                continue
            low = (
                device.power.battery_percent is not None
                and device.power.battery_percent <= 20
            )
            state = {"status": device.status, "low_battery": low}
            key = "device-observation:" + id
            row = self.service.db.one(
                "SELECT payload FROM app_config WHERE key=?", (key,)
            )
            previous = json.loads(row["payload"]) if row else None
            if previous == state:
                continue
            if previous:
                if previous["status"] != state["status"] and state["status"] in (
                    "online",
                    "offline",
                ):
                    self.service.db.event(
                        "device." + state["status"], {"device_id": id}
                    )
                if low and not previous["low_battery"]:
                    self.service.db.event(
                        "device.low_battery",
                        {
                            "device_id": id,
                            "battery_percent": device.power.battery_percent,
                        },
                    )
            self.service.db.execute(
                "INSERT OR REPLACE INTO app_config VALUES(?,?)", (key, encode(state))
            )