"""Vehicle truth and command transactions; independent of UI/animation clocks."""
import time


class VehicleState:
    HEARTBEAT_TIMEOUT = 3.0
    COMMAND_TIMEOUT = 5.0

    def __init__(self, clock=time.monotonic):
        self.clock = clock
        self.connected = False
        self.armed = None
        self.heartbeat_at = None
        self.pending = None
        self.error = ""

    def fresh(self):
        return (self.connected and self.heartbeat_at is not None
                and self.clock() - self.heartbeat_at <= self.HEARTBEAT_TIMEOUT)

    def heartbeat(self, armed, received_at=None):
        received_at = self.clock() if received_at is None else received_at
        if self.heartbeat_at is not None and received_at < self.heartbeat_at:
            return
        self.connected = True
        self.heartbeat_at = received_at
        self.armed = bool(armed)
        # A fresh heartbeat is the authoritative confirmation of actual state.
        # ACK ACCEPTED alone only confirms that a command was accepted.
        if (self.pending and received_at > self.pending["started"]
                and self.armed == (self.pending["name"] == "arm")):
            self.pending = None

    def request(self, name):
        self.tick()
        if name == "arm" and (not self.fresh() or self.armed is not False):
            raise RuntimeError("Fresh, disarmed vehicle telemetry is required to engage")
        if self.pending and (name == "arm" or self.pending["name"] == name):
            raise RuntimeError("A vehicle confirmation is already pending")
        now = self.clock()
        self.pending = {"name": name, "started": now,
                        "deadline": now + self.COMMAND_TIMEOUT, "ack": None}
        self.error = ""

    def ack(self, result, received_at=None):
        received_at = self.clock() if received_at is None else received_at
        if not self.pending or received_at <= self.pending["started"]:
            return
        self.pending["ack"] = result
        if result not in ("MAV_RESULT_ACCEPTED", "MAV_RESULT_IN_PROGRESS"):
            self.fail(f"{self.pending['name'].capitalize()} rejected: {result}")

    def fail(self, message):
        self.error = message
        self.pending = None

    def disconnect(self):
        self.connected = False
        if self.pending:
            self.fail("Connection lost before vehicle confirmation. State unknown.")

    def tick(self):
        if self.pending and self.clock() >= self.pending["deadline"]:
            name = self.pending["name"].capitalize()
            self.fail(f"{name} confirmation timed out. Check current vehicle state.")

    def snapshot(self):
        self.tick()
        fresh = self.fresh()
        return {"armed": self.armed, "armed_known": fresh,
                "heartbeat_age": None if self.heartbeat_at is None else self.clock() - self.heartbeat_at,
                "pending_command": self.pending["name"] if self.pending else None,
                "command_ack": self.pending["ack"] if self.pending else None,
                "command_error": self.error,
                "controls_allowed": fresh and self.armed is True and self.pending is None}