"""Crash auto-restart back-off policy (pure, GUI-independent)."""

from __future__ import annotations

import math
from dataclasses import dataclass


@dataclass(frozen=True, slots=True)
class RestartPolicy:
    """Decide when to relaunch a crashed pipeline — retry forever with capped back-off.

    This is an unattended appliance, so the pipeline never permanently gives up. The
    wait between restart attempts grows with the number of consecutive failures (so a
    persistently broken pipeline is not hammered) but is capped at ``max_interval_s``,
    and the failure streak resets to zero once genuine data flows again. A burst of
    crash signals within the current back-off window collapses to a single restart.
    """

    min_interval_s: float = 3.0
    max_interval_s: float = 60.0
    backoff_factor: float = 2.0

    def backoff_for(self, consecutive_failures: int) -> float:
        """Return the seconds to wait before the next restart for this failure streak.

        ``consecutive_failures`` is the number of restarts already attempted without a
        recovery: 0 → ``min_interval_s``, then each additional failure multiplies the
        wait by ``backoff_factor``, capped at ``max_interval_s``.
        """
        if consecutive_failures <= 0:
            return self.min_interval_s
        # Beyond this many doublings the wait is always capped; clamp the exponent so a
        # long streak cannot overflow ``factor ** n``.
        max_exponent = max(1, math.ceil(math.log(self.max_interval_s / self.min_interval_s, self.backoff_factor)))
        exponent = min(int(consecutive_failures), max_exponent)
        return min(self.min_interval_s * (self.backoff_factor ** exponent), self.max_interval_s)

    def should_restart_now(
        self,
        *,
        now_s: float,
        last_restart_s: float,
        consecutive_failures: int,
    ) -> bool:
        """Return whether enough back-off has elapsed since the last restart to retry."""
        return now_s - last_restart_s >= self.backoff_for(consecutive_failures)