"""A counter's previous sample, to turn the next one into a per-second rate.

Scrapers that read monotonic counters (SNMP octets, Prometheus *_total)
store the rate, not the running total. Keys are tuples whose first element
is the owner (a device, a target), so forgetting an owner drops all of its
counters at once.
"""
from __future__ import annotations

from datetime import datetime


class CounterRates:
    """In memory: after a restart the first sample of each counter gives no
    rate and the second does. A sample lower than the one before (the source
    restarted, or a 32-bit counter wrapped) gives no rate rather than a huge
    negative or wrapped one."""

    def __init__(self) -> None:
        self._last: dict[tuple, tuple[float, datetime]] = {}

    def rate(self, key: tuple, value: float, at: datetime) -> float | None:
        prev = self._last.get(key)
        self._last[key] = (value, at)
        if prev is None:
            return None
        seconds = (at - prev[1]).total_seconds()
        if seconds <= 0 or value < prev[0]:
            return None
        return (value - prev[0]) / seconds

    def forget(self, owner) -> None:
        for key in [k for k in self._last if k[0] == owner]:
            del self._last[key]