import time
from collections import deque

from app.sample import Sample


class HistoryStore:
    """In-memory ring buffer of Sample points, oldest dropped first.

    `maxlen` is derived from `DASH_RETENTION_MINUTES` / `DASH_SAMPLE_INTERVAL`
    (see `Settings.history_maxlen`). All methods are called from the event
    loop thread; the sampler's collection work happens in a worker thread
    before `record` is called, so no locking is needed.
    """

    def __init__(self, maxlen: int) -> None:
        """Create an empty store.

        Args:
            maxlen: maximum number of samples to keep.
        """
        self._buf: deque[Sample] = deque(maxlen=maxlen)

    def record(self, sample: Sample) -> None:
        """Stamp the sample with the current unix time and append it.

        Args:
            sample: sample to store; its `ts` field is overwritten.
        """
        sample.ts = time.time()
        self._buf.append(sample)

    def snapshot(self) -> list[Sample]:
        """Return all stored samples, oldest first.

        Returns:
            A copy of the buffer contents as a list.
        """
        return list(self._buf)

    def latest(self) -> Sample | None:
        """Return the newest sample, or None if the store is empty."""
        return self._buf[-1] if self._buf else None

    def __len__(self) -> int:
        """Number of samples currently stored."""
        return len(self._buf)