"""FC-3d: pure logic for deciding which sources are due for a check.

The Celery tick task wraps `select_due_sources` and fires
`download_source.delay()` per result; this module knows nothing about
Celery, gallery-dl, or any side effect.
"""
from __future__ import annotations

from datetime import UTC, datetime, timedelta

from sqlalchemy import select
from sqlalchemy.ext.asyncio import AsyncSession
from sqlalchemy.orm import selectinload

from ..models import Artist, ImportSettings, Source

MIN_INTERVAL_SECONDS = 60
MAX_INTERVAL_SECONDS = 86400
MAX_BACKOFF_EXPONENT = 6


def compute_effective_interval(
    source: Source, artist: Artist, settings: ImportSettings,
) -> int:
    """Return the seconds between scheduled checks for one source.

    Precedence: source.check_interval_override > artist.check_interval_seconds
    > settings.download_schedule_default_seconds. Multiplied by
    2 ** min(consecutive_failures, MAX_BACKOFF_EXPONENT) and clamped to
    [MIN_INTERVAL_SECONDS, MAX_INTERVAL_SECONDS].
    """
    base = (
        source.check_interval_override
        or artist.check_interval_seconds
        or settings.download_schedule_default_seconds
    )
    exponent = min(max(0, source.consecutive_failures or 0), MAX_BACKOFF_EXPONENT)
    factor = 2 ** exponent
    raw = base * factor
    return max(MIN_INTERVAL_SECONDS, min(MAX_INTERVAL_SECONDS, raw))


async def select_due_sources(session: AsyncSession) -> list[Source]:
    """Sources where (enabled, artist.auto_check) and now >= last_checked_at + effective_interval.

    Never-checked sources (last_checked_at IS NULL) are always due.
    """
    rows = (await session.execute(
        select(Source)
        .options(selectinload(Source.artist))
        .join(Artist, Source.artist_id == Artist.id)
        .where(Source.enabled.is_(True))
        .where(Artist.auto_check.is_(True))
    )).scalars().all()

    settings = (await session.execute(
        select(ImportSettings).where(ImportSettings.id == 1)
    )).scalar_one()

    now = datetime.now(UTC)
    due: list[Source] = []
    for s in rows:
        interval = compute_effective_interval(s, s.artist, settings)
        if s.last_checked_at is None:
            due.append(s)
            continue
        elapsed = (now - s.last_checked_at).total_seconds()
        if elapsed >= interval:
            due.append(s)
    return due


def compute_next_check_at(
    source: Source, artist: Artist, settings: ImportSettings,
) -> datetime | None:
    """Return the projected datetime of the next check, or None if never checked."""
    if source.last_checked_at is None:
        return None
    interval = compute_effective_interval(source, artist, settings)
    return source.last_checked_at + timedelta(seconds=interval)