"""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 AppSetting, Artist, ImportSettings, Source

MIN_INTERVAL_SECONDS = 60
MAX_INTERVAL_SECONDS = 86400
MAX_BACKOFF_EXPONENT = 6

# AppSetting key stamped every time the Beat tick fires (see scan.py). The
# tick runs every 60s; the UI flags the scheduler as stalled if the last
# stamp is older than a few minutes.
SCHEDULER_LAST_TICK_KEY = "scheduler_last_tick_at"


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)


async def record_tick(session: AsyncSession) -> None:
    """Stamp the current time on the SCHEDULER_LAST_TICK_KEY AppSetting.

    Called once per Beat tick so the UI can prove the scheduler is alive.
    Commits its own write so the stamp survives even if the rest of the
    tick errors out.
    """
    now_iso = datetime.now(UTC).isoformat()
    row = (await session.execute(
        select(AppSetting).where(AppSetting.key == SCHEDULER_LAST_TICK_KEY)
    )).scalar_one_or_none()
    if row is None:
        session.add(AppSetting(key=SCHEDULER_LAST_TICK_KEY, value=now_iso))
    else:
        row.value = now_iso
    await session.commit()


async def scheduler_status(session: AsyncSession) -> dict:
    """Summarise scheduler health for the dashboard.

    Returns last_tick_at (when Beat last fired), next_due_at (earliest
    upcoming scheduled check across enabled auto-check sources), due_now
    (how many are due right now), and auto_sources (total under schedule).
    """
    last_tick_at = (await session.execute(
        select(AppSetting.value).where(AppSetting.key == SCHEDULER_LAST_TICK_KEY)
    )).scalar_one_or_none()

    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_now = 0
    next_due_at: datetime | None = None
    for s in rows:
        if s.last_checked_at is None:
            due_now += 1
            continue
        nca = compute_next_check_at(s, s.artist, settings)
        if nca is None or nca <= now:
            due_now += 1
        elif next_due_at is None or nca < next_due_at:
            next_due_at = nca

    return {
        "last_tick_at": last_tick_at,
        "next_due_at": next_due_at.isoformat() if next_due_at else None,
        "due_now": due_now,
        "auto_sources": len(rows),
    }