"""CalDAV → local SQLite sync.

The Settings UI lets users save CalDAV credentials, but the original
sync path was removed when calendar storage was migrated to SQLite.
This module re-wires that gap as a one-way pull (remote → local),
called on calendar open and from a periodic scheduler loop.

Design notes:
- We use the `caldav` lib so PROPFIND discovery + REPORT XML work
  across Radicale / Nextcloud / Apple / Fastmail without us
  reinventing the protocol. It's pure Python.
- The lib is synchronous; we run it in a threadpool via
  `asyncio.to_thread` so the FastAPI event loop stays free.
- Each remote calendar maps to one local `CalendarCal` row with
  `source="caldav"` and `id` = a stable hash of the remote URL so
  re-syncs idempotently target the same row.
- Events upsert by VEVENT UID (kept as the local `uid`). Local
  CalDAV-sourced events not seen in the latest pull are deleted so
  remote deletions propagate.
- Datetimes are converted to UTC and the row is flagged `is_utc=True`
  so the serializer adds the Z suffix and the frontend renders in the
  user's local TZ correctly.
"""

import asyncio
import hashlib
import ipaddress
import logging
import os
import socket
import uuid
from datetime import date, datetime, timedelta, timezone
from urllib.parse import urlparse, urlunparse

logger = logging.getLogger(__name__)

# Pull window: 90 days back, 1 year forward. Keeps the REPORT cheap and
# matches what the calendar UI typically renders. Far-future recurring
# events still come through via RRULE expansion on the frontend.
_LOOKBACK_DAYS = 90
_LOOKAHEAD_DAYS = 365
_BLOCKED_HOSTS = {
    "localhost",
    "localhost.",
    "ip6-localhost",
    "metadata.google.internal",
}


def _private_caldav_allowed() -> bool:
    return os.environ.get("ODYSSEUS_ALLOW_PRIVATE_CALDAV", "0").lower() in {"1", "true", "yes"}


def _validate_caldav_address(addr: ipaddress._BaseAddress) -> None:
    if isinstance(addr, ipaddress.IPv6Address) and addr.ipv4_mapped is not None:
        addr = addr.ipv4_mapped
    if (
        addr.is_loopback
        or addr.is_link_local
        or addr.is_multicast
        or addr.is_unspecified
        or addr.is_reserved
    ):
        raise ValueError("CalDAV URL host is not allowed")
    if addr.is_private and not _private_caldav_allowed():
        raise ValueError("Private CalDAV IPs require ODYSSEUS_ALLOW_PRIVATE_CALDAV=1")


def _validate_caldav_ip(host: str) -> None:
    try:
        ip = ipaddress.ip_address(host.strip("[]"))
    except ValueError:
        return
    _validate_caldav_address(ip)


def _resolve_caldav_host_ips(host: str) -> list[ipaddress._BaseAddress]:
    addrs: list[ipaddress._BaseAddress] = []
    for family, _, _, _, sockaddr in socket.getaddrinfo(host, None):
        if family not in (socket.AF_INET, socket.AF_INET6):
            continue
        try:
            addrs.append(ipaddress.ip_address(sockaddr[0].split("%", 1)[0]))
        except ValueError:
            continue
    return addrs


def _validate_caldav_hostname(host: str) -> None:
    try:
        ipaddress.ip_address(host.strip("[]"))
        return
    except ValueError:
        pass
    try:
        addrs = _resolve_caldav_host_ips(host)
    except OSError:
        raise ValueError("CalDAV URL host does not resolve")
    if not addrs:
        raise ValueError("CalDAV URL host does not resolve")
    for addr in addrs:
        _validate_caldav_address(addr)


def validate_caldav_url(raw_url: str) -> str:
    """Validate and normalize a user-provided CalDAV URL before server-side use."""
    url = (raw_url if isinstance(raw_url, str) else "").strip()
    if not url:
        raise ValueError("CalDAV URL is required")
    parsed = urlparse(url)
    if parsed.scheme not in {"http", "https"}:
        raise ValueError("CalDAV URL must start with http:// or https://")
    if not parsed.hostname:
        raise ValueError("CalDAV URL must include a host")
    if parsed.username or parsed.password:
        raise ValueError("Put CalDAV credentials in the username/password fields, not the URL")
    if parsed.fragment:
        raise ValueError("CalDAV URL fragments are not allowed")
    try:
        parsed.port
    except ValueError:
        raise ValueError("CalDAV URL has an invalid port")
    host = (parsed.hostname or "").lower()
    if host in _BLOCKED_HOSTS or host.endswith(".localhost"):
        raise ValueError("CalDAV URL host is not allowed")
    _validate_caldav_ip(host)
    _validate_caldav_hostname(host)
    return urlunparse(parsed._replace(fragment="")).rstrip("/")


def _stable_cal_id(remote_url: str, owner: str = "", account_id: str = "") -> str:
    """Deterministic local id for a remote CalDAV calendar, scoped to owner
    and account so two users — or one user with two accounts — pointing at
    the same server URL get distinct local rows (avoids PK collision, #2765).
    The owner and account_id default to "" for the legacy/URL-only path so
    existing callers without those arguments keep working."""
    key = f"{owner}\n{account_id}\n{remote_url}"
    h = hashlib.sha256(key.encode("utf-8")).hexdigest()[:24]
    return f"caldav-{h}"


def _to_utc_naive(dt):
    """CalDAV datetimes can be tz-aware (with a TZID) or naive. The DB
    column is naive but we set is_utc=True so the serializer adds Z.
    All-day events stay as date and get widened to datetime here."""
    if isinstance(dt, datetime):
        if dt.tzinfo is not None:
            return dt.astimezone(timezone.utc).replace(tzinfo=None), False
        return dt, False  # naive → treat as local
    # date-only (all-day)
    return datetime(dt.year, dt.month, dt.day), True


def _find_existing_event(db, pending, uid_val, calendar_id):
    """Find the event to update for THIS calendar.

    CalendarEvent.uid is the global primary key, so an unscoped lookup by uid
    returns whatever row holds that VEVENT uid — including another owner's.
    The old code then reassigned that row's calendar_id, moving (stealing)
    another user's event into the syncing calendar whenever the two share a
    uid (shared/subscribed/public calendars, or two accounts on one server).
    Scope the lookup to the calendar being synced; a genuine cross-user uid
    collision then fails the PK insert inside the per-calendar try/except
    instead of hijacking the row. (import_ics was already fixed this way.)
    """
    from core.database import CalendarEvent
    return pending.get(uid_val) or db.query(CalendarEvent).filter(
        CalendarEvent.uid == uid_val,
        CalendarEvent.calendar_id == calendar_id,
    ).first()


def _google_caldav_events_url(url: str) -> str | None:
    """Map a Google CalDAV *principal* URL to its event-collection URL.

    Google serves the principal at ``…/user`` but events live under ``…/events``
    — the ``/user`` resource holds no VEVENTs. The `caldav` library's
    principal→home-set discovery does not reliably enumerate calendars from
    Google's ``/user`` endpoint, so the sync falls into the "treat the URL as a
    single calendar" fallback below. Pointed at ``/user`` that fallback issues
    every calendar-query REPORT against the principal, which returns a clean but
    empty 200 for all date ranges — the calendar shows no events even though
    auth succeeded (issue #2507).

    Both Google CalDAV endpoint forms are handled, since some accounts only
    authenticate against one of them:
      - newer:  ``https://apidata.googleusercontent.com/caldav/v2/<id>/user``
      - legacy: ``https://www.google.com/calendar/dav/<id>/user``

    Returns the events URL for a recognised Google principal URL, else None so
    the caller keeps the original URL unchanged.
    """
    parts = urlparse(url)
    host = (parts.hostname or "").lower()
    path = parts.path.rstrip("/")
    if not path.endswith("/user"):
        return None
    is_google = (
        host.endswith("googleusercontent.com")                       # newer /caldav/v2 form
        or (host in ("www.google.com", "google.com") and "/calendar/dav/" in path)  # legacy form
    )
    if not is_google:
        return None
    new_path = path[: -len("/user")] + "/events"
    return urlunparse(parts._replace(path=new_path))


def _open_url_as_calendar(client, url: str):
    """Open ``url`` as a single calendar collection.

    Used when principal discovery yields no calendars. Google's principal URL
    is not an event collection, so map it to the events URL first
    (see ``_google_caldav_events_url``); other servers' URLs are used as-is.
    """
    target = _google_caldav_events_url(url) or url
    return client.calendar(url=target)


def _build_dav_client(url: str, username: str, password: str):
    """Construct a CalDAV client with automatic redirects disabled.

    ``validate_caldav_url`` resolves and vets the *initial* host, but caldav's
    underlying HTTP session follows 3xx redirects by default. So a URL that
    passes validation can still be redirected — at request time — to
    loopback / link-local / private space, re-opening the SSRF the host check
    closes. Pin the session to zero redirects: any 3xx then raises instead of
    silently following an attacker-chosen ``Location``. This mirrors the
    test-connection path in ``routes/calendar_routes.py``, which already sets
    ``follow_redirects=False``.

    DAVClient exposes no per-request redirect flag, so we set it on the session
    after construction (the session is created in ``__init__``).
    """
    import caldav

    client = caldav.DAVClient(url=url, username=username, password=password)
    # Unconditional: a redirect-disable that only sometimes applies is not a
    # control. The session exists right after __init__ on every real client;
    # test_build_dav_client_disables_redirects asserts it against installed
    # caldav in CI.
    client.session.max_redirects = 0
    return client


def _should_prune_window(seen_uids: set, parse_failed: bool) -> bool:
    """Whether the post-sync prune of vanished CalDAV events is safe to run.

    The prune deletes local ``origin=="caldav"`` rows in the window whose UID the
    server did not just return. Any parse failure (total or partial) makes
    ``seen_uids`` an incomplete view of the server, so pruning against it can
    delete events that still exist upstream but could not be read: a total
    failure wipes the whole window, a partial failure deletes just the
    unreadable ones. Only prune on a clean read. An empty ``seen_uids`` after a
    clean read is a genuinely empty window, which is safe to prune.
    """
    return not parse_failed


def _sync_blocking(owner: str, url: str, username: str, password: str, account_id: str = "") -> dict:
    """The actual sync — synchronous, intended to run in a threadpool.
    Returns counts: {calendars, events, deleted, errors}."""
    # Lazy imports so a missing `caldav` dep doesn't break app startup —
    # the integrations form still works, sync just no-ops with an error.
    from caldav.lib.error import AuthorizationError, NotFoundError
    from core.database import CalendarCal, CalendarEvent, SessionLocal

    result = {"calendars": 0, "events": 0, "deleted": 0, "errors": []}

    client = _build_dav_client(url, username, password)

    # Discovery: try principal → calendars first; if the server doesn't
    # support discovery (or the URL points directly at a calendar), fall
    # back to treating the URL as a single calendar.
    calendars = []
    try:
        principal = client.principal()
        calendars = principal.calendars()
    except (AuthorizationError, NotFoundError) as e:
        result["errors"].append(f"Discovery failed: {e}")
        return result
    except Exception as e:
        logger.info(f"CalDAV principal discovery failed, trying URL as calendar: {e}")
        try:
            calendars = [_open_url_as_calendar(client, url)]
        except Exception as e2:
            result["errors"].append(f"Could not open URL as calendar: {e2}")
            return result

    if not calendars:
        try:
            calendars = [_open_url_as_calendar(client, url)]
        except Exception as e:
            result["errors"].append(f"No calendars and URL fallback failed: {e}")
            return result

    start = datetime.utcnow() - timedelta(days=_LOOKBACK_DAYS)
    end = datetime.utcnow() + timedelta(days=_LOOKAHEAD_DAYS)

    db = SessionLocal()
    try:
        for remote_cal in calendars:
            try:
                remote_url = str(remote_cal.url)
                cal_id = _stable_cal_id(remote_url, owner=owner, account_id=account_id)
                display_name = (remote_cal.name or "").strip() or "CalDAV"

                local_cal = db.query(CalendarCal).filter(
                    CalendarCal.id == cal_id,
                    CalendarCal.owner == owner,
                ).first()
                if not local_cal:
                    local_cal = CalendarCal(
                        id=cal_id,
                        owner=owner,
                        name=display_name,
                        color="#5b8abf",
                        source="caldav",
                        account_id=account_id or None,
                    )
                    db.add(local_cal)
                    db.commit()
                else:
                    # Refresh display name and stamp account_id if missing.
                    changed = False
                    if local_cal.name != display_name:
                        local_cal.name = display_name
                        changed = True
                    if account_id and not local_cal.account_id:
                        local_cal.account_id = account_id
                        changed = True
                    if changed:
                        db.commit()
                result["calendars"] += 1

                # Fetch events in window. `date_search` returns CalendarObject
                # resources; each may contain one VEVENT (most servers) or
                # several (rare).
                from icalendar import Calendar as iCal

                seen_uids = set()
                # Track events added to the session but not yet committed so
                # duplicate UIDs within the same batch are updated, not re-inserted
                # (which would violate the UNIQUE constraint on commit).
                pending: dict = {}
                parse_failed = False
                try:
                    objs = remote_cal.date_search(start=start, end=end, expand=False)
                except Exception as e:
                    result["errors"].append(f"{display_name}: date_search failed ({e})")
                    continue

                for obj in objs:
                    try:
                        ical = iCal.from_ical(obj.data)
                    except Exception as e:
                        result["errors"].append(f"{display_name}: parse failed ({e})")
                        parse_failed = True
                        continue

                    for comp in ical.walk():
                        if comp.name != "VEVENT":
                            continue
                        uid_val = str(comp.get("uid", "")) or str(uuid.uuid4())
                        seen_uids.add(uid_val)

                        dtstart_p = comp.get("dtstart")
                        if not dtstart_p:
                            continue
                        start_dt, all_day = _to_utc_naive(dtstart_p.dt)

                        dtend_p = comp.get("dtend")
                        if dtend_p:
                            end_dt, _ = _to_utc_naive(dtend_p.dt)
                        elif all_day:
                            end_dt = start_dt + timedelta(days=1)
                        else:
                            end_dt = start_dt + timedelta(hours=1)

                        # is_utc reflects whether the source carried a TZ
                        # we converted from. All-day = no TZ semantics.
                        row_is_utc = (
                            not all_day
                            and isinstance(dtstart_p.dt, datetime)
                            and dtstart_p.dt.tzinfo is not None
                        )

                        summary = str(comp.get("summary", ""))
                        description = str(comp.get("description", ""))
                        location = str(comp.get("location", ""))
                        rrule = (
                            comp.get("rrule").to_ical().decode()
                            if comp.get("rrule")
                            else ""
                        )

                        existing = _find_existing_event(db, pending, uid_val, local_cal.id)
                        if existing:
                            existing.calendar_id = local_cal.id
                            existing.summary = summary
                            existing.description = description
                            existing.location = location
                            existing.dtstart = start_dt
                            existing.dtend = end_dt
                            existing.all_day = all_day
                            existing.is_utc = row_is_utc
                            existing.rrule = rrule
                            existing.origin = "caldav"
                        else:
                            new_ev = CalendarEvent(
                                uid=uid_val,
                                calendar_id=local_cal.id,
                                summary=summary,
                                description=description,
                                location=location,
                                dtstart=start_dt,
                                dtend=end_dt,
                                all_day=all_day,
                                is_utc=row_is_utc,
                                rrule=rrule,
                                origin="caldav",
                            )
                            db.add(new_ev)
                            pending[uid_val] = new_ev
                        result["events"] += 1
                db.commit()

                # Prune locally-cached CalDAV events that vanished
                # upstream (only within our sync window — events outside
                # the window aren't in `objs`, so we'd false-delete them).
                # Only rows we previously pulled from the server (origin=="caldav")
                # are prunable; locally-created events (agent / email triage / a
                # UI event whose write-back failed) carry origin NULL and must
                # never be deleted just because the server didn't return them.
                # Skip the prune on any parse failure: seen_uids is then an
                # incomplete view of the server, so pruning against it would
                # delete events that still exist upstream but could not be read
                # (the empty-seen_uids case wipes the whole window; a partial
                # failure deletes just the unreadable rows).
                if _should_prune_window(seen_uids, parse_failed):
                    stale = db.query(CalendarEvent).filter(
                        CalendarEvent.calendar_id == local_cal.id,
                        CalendarEvent.origin == "caldav",
                        CalendarEvent.dtstart >= start,
                        CalendarEvent.dtstart <= end,
                        ~CalendarEvent.uid.in_(seen_uids) if seen_uids else CalendarEvent.uid.isnot(None),
                    ).all()
                    for ev in stale:
                        db.delete(ev)
                    result["deleted"] += len(stale)
                    db.commit()
            except Exception as e:
                logger.exception("CalDAV sync failed for one calendar")
                result["errors"].append(str(e)[:200])
                db.rollback()
    finally:
        db.close()

    return result


def _load_caldav_accounts(owner: str) -> list:
    """Return the list of CalDAV accounts for *owner*, auto-migrating the legacy
    single-account ``caldav`` key to the new ``caldav_accounts`` list on first call.

    The save step is best-effort: if ``_save_for_user`` is unavailable (e.g. in a
    test with a minimal prefs mock) the migrated accounts are still returned; the
    next real call will just re-run the cheap migration again.
    """
    import uuid as _uuid
    from routes.prefs_routes import _load_for_user

    prefs = _load_for_user(owner) or {}
    if "caldav_accounts" in prefs:
        return list(prefs["caldav_accounts"] or [])
    # Migrate legacy single-account config to the list format.
    legacy = prefs.get("caldav", {}) or {}
    if legacy.get("url"):
        accounts = [{
            "id": str(_uuid.uuid4()),
            "label": "CalDAV",
            "url": legacy["url"],
            "username": legacy.get("username", ""),
            "password": legacy.get("password", ""),
        }]
        prefs["caldav_accounts"] = accounts
        prefs.pop("caldav", None)
        try:
            from routes.prefs_routes import _save_for_user
            _save_for_user(owner, prefs)
        except (ImportError, AttributeError):
            pass  # best-effort; next call re-migrates from the still-present legacy key
        return accounts
    return []


async def sync_caldav(owner: str) -> dict:
    """Pull CalDAV state into local DB for `owner` across all configured accounts.
    Returns aggregated counts + per-account errors."""
    from src.secret_storage import decrypt

    accounts = _load_caldav_accounts(owner)
    if not accounts:
        return {
            "calendars": 0, "events": 0, "deleted": 0,
            "errors": ["CalDAV is not configured"],
        }

    totals: dict = {"calendars": 0, "events": 0, "deleted": 0, "errors": []}
    for acc in accounts:
        url = (acc.get("url") or "").strip()
        user = (acc.get("username") or "").strip()
        pw = acc.get("password") or ""
        account_id = acc.get("id") or ""
        label = acc.get("label") or url or account_id
        try:
            pw = decrypt(pw)
        except Exception:
            pass
        if not (url and user and pw):
            totals["errors"].append(f"{label}: missing URL, username, or password")
            continue
        try:
            url = validate_caldav_url(url)
            result = await asyncio.to_thread(_sync_blocking, owner, url, user, pw, account_id)
        except ValueError as e:
            result = {"calendars": 0, "events": 0, "deleted": 0, "errors": [str(e)]}
        except Exception as e:
            logger.exception("CalDAV sync raised for account %s", label)
            result = {"calendars": 0, "events": 0, "deleted": 0, "errors": [str(e)[:200]]}
        totals["calendars"] += result.get("calendars", 0)
        totals["events"] += result.get("events", 0)
        totals["deleted"] += result.get("deleted", 0)
        for err in result.get("errors", []):
            totals["errors"].append(f"{label}: {err}")
    return totals