import os
import caldav
import json
import traceback
from datetime import datetime
from zoneinfo import ZoneInfo
from app.db.session import SessionLocal
from app.models import Event, Task

# Berlin timezone for display
BERLIN_TZ = ZoneInfo("Europe/Berlin")
UTC_TZ = ZoneInfo("UTC")


# ---------------------------------------------------------------------------
# Helpers
# ---------------------------------------------------------------------------

def _get_caldav_principal():
    url = os.getenv("NEXTCLOUD_URL")
    username = os.getenv("NEXTCLOUD_USERNAME")
    password = os.getenv("NEXTCLOUD_PASSWORD")

    if not all([url, username, password]):
        raise ValueError("Nextcloud credentials missing. Set NEXTCLOUD_URL, NEXTCLOUD_USERNAME, NEXTCLOUD_PASSWORD.")

    caldav_url = url if url.endswith("/remote.php/dav/") else f"{url.rstrip('/')}/remote.php/dav/"
    client = caldav.DAVClient(url=caldav_url, username=username, password=password)
    return client.principal()


def _to_berlin(dt):
    """Convert a datetime to Berlin timezone. Handles naive and aware datetimes."""
    if dt is None:
        return None
    if not isinstance(dt, datetime):
        # date-only: treat as midnight Berlin time
        return datetime.combine(dt, datetime.min.time()).replace(tzinfo=BERLIN_TZ)
    if dt.tzinfo is None:
        # Naive datetime — assume UTC
        dt = dt.replace(tzinfo=UTC_TZ)
    return dt.astimezone(BERLIN_TZ)


def _is_own_calendar(cal_url_str: str) -> bool:
    """Check if a calendar belongs to the authenticated user (not shared by others)."""
    return "_shared_by_" not in cal_url_str


def _get_nextcloud_base_url() -> str:
    """Get the base Nextcloud URL for building links."""
    url = os.getenv("NEXTCLOUD_URL", "")
    return url.rstrip("/")


# ---------------------------------------------------------------------------
# Calendar events
# ---------------------------------------------------------------------------

def sync_calendar_events():
    try:
        principal = _get_caldav_principal()
        calendars = principal.calendars()
    except Exception as e:
        print(f"Failed to connect to Nextcloud CalDAV: {e}")
        return False

    db = SessionLocal()

    for calendar in calendars:
        cal_url_str = str(calendar.url)
        if not _is_own_calendar(cal_url_str):
            print(f"Skipping shared calendar: {calendar.get_display_name()}")
            continue

        try:
            events = calendar.events()
            for event in events:
                ical = event.icalendar_component
                if ical.name == "VEVENT":
                    title = str(ical.get("SUMMARY", "Untitled Event"))
                    dtstart = ical.get("DTSTART")
                    dtend = ical.get("DTEND")

                    if not dtstart or not dtend:
                        continue

                    start_time = _to_berlin(dtstart.dt)
                    end_time = _to_berlin(dtend.dt)

                    existing = db.query(Event).filter(Event.title == title, Event.source == "nextcloud").first()
                    if not existing:
                        db.add(Event(title=title, start_time=start_time, end_time=end_time, source="nextcloud"))
        except Exception as e:
            print(f"Error reading calendar {calendar}: {e}")

    db.commit()
    db.close()
    return True


# ---------------------------------------------------------------------------
# ToDo sync (two-way, only own tasks)
# ---------------------------------------------------------------------------

def _parse_vtodo_datetime(val):
    """Parse a VTODO datetime and convert to Berlin timezone."""
    if val is None:
        return None
    return _to_berlin(val.dt)


def _has_local_changes(task, remote_title: str, remote_description: str, remote_status: str) -> bool:
    """Check if the local task differs from what was last synced (i.e., user edited locally)."""
    # If never synced, treat as no local changes (first sync)
    if task.last_synced_at is None:
        return False
    # If the task was modified after last sync, local has changes
    # We compare local fields against remote to detect divergence
    local_title = task.title or ""
    local_desc = task.description or ""
    local_status = task.status or "open"
    # If local differs from remote, and we have a last_synced_at, local was changed
    if local_title != remote_title:
        return True
    if local_desc != (remote_description or ""):
        return True
    if local_status != remote_status:
        return True
    return False


def _has_remote_changes(task, remote_title: str, remote_description: str, remote_status: str) -> bool:
    """Check if remote data differs from what's currently stored locally.
    
    This detects whether the remote side changed since our last sync.
    If the remote values differ from what we have locally, remote changed.
    """
    local_title = task.title or ""
    local_desc = task.description or ""
    local_status = task.status or "open"
    if local_title != remote_title:
        return True
    if local_desc != (remote_description or ""):
        return True
    if local_status != remote_status:
        return True
    return False


def sync_todo_tasks():
    """Import VTODOs from own Nextcloud todo lists into local Task table.
    
    Only imports from calendars owned by the authenticated user (skips shared).
    Detects conflicts when both local and remote changed since last sync.
    """
    try:
        principal = _get_caldav_principal()
        calendars = principal.calendars()
    except Exception as e:
        print(f"Failed to connect to Nextcloud CalDAV: {e}")
        return False

    db = SessionLocal()
    nc_username = os.getenv("NEXTCLOUD_USERNAME", "")
    base_url = _get_nextcloud_base_url()

    for calendar in calendars:
        cal_url_str = str(calendar.url)
        cal_name = calendar.get_display_name() or "Tasks"

        # Skip shared calendars (these belong to coworkers)
        if not _is_own_calendar(cal_url_str):
            print(f"Skipping shared todo list: {cal_name}")
            continue

        try:
            todos = calendar.todos(include_completed=True)
        except Exception:
            continue  # calendar may not support VTODOs

        for todo in todos:
            try:
                ical = todo.icalendar_component
                if ical.name != "VTODO":
                    continue

                uid = str(ical.get("UID", ""))
                if not uid:
                    continue

                title = str(ical.get("SUMMARY", "Untitled Task"))
                description = str(ical.get("DESCRIPTION", "") or "")
                due_date = _parse_vtodo_datetime(ical.get("DUE"))
                vtodo_status = str(ical.get("STATUS", "NEEDS-ACTION")).upper()
                status = "completed" if vtodo_status == "COMPLETED" else "open"

                # Build origin label with calendar name
                origin_label = f"nextcloud_todo:{cal_name}"

                # Build link to Nextcloud tasks app
                source_url = f"{base_url}/apps/tasks/#/calendars/{cal_name}" if base_url else None

                existing = db.query(Task).filter(Task.source_id == uid).first()
                if existing:
                    # Don't touch dismissed or completed tasks (user already handled them)
                    if existing.status in ("dismissed", "completed"):
                        continue

                    # --- Conflict detection ---
                    # Check if local was modified since last sync
                    local_changed = _has_local_changes(existing, title, description, status)
                    remote_changed = _has_remote_changes(existing, title, description, status)

                    if local_changed and remote_changed:
                        # Both sides changed → conflict
                        existing.has_conflict = True
                        existing.conflict_data = json.dumps({
                            "title": title,
                            "status": status,
                            "description": description,
                        })
                        continue

                    if not remote_changed:
                        # Only local changed (or nothing changed) → push local to remote if needed
                        if local_changed and existing.source_id:
                            push_updates = {}
                            if existing.title != title:
                                push_updates["title"] = existing.title
                            if existing.description != (description or ""):
                                push_updates["description"] = existing.description
                            if existing.status != status:
                                push_updates["status"] = existing.status
                            if push_updates:
                                update_remote_todo(existing.source_id, push_updates)
                        existing.last_synced_at = datetime.now(tz=UTC_TZ)
                        continue

                    # Only remote changed → update local normally
                    # Don't revert in_progress or review tasks back to open
                    if existing.status in ("in_progress", "review") and status == "open":
                        existing.title = title
                        existing.description = description
                        existing.due_date = due_date
                        existing.origin = origin_label
                        existing.source_url = source_url
                        existing.last_synced_at = datetime.now(tz=UTC_TZ)
                        continue

                    existing.title = title
                    existing.description = description
                    existing.due_date = due_date
                    existing.status = status
                    existing.origin = origin_label
                    existing.source_url = source_url
                    existing.last_synced_at = datetime.now(tz=UTC_TZ)
                else:
                    max_pos = db.query(Task).count()
                    db.add(Task(
                        title=title,
                        description=description,
                        priority=4,
                        position=max_pos,
                        status=status,
                        origin=origin_label,
                        source_id=uid,
                        source_url=source_url,
                        due_date=due_date,
                        last_synced_at=datetime.now(tz=UTC_TZ),
                    ))
            except Exception as e:
                print(f"Error processing VTODO: {e}")

    db.commit()
    db.close()
    return True


def update_remote_todo(source_id: str, updates: dict):
    """Push local task changes back to the Nextcloud server.

    updates: dict with any of: title, description, status
    """
    try:
        principal = _get_caldav_principal()
        calendars = principal.calendars()
    except Exception as e:
        print(f"Failed to connect to Nextcloud CalDAV for push: {e}")
        return False

    for calendar in calendars:
        cal_url_str = str(calendar.url)
        # Only push to own calendars
        if not _is_own_calendar(cal_url_str):
            continue

        try:
            todos = calendar.todos(include_completed=True)
        except Exception:
            continue

        for todo in todos:
            try:
                ical = todo.icalendar_component
                if ical.name != "VTODO":
                    continue
                if str(ical.get("UID", "")) != source_id:
                    continue

                # Apply updates
                if "title" in updates:
                    ical["SUMMARY"] = updates["title"]
                if "description" in updates:
                    ical["DESCRIPTION"] = updates["description"]
                if "status" in updates:
                    ical["STATUS"] = "COMPLETED" if updates["status"] == "completed" else "NEEDS-ACTION"

                todo.save()
                return True
            except Exception as e:
                print(f"Error updating remote VTODO {source_id}: {e}")

    print(f"Remote VTODO with UID {source_id} not found.")
    return False