from __future__ import annotations

from dataclasses import dataclass
from datetime import UTC, datetime, time, timedelta
from zoneinfo import ZoneInfo, ZoneInfoNotFoundError
from urllib.parse import urlparse

import httpx

MAX_RECURRENCE_OCCURRENCES = 120
RECURRENCE_LOOKAHEAD_DAYS = 365


class CalendarFeedError(RuntimeError):
    pass


@dataclass(frozen=True)
class CalendarEvent:
    title: str
    starts_at: datetime
    source: str
    mode: str


@dataclass(frozen=True)
class CalendarFeed:
    url: str
    source_name: str
    events: list[CalendarEvent]


async def fetch_calendar_feed(url: str, timeout_seconds: float) -> CalendarFeed:
    normalized_url = _normalize_calendar_url(url)
    async with httpx.AsyncClient(timeout=timeout_seconds, follow_redirects=True) as client:
        response = await client.get(
            normalized_url,
            headers={"User-Agent": "RobotUCalendar/0.1 (+https://aksal.cloud)"},
        )

    if not response.is_success:
        raise CalendarFeedError(f"{response.status_code} from calendar feed {normalized_url}")

    calendar_name, events = _parse_ics(response.text, normalized_url)
    return CalendarFeed(url=normalized_url, source_name=calendar_name, events=events)


def _normalize_calendar_url(raw_url: str) -> str:
    value = raw_url.strip()
    if not value:
        raise CalendarFeedError("Calendar feed URL is empty.")

    parsed = urlparse(value)
    if parsed.scheme == "webcal":
        return parsed._replace(scheme="https").geturl()
    if parsed.scheme in {"http", "https"}:
        return value
    raise CalendarFeedError(f"Unsupported calendar feed scheme: {parsed.scheme or 'missing'}")


def _parse_ics(raw_text: str, feed_url: str) -> tuple[str, list[CalendarEvent]]:
    lines = _unfold_ics_lines(raw_text)
    calendar_name = _calendar_name(lines, feed_url)
    now = datetime.now(UTC)
    recurrence_until = now + timedelta(days=RECURRENCE_LOOKAHEAD_DAYS)
    events: list[CalendarEvent] = []

    current_event: dict[str, str] | None = None
    for line in lines:
        if line == "BEGIN:VEVENT":
            current_event = {}
            continue
        if line == "END:VEVENT":
            events.extend(
                _events_from_properties(
                    current_event or {},
                    calendar_name,
                    now=now,
                    recurrence_until=recurrence_until,
                ),
            )
            current_event = None
            continue
        if current_event is None or ":" not in line:
            continue

        raw_key, value = line.split(":", 1)
        current_event[raw_key] = value.strip()

    events.sort(key=lambda event: event.starts_at)
    return calendar_name, events


def _unfold_ics_lines(raw_text: str) -> list[str]:
    lines = raw_text.replace("\r\n", "\n").replace("\r", "\n").split("\n")
    unfolded: list[str] = []

    for line in lines:
        if not line:
            continue
        if unfolded and line[:1] in {" ", "\t"}:
            unfolded[-1] = f"{unfolded[-1]}{line[1:]}"
        else:
            unfolded.append(line)

    return unfolded


def _calendar_name(lines: list[str], feed_url: str) -> str:
    for line in lines:
        if line.startswith("X-WR-CALNAME:"):
            return _decode_ics_text(line.split(":", 1)[1].strip()) or _calendar_host(feed_url)
    return _calendar_host(feed_url)


def _calendar_host(feed_url: str) -> str:
    parsed = urlparse(feed_url)
    return parsed.hostname or "Calendar"


def _events_from_properties(
    properties: dict[str, str],
    calendar_name: str,
    *,
    now: datetime,
    recurrence_until: datetime,
) -> list[CalendarEvent]:
    title = _decode_ics_text(properties.get("SUMMARY", "").strip())
    start_key = next((key for key in properties if key.startswith("DTSTART")), None)
    if not title or not start_key:
        return []

    starts_at = _parse_ics_datetime(start_key, properties[start_key])
    if starts_at is None:
        return []

    location = _decode_ics_text(properties.get("LOCATION", "").strip())
    event_template = {
        "title": title,
        "source": calendar_name,
        "mode": location or "Calendar",
    }
    recurrence_rule = properties.get("RRULE")
    if not recurrence_rule:
        if starts_at >= now:
            return [CalendarEvent(starts_at=starts_at, **event_template)]
        return []

    return [
        CalendarEvent(starts_at=occurrence, **event_template)
        for occurrence in _recurrence_occurrences(
            starts_at,
            recurrence_rule,
            now=now,
            recurrence_until=recurrence_until,
            start_key=start_key,
        )
    ]


def _recurrence_occurrences(
    starts_at: datetime,
    raw_rule: str,
    *,
    now: datetime,
    recurrence_until: datetime,
    start_key: str,
) -> list[datetime]:
    rule = _parse_recurrence_rule(raw_rule)
    frequency = rule.get("FREQ", "").upper()
    interval = _positive_int(rule.get("INTERVAL"), default=1)
    count = _positive_int(rule.get("COUNT"), default=MAX_RECURRENCE_OCCURRENCES)
    until = _parse_recurrence_until(rule.get("UNTIL"), start_key)
    effective_until = min(
        [candidate for candidate in (until, recurrence_until) if candidate is not None],
        default=recurrence_until,
    )
    occurrences: list[datetime] = []
    occurrence = starts_at

    for index in range(count):
        if occurrence > effective_until:
            break
        if occurrence >= now:
            occurrences.append(occurrence)
        if len(occurrences) >= MAX_RECURRENCE_OCCURRENCES:
            break

        next_occurrence = _next_recurrence_occurrence(occurrence, frequency, interval)
        if next_occurrence is None or next_occurrence <= occurrence:
            break
        occurrence = next_occurrence

    return occurrences


def _parse_recurrence_rule(raw_rule: str) -> dict[str, str]:
    rule: dict[str, str] = {}
    for segment in raw_rule.split(";"):
        if "=" not in segment:
            continue
        key, value = segment.split("=", 1)
        rule[key.strip().upper()] = value.strip()
    return rule


def _positive_int(value: str | None, *, default: int) -> int:
    if value is None:
        return default
    try:
        parsed = int(value)
    except ValueError:
        return default
    return parsed if parsed > 0 else default


def _parse_recurrence_until(value: str | None, start_key: str) -> datetime | None:
    if not value:
        return None
    return _parse_ics_datetime(start_key, value)


def _next_recurrence_occurrence(
    occurrence: datetime,
    frequency: str,
    interval: int,
) -> datetime | None:
    if frequency == "DAILY":
        return occurrence + timedelta(days=interval)
    if frequency == "WEEKLY":
        return occurrence + timedelta(weeks=interval)
    if frequency == "MONTHLY":
        return _add_months(occurrence, interval)
    if frequency == "YEARLY":
        return _add_months(occurrence, interval * 12)
    return None


def _add_months(value: datetime, months: int) -> datetime:
    month_index = value.month - 1 + months
    year = value.year + month_index // 12
    month = month_index % 12 + 1
    day = min(value.day, _days_in_month(year, month))
    return value.replace(year=year, month=month, day=day)


def _days_in_month(year: int, month: int) -> int:
    if month == 12:
        next_month = datetime(year + 1, 1, 1)
    else:
        next_month = datetime(year, month + 1, 1)
    this_month = datetime(year, month, 1)
    return (next_month - this_month).days


def _parse_ics_datetime(key: str, value: str) -> datetime | None:
    try:
        if "VALUE=DATE" in key:
            parsed_date = datetime.strptime(value, "%Y%m%d").date()
            return datetime.combine(parsed_date, time.min, tzinfo=UTC)
        if value.endswith("Z"):
            return datetime.strptime(value, "%Y%m%dT%H%M%SZ").replace(tzinfo=UTC)
        if "T" in value:
            return _with_ics_timezone(datetime.strptime(value, "%Y%m%dT%H%M%S"), key)
        parsed_date = datetime.strptime(value, "%Y%m%d").date()
        return datetime.combine(parsed_date, time.min, tzinfo=UTC)
    except ValueError:
        return None


def _with_ics_timezone(value: datetime, key: str) -> datetime:
    timezone_name = _ics_timezone_name(key)
    if not timezone_name:
        return value.replace(tzinfo=UTC)

    try:
        return value.replace(tzinfo=ZoneInfo(timezone_name))
    except ZoneInfoNotFoundError:
        return value.replace(tzinfo=UTC)


def _ics_timezone_name(key: str) -> str | None:
    for segment in key.split(";")[1:]:
        if segment.startswith("TZID="):
            return segment.split("=", 1)[1].strip() or None
    return None


def _decode_ics_text(value: str) -> str:
    return (
        value.replace("\\n", "\n")
        .replace("\\N", "\n")
        .replace("\\,", ",")
        .replace("\\;", ";")
        .replace("\\\\", "\\")
    )