"""Recurrence engine for calendar entries — daily/weekly/monthly/yearly + custom + exceptions."""

from __future__ import annotations

from datetime import date, datetime, timedelta
from typing import Any

WEEKDAYS = ["MO", "TU", "WE", "TH", "FR", "SA", "SU"]


def _parse_date(val: Any) -> date | None:
    """Parse a value into a date object. Accepts date, datetime, or ISO string."""
    if val is None:
        return None
    if isinstance(val, date) and not isinstance(val, datetime):
        return val
    if isinstance(val, datetime):
        return val.date()
    if isinstance(val, str):
        try:
            return datetime.fromisoformat(val).date()
        except ValueError:
            try:
                return date.fromisoformat(val)
            except ValueError:
                return None
    return None


def _parse_datetime(val: Any) -> datetime | None:
    """Parse a value into a datetime object."""
    if val is None:
        return None
    if isinstance(val, datetime):
        return val
    if isinstance(val, date) and not isinstance(val, datetime):
        return datetime(val.year, val.month, val.day)
    if isinstance(val, str):
        try:
            return datetime.fromisoformat(val)
        except ValueError:
            return None
    return None


def generate_occurrences(
    recurrence: dict[str, Any],
    base_start: datetime | None,
    range_start: date,
    range_end: date,
) -> list[datetime]:
    """Generate occurrence start datetimes within [range_start, range_end].

    Args:
        recurrence: {pattern, custom_rule, end_date, exceptions}
        base_start: the original start_at datetime of the entry
        range_start: query range start date
        range_end: query range end date

    Returns:
        List of datetime occurrences within the range, excluding exception dates.
    """
    if not recurrence or not base_start:
        return [base_start] if base_start and range_start <= base_start.date() <= range_end else []

    pattern = recurrence.get("pattern", "daily")
    custom_rule = recurrence.get("custom_rule")
    end_date = _parse_date(recurrence.get("end_date"))
    exceptions_raw = recurrence.get("exceptions", [])
    exception_dates = {_parse_date(d) for d in exceptions_raw if _parse_date(d) is not None}

    # Max 2 years forward
    max_end = base_start.date() + timedelta(days=730)
    effective_end = range_end
    if end_date and end_date < effective_end:
        effective_end = end_date
    if effective_end > max_end:
        effective_end = max_end
    if effective_end < range_start:
        return []

    occurrences: list[datetime] = []

    if pattern == "daily":
        interval = 1
        if custom_rule and "INTERVAL=" in custom_rule:
            interval = _extract_int(custom_rule, "INTERVAL")
        current = base_start
        while current.date() <= effective_end:
            if range_start <= current.date() <= range_end and current.date() not in exception_dates:
                occurrences.append(current)
            current = current + timedelta(days=interval)

    elif pattern == "weekly":
        interval = 1
        bydays: list[str] | None = None
        if custom_rule:
            interval = _extract_int(custom_rule, "INTERVAL")
            bydays = _extract_bydays(custom_rule)
        if bydays is None:
            bydays = [WEEKDAYS[base_start.weekday()]]
        weekday_nums = {d: i for i, d in enumerate(WEEKDAYS)}
        current_week_start = base_start - timedelta(days=base_start.weekday())
        while current_week_start.date() <= effective_end:
            for wd in bydays:
                wd_num = weekday_nums.get(wd)
                if wd_num is None:
                    continue
                occ_date = current_week_start + timedelta(days=wd_num)
                if occ_date.date() > effective_end:
                    continue
                if occ_date.date() < base_start.date():
                    continue
                if (
                    range_start <= occ_date.date() <= range_end
                    and occ_date.date() not in exception_dates
                ):
                    occurrences.append(occ_date)
            current_week_start = current_week_start + timedelta(weeks=interval)

    elif pattern == "monthly":
        interval = 1
        if custom_rule and "INTERVAL=" in custom_rule:
            interval = _extract_int(custom_rule, "INTERVAL")
        bysetpos: list[int] | None = None
        if custom_rule and "BYSETPOS=" in custom_rule:
            bysetpos = _extract_setpos(custom_rule)
        current = base_start
        month_offset = 0
        while True:
            year = base_start.year + (base_start.month - 1 + month_offset) // 12
            month = (base_start.month - 1 + month_offset) % 12 + 1
            if date(year, month, 1) > effective_end:
                break
            if bysetpos:
                for pos in bysetpos:
                    occ_date = _nth_weekday_of_month(year, month, pos, base_start.weekday())
                    if occ_date is None:
                        continue
                    occ_dt = datetime(
                        occ_date.year,
                        occ_date.month,
                        occ_date.day,
                        base_start.hour,
                        base_start.minute,
                    )
                    if occ_dt.date() < base_start.date():
                        continue
                    if (
                        range_start <= occ_dt.date() <= range_end
                        and occ_dt.date() not in exception_dates
                    ):
                        occurrences.append(occ_dt)
            else:
                day = min(base_start.day, _days_in_month(year, month))
                occ_date = date(year, month, day)
                if (
                    occ_date >= base_start.date()
                    and range_start <= occ_date <= range_end
                    and occ_date not in exception_dates
                ):
                    occurrences.append(
                        datetime(
                            occ_date.year,
                            occ_date.month,
                            occ_date.day,
                            base_start.hour,
                            base_start.minute,
                        )
                    )
            month_offset += interval

    elif pattern == "yearly":
        interval = 1
        if custom_rule and "INTERVAL=" in custom_rule:
            interval = _extract_int(custom_rule, "INTERVAL")
        current = base_start
        year_offset = 0
        while True:
            year = base_start.year + year_offset
            if date(year, base_start.month, 1) > effective_end:
                break
            day = min(base_start.day, _days_in_month(year, base_start.month))
            occ_date = date(year, base_start.month, day)
            if (
                occ_date >= base_start.date()
                and range_start <= occ_date <= range_end
                and occ_date not in exception_dates
            ):
                occurrences.append(
                    datetime(
                        occ_date.year,
                        occ_date.month,
                        occ_date.day,
                        base_start.hour,
                        base_start.minute,
                    )
                )
            year_offset += interval

    elif pattern == "custom":
        if custom_rule:
            interval = _extract_int(custom_rule, "INTERVAL")
            bydays = _extract_bydays(custom_rule)
            if bydays:
                weekday_nums = {d: i for i, d in enumerate(WEEKDAYS)}
                current_week_start = base_start - timedelta(days=base_start.weekday())
                while current_week_start.date() <= effective_end:
                    for wd in bydays:
                        wd_num = weekday_nums.get(wd)
                        if wd_num is None:
                            continue
                        occ_date = current_week_start + timedelta(days=wd_num)
                        if occ_date.date() > effective_end or occ_date.date() < base_start.date():
                            continue
                        if (
                            range_start <= occ_date.date() <= range_end
                            and occ_date.date() not in exception_dates
                        ):
                            occurrences.append(occ_date)
                    current_week_start = current_week_start + timedelta(weeks=interval)

    occurrences.sort()
    return occurrences


def _extract_int(rule: str, key: str) -> int:
    """Extract an integer value from a rule string like 'INTERVAL=2'."""
    parts = rule.split(";")
    for part in parts:
        if part.startswith(f"{key}="):
            try:
                return int(part.split("=")[1])
            except (ValueError, IndexError):
                return 1
    return 1


def _extract_bydays(rule: str) -> list[str] | None:
    """Extract BYDAY values from a rule string like 'BYDAY=MO,WE,FR'."""
    parts = rule.split(";")
    for part in parts:
        if part.startswith("BYDAY="):
            days = part.split("=")[1].split(",")
            return [d.strip().upper() for d in days if d.strip()]
    return None


def _extract_setpos(rule: str) -> list[int] | None:
    """Extract BYSETPOS values from a rule string like 'BYSETPOS=2'."""
    parts = rule.split(";")
    for part in parts:
        if part.startswith("BYSETPOS="):
            try:
                return [int(x) for x in part.split("=")[1].split(",")]
            except (ValueError, IndexError):
                return None
    return None


def _days_in_month(year: int, month: int) -> int:
    """Return number of days in a given month."""
    if month == 12:
        return 31
    next_month = date(year, month + 1, 1)
    return (next_month - timedelta(days=1)).day


def _nth_weekday_of_month(year: int, month: int, n: int, weekday: int) -> date | None:
    """Get the nth occurrence of a weekday in a month.

    n=1 → first, n=2 → second, n=-1 → last, n=-2 → second-to-last.
    """
    if n > 0:
        first = date(year, month, 1)
        first_offset = (weekday - first.weekday()) % 7
        day = 1 + first_offset + (n - 1) * 7
        try:
            return date(year, month, day)
        except ValueError:
            return None
    else:
        last_day = _days_in_month(year, month)
        last = date(year, month, last_day)
        last_offset = (last.weekday() - weekday) % 7
        day = last_day - last_offset + (n + 1) * 7
        try:
            return date(year, month, day)
        except ValueError:
            return None