UMBRA/backend/app/services/recurrence.py

"""
Recurrence service — generates materialized child CalendarEvent ORM objects
from a parent event's recurrence_rule JSON string.

All datetimes are naive (no timezone) to match TIMESTAMP WITHOUT TIME ZONE in DB.
"""
import json
from datetime import datetime, timedelta
from typing import Optional

from app.models.calendar_event import CalendarEvent


def _nth_weekday_of_month(year: int, month: int, weekday: int, week: int) -> Optional[datetime]:
    """
    Return the date of the Nth (1-4) occurrence of a given weekday (0=Mon…6=Sun)
    in a month, or None if it doesn't exist (e.g. 5th Monday in a short month).
    """
    # Find the first occurrence of the weekday in the month
    first_day = datetime(year, month, 1)
    # days_ahead: how many days until the target weekday
    days_ahead = (weekday - first_day.weekday()) % 7
    first_occurrence = first_day + timedelta(days=days_ahead)
    # Advance by (week - 1) full weeks
    target = first_occurrence + timedelta(weeks=week - 1)
    if target.month != month:
        return None
    return target


def generate_occurrences(
    parent: CalendarEvent,
    horizon_days: int = 365,
) -> list[CalendarEvent]:
    """
    Parse parent.recurrence_rule (JSON string) and materialise child CalendarEvent
    objects up to horizon_days from parent.start_datetime.

    Supported rule types:
      - every_n_days  : {"type": "every_n_days", "interval": N}
      - weekly        : {"type": "weekly", "weekday": 0-6}  (0=Mon, 6=Sun)
      - monthly_nth_weekday: {"type": "monthly_nth_weekday", "week": 1-4, "weekday": 0-6}
      - monthly_date  : {"type": "monthly_date", "day": 1-31}

    Returns unsaved ORM objects — the caller must add them to the session.
    """
    if not parent.recurrence_rule:
        return []

    try:
        rule: dict = json.loads(parent.recurrence_rule)
    except (json.JSONDecodeError, TypeError):
        return []

    rule_type: str = rule.get("type", "")
    parent_start: datetime = parent.start_datetime
    parent_end: datetime = parent.end_datetime
    duration: timedelta = parent_end - parent_start
    horizon: datetime = parent_start + timedelta(days=horizon_days)

    # Fields to copy verbatim from parent to each child
    shared_fields = dict(
        title=parent.title,
        description=parent.description,
        all_day=parent.all_day,
        color=parent.color,
        location_id=parent.location_id,
        is_starred=parent.is_starred,
        calendar_id=parent.calendar_id,
        # Children do not carry the recurrence_rule; parent is the template
        recurrence_rule=None,
        parent_event_id=parent.id,
        is_recurring=True,
    )

    occurrences: list[CalendarEvent] = []

    def _make_child(occ_start: datetime) -> CalendarEvent:
        occ_end = occ_start + duration
        return CalendarEvent(
            **shared_fields,
            start_datetime=occ_start,
            end_datetime=occ_end,
            original_start=occ_start,
        )

    if rule_type == "every_n_days":
        interval: int = int(rule.get("interval", 1))
        if interval < 1:
            interval = 1
        current = parent_start + timedelta(days=interval)
        while current < horizon:
            occurrences.append(_make_child(current))
            current += timedelta(days=interval)

    elif rule_type == "weekly":
        weekday: int = int(rule.get("weekday", parent_start.weekday()))
        # Start from the week after the parent
        days_ahead = (weekday - parent_start.weekday()) % 7
        if days_ahead == 0:
            days_ahead = 7  # skip the parent's own week
        current = parent_start + timedelta(days=days_ahead)
        while current < horizon:
            occurrences.append(_make_child(current))
            current += timedelta(weeks=1)

    elif rule_type == "monthly_nth_weekday":
        week: int = int(rule.get("week", 1))
        weekday = int(rule.get("weekday", parent_start.weekday()))
        # Advance month by month
        year, month = parent_start.year, parent_start.month
        # Move to the next month from the parent
        month += 1
        if month > 12:
            month = 1
            year += 1
        while True:
            target = _nth_weekday_of_month(year, month, weekday, week)
            if target is None:
                # Skip months where Nth weekday doesn't exist
                pass
            else:
                # Preserve the time-of-day from the parent
                occ_start = target.replace(
                    hour=parent_start.hour,
                    minute=parent_start.minute,
                    second=parent_start.second,
                    microsecond=0,
                )
                if occ_start >= horizon:
                    break
                occurrences.append(_make_child(occ_start))
            month += 1
            if month > 12:
                month = 1
                year += 1
            # Safety: if we've passed the horizon year by 2, stop
            if year > horizon.year + 1:
                break

    elif rule_type == "monthly_date":
        day: int = int(rule.get("day", parent_start.day))
        year, month = parent_start.year, parent_start.month
        month += 1
        if month > 12:
            month = 1
            year += 1
        while True:
            # Some months don't have day 29-31
            try:
                occ_start = datetime(
                    year, month, day,
                    parent_start.hour,
                    parent_start.minute,
                    parent_start.second,
                )
            except ValueError:
                # Day out of range for this month — skip
                month += 1
                if month > 12:
                    month = 1
                    year += 1
                if year > horizon.year + 1:
                    break
                continue
            if occ_start >= horizon:
                break
            occurrences.append(_make_child(occ_start))
            month += 1
            if month > 12:
                month = 1
                year += 1
            if year > horizon.year + 1:
                break

    return occurrences