from dataclasses import replace
from typing import Optional

from .dates_add_days import add_days
from .todo_item import Recurrence


def reschedule_recurrence(
    recurrence: Optional[Recurrence], from_due: Optional[str], to_due: Optional[str]
) -> Optional[Recurrence]:
    """The repeat a todo keeps when its due date is edited: a moved date becomes the anchor."""
    # add_days(x, 0) is the date check: it raises dates.add-days's message.
    if from_due is not None:
        add_days(from_due, 0)
    if to_due is not None:
        add_days(to_due, 0)
    if recurrence is None:
        return None
    add_days(recurrence.anchor, 0)
    # No date: todo.item's validate_draft says a repeating todo needs one.
    if to_due is None or to_due == from_due:
        return recurrence
    return replace(recurrence, anchor=to_due)