from datetime import date


def months_before(d, n):
    """Return a date on the 1st of the month n months before d."""
    m = (d.month - 1 - n) % 12 + 1
    y = d.year + (d.month - 1 - n) // 12
    return date(y, m, 1)


def pruning_status(plant, today=None):
    """
    Returns one of: 'overdue', 'due_this_month', 'upcoming', 'no_schedule'.

    A scheduled month is 'overdue' only if its most recent occurrence is within
    the past 6 months and was not logged. Beyond 6 months, the next occurrence
    is sooner than the missed one, so it is 'upcoming'.
    """
    if today is None:
        today = date.today()

    months = plant.pruning_months or []
    if not months:
        return 'no_schedule'

    logs = plant.pruning_logs.all()
    logged = {(log.pruned_on.year, log.pruned_on.month) for log in logs}

    if today.month in months:
        if (today.year, today.month) not in logged:
            return 'due_this_month'
        return 'upcoming'

    for i in range(1, 13):
        past = months_before(today, i)
        if past.month in months:
            if i >= 6:
                return 'upcoming'
            if (past.year, past.month) not in logged:
                return 'overdue'
            return 'upcoming'

    return 'upcoming'