certain/backend/app/services/budget.py

"""Budget-Engine: Perioden-Grenzen, Standard-Vorlage vs. Monats-Override, Soll/Ist.

Der Budget-Monat hat einen einstellbaren Starttag D. Ein Zeitraum ist nach seinem
End-/Hauptmonat benannt (YYYY-MM):
- D <= 1: der Kalendermonat.
- D >= 2: [Tag D des Vormonats … Tag D-1 des Hauptmonats].
Planned-Beträge liegen in der Hauptwährung des Nutzers; Ist-Werte werden über den
eingefrorenen EUR-Wert je Buchung in die Hauptwährung umgerechnet.
"""

import calendar
from collections import defaultdict
from datetime import date, timedelta

from dateutil.relativedelta import relativedelta
from sqlalchemy.orm import Session

from ..models import (
    Account,
    BudgetPeriod,
    BudgetPeriodLine,
    BudgetTemplateLine,
    BudgetTreatment,
    Category,
    CategoryKind,
    SAVINGS_ACCOUNT_TYPES,
    Transaction,
    TxnType,
    User,
)
from . import currency


# --- Perioden --------------------------------------------------------------


def _clamp_day(year: int, month: int, day: int) -> date:
    last = calendar.monthrange(year, month)[1]
    return date(year, month, min(day, last))


def parse_label(label: str) -> tuple[int, int]:
    y, m = label.split("-")
    return int(y), int(m)


def period_bounds(label: str, start_day: int) -> tuple[date, date]:
    """(start, end) inklusive für einen Budget-Monat."""
    y, m = parse_label(label)
    if start_day <= 1:
        return date(y, m, 1), date(y, m, calendar.monthrange(y, m)[1])
    prev = date(y, m, 1) - relativedelta(months=1)
    start = _clamp_day(prev.year, prev.month, start_day)
    end = _clamp_day(y, m, start_day) - timedelta(days=1)
    return start, end


def period_of(d: date, start_day: int) -> str:
    """Label des Budget-Monats, in den ein Datum fällt."""
    if start_day <= 1:
        return f"{d.year:04d}-{d.month:02d}"
    if d.day >= start_day:
        nxt = date(d.year, d.month, 1) + relativedelta(months=1)
        return f"{nxt.year:04d}-{nxt.month:02d}"
    return f"{d.year:04d}-{d.month:02d}"


def shift_label(label: str, months: int) -> str:
    y, m = parse_label(label)
    d = date(y, m, 1) + relativedelta(months=months)
    return f"{d.year:04d}-{d.month:02d}"


def current_label(start_day: int) -> str:
    return period_of(date.today(), start_day)


# --- Vorlage vs. Monats-Override ------------------------------------------


def _period(db: Session, user: User, label: str) -> BudgetPeriod | None:
    return (
        db.query(BudgetPeriod)
        .filter(BudgetPeriod.owner_id == user.id, BudgetPeriod.label == label)
        .first()
    )


def effective_planned(db: Session, user: User, label: str) -> dict[int, int]:
    """Wirksame Soll-Beträge je Kategorie: Monats-Override falls vorhanden, sonst Vorlage."""
    period = _period(db, user, label)
    if period is not None:
        return {ln.category_id: ln.planned_cents for ln in period.lines}
    return {
        tl.category_id: tl.planned_cents
        for tl in db.query(BudgetTemplateLine).filter(BudgetTemplateLine.owner_id == user.id)
    }


def is_customized(db: Session, user: User, label: str) -> bool:
    return _period(db, user, label) is not None


def ensure_customized(db: Session, user: User, label: str) -> BudgetPeriod:
    """Legt (falls nötig) die Monats-Periode an und übernimmt einmalig die Vorlage."""
    period = _period(db, user, label)
    if period is None:
        period = BudgetPeriod(owner_id=user.id, label=label, customized=True)
        db.add(period)
        db.flush()
        for tl in db.query(BudgetTemplateLine).filter(BudgetTemplateLine.owner_id == user.id):
            period.lines.append(
                BudgetPeriodLine(category_id=tl.category_id, planned_cents=tl.planned_cents)
            )
        db.flush()
    return period


def set_month_category(db: Session, user: User, label: str, category_id: int, planned_cents: int) -> None:
    period = ensure_customized(db, user, label)
    line = next((ln for ln in period.lines if ln.category_id == category_id), None)
    if line is None:
        period.lines.append(BudgetPeriodLine(category_id=category_id, planned_cents=planned_cents))
    else:
        line.planned_cents = planned_cents
    db.commit()


def reset_month(db: Session, user: User, label: str) -> None:
    period = _period(db, user, label)
    if period is not None:
        db.delete(period)
        db.commit()


def copy_month_to_template(db: Session, user: User, label: str) -> None:
    """Setzt die Standard-Vorlage auf die wirksamen Soll-Werte dieses Monats
    („diesen Monat als Standard festlegen"). Einnahmen bleiben unberührt."""
    planned = effective_planned(db, user, label)  # {category_id: cents}
    db.query(BudgetTemplateLine).filter(BudgetTemplateLine.owner_id == user.id).delete()
    for category_id, cents in planned.items():
        if cents > 0:
            db.add(BudgetTemplateLine(owner_id=user.id, category_id=category_id, planned_cents=cents))
    db.commit()


def copy_template_to_month(db: Session, user: User, label: str) -> BudgetPeriod:
    """Überschreibt den Monat mit der aktuellen Vorlage (auch wenn er schon
    angepasst war): vorhandene Monatszeilen weg, Vorlage frisch übernehmen."""
    period = _period(db, user, label)
    if period is None:
        period = BudgetPeriod(owner_id=user.id, label=label, customized=True)
        db.add(period)
        db.flush()
    else:
        for ln in list(period.lines):
            db.delete(ln)
        db.flush()
        period.customized = True
    for tl in db.query(BudgetTemplateLine).filter(BudgetTemplateLine.owner_id == user.id):
        period.lines.append(BudgetPeriodLine(category_id=tl.category_id, planned_cents=tl.planned_cents))
    db.commit()
    return period


# --- Ist-Werte -------------------------------------------------------------


def _to_base(db: Session, eur_cents: int, base: str, on: date) -> int:
    if base == "EUR":
        return eur_cents
    conv = currency.from_eur_cents(db, eur_cents, base, on)
    return conv if conv is not None else eur_cents


def _neutrale_kategorien(db: Session, user: User) -> set[int]:
    """Kategorien, die nicht in Auswertungen zählen (z. B. „Kontoübertrag").

    Eine Umbuchung zwischen eigenen Konten ist keine Ausgabe – das Geld liegt nur
    woanders. Auf den Kontostand wirkt sie weiterhin, nur Budget und Statistik
    lassen sie außen vor."""
    return {
        cid
        for (cid,) in db.query(Category.id).filter(
            Category.owner_id == user.id, Category.excluded_from_stats.is_(True)
        )
    }


def actual_expense_by_category(db: Session, user: User, start: date, end: date, base: str) -> dict[int, int]:
    """Ist-Ausgaben je Kategorie in Hauptwährung (inkl. budgetierter Sparen-Transfers)."""
    result: dict[int, int] = defaultdict(int)
    neutral = _neutrale_kategorien(db, user)
    expenses = (
        db.query(Transaction)
        .filter(
            Transaction.owner_id == user.id,
            Transaction.type == TxnType.expense,
            Transaction.date >= start,
            Transaction.date <= end,
            Transaction.category_id.isnot(None),
        )
        .all()
    )
    for t in expenses:
        if t.category_id in neutral:
            continue
        result[t.category_id] += _to_base(db, t.amount_eur_cents, base, t.date)

    # Transfers auf Sparen/Depot zählen (sofern nicht auf "neutral" gesetzt) als
    # budgetierte Ausgabe gegen budget_category_id.
    transfers = (
        db.query(Transaction)
        .filter(
            Transaction.owner_id == user.id,
            Transaction.type == TxnType.transfer,
            Transaction.date >= start,
            Transaction.date <= end,
            Transaction.budget_category_id.isnot(None),
        )
        .all()
    )
    for t in transfers:
        if t.budget_treatment == BudgetTreatment.neutral:
            continue
        to_acc = db.get(Account, t.to_account_id) if t.to_account_id else None
        if to_acc is not None and to_acc.type in SAVINGS_ACCOUNT_TYPES:
            result[t.budget_category_id] += _to_base(db, t.amount_eur_cents, base, t.date)
    return result


def income_by_category(db: Session, user: User, start: date, end: date, base: str) -> dict[int | None, int]:
    result: dict[int | None, int] = defaultdict(int)
    neutral = _neutrale_kategorien(db, user)
    incomes = (
        db.query(Transaction)
        .filter(
            Transaction.owner_id == user.id,
            Transaction.type == TxnType.income,
            Transaction.date >= start,
            Transaction.date <= end,
        )
        .all()
    )
    for t in incomes:
        if t.category_id in neutral:
            continue
        result[t.category_id] += _to_base(db, t.amount_eur_cents, base, t.date)
    return result