feat: Recurrence-Terminberechnung

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

finance/app/engine/recurrence.py

@@ -0,0 +1,25 @@
+import calendar
+from datetime import date
+
+STEP = {"monthly": 1, "quarterly": 3, "yearly": 12}
+
+
+def _clamp(y: int, m: int, day: int) -> date:
+    return date(y, m, min(day, calendar.monthrange(y, m)[1]))
+
+
+def occurrences(rhythm: str, due_day: int, window_start: date, window_end: date,
+                item_start: date | None = None, item_end: date | None = None) -> list[date]:
+    step = STEP[rhythm]
+    lo = max(window_start, item_start) if item_start else window_start
+    hi = min(window_end, item_end) if item_end else window_end
+    anchor = item_start or window_start
+    d = _clamp(anchor.year, anchor.month, due_day)
+    out: list[date] = []
+    while d <= hi:
+        if d >= lo:
+            out.append(d)
+        total = d.year * 12 + d.month - 1 + step
+        y, m0 = divmod(total, 12)
+        d = _clamp(y, m0 + 1, due_day)
+    return out

finance/tests/test_recurrence.py

@@ -0,0 +1,19 @@
+from datetime import date
+
+from app.engine.recurrence import occurrences
+
+
+def test_monthly_clamps_short_months():
+    got = occurrences("monthly", 31, date(2026, 1, 1), date(2026, 3, 31))
+    assert got == [date(2026, 1, 31), date(2026, 2, 28), date(2026, 3, 31)]
+
+
+def test_quarterly_respects_item_bounds():
+    got = occurrences("quarterly", 15, date(2026, 1, 1), date(2026, 12, 31),
+                      item_start=date(2026, 4, 1), item_end=date(2026, 10, 31))
+    assert got == [date(2026, 4, 15), date(2026, 7, 15), date(2026, 10, 15)]
+
+
+def test_yearly():
+    got = occurrences("yearly", 1, date(2026, 1, 1), date(2028, 12, 31))
+    assert got == [date(2026, 1, 1), date(2027, 1, 1), date(2028, 1, 1)]