fix(analytics): TWR не цепляет остаток после полного вывода из портфеля

День, выводящий всё, кроме 1 % стоимости (EMPTIED_SHARE), не участвует в цепочке: остаток в пару рублей превращал комиссию или округление в −60 % за день, и несколько таких опустошений обнуляли всю историю.

backend/src/fintracker/analytics/returns.py

@@ -68,6 +68,11 @@ PERIODS: tuple[str, ...] = ("1m", "3m", "6m", "ytd", "1y", "3y", "all")
 #: Periods reported even when the data is younger than their nominal length.
 ELASTIC_PERIODS = frozenset({"ytd", "all"})
 
+EMPTIED_SHARE = Decimal("0.1")
+"""A day that withdraws all but this share of the previous value has emptied the portfolio: what
+is left is a residue of rounding, fees and dust, and its daily change says nothing about the
+market. See `twr`."""
+
 MIN_XIRR_DAYS = 30
 """Below this, annualising a per-instrument return turns a day of noise into a percent a
 year. A paper bought yesterday gets no XIRR rather than a triple-digit one."""
@@ -178,8 +183,16 @@ def twr(points: Sequence[Point], *, opening_value: Decimal, opening_missing: int
 
     One case is still skipped rather than adjusted: a day where the NUMBER of unvalued
     positions changed. The value then jumps by a whole position that neither gained nor lost
-    anything, and no flow describes it. `TwrResult.days_skipped` reports how many such days
-    a period contains, because a TWR with holes has to say so.
+    anything, and no flow describes it.
+
+    A day that withdraws all but 10 % of what the portfolio was worth (`EMPTIED_SHARE`) is not
+    chained either: the base left is dust, and the fee, rounding or unrecorded small item that eats
+    a share of it reads as -60 %. Chained, a few such emptyings over the years multiply the whole
+    history to -99.9 %. The day is neither used nor skipped — like a zero base, there was
+    nothing invested to earn a return on — and the chain carries on from the next day.
+
+    `TwrResult.days_skipped` reports how many days of the first kind a period contains, because
+    a TWR with holes has to say so.
     """
     factor = ONE
     previous = opening_value
@@ -189,6 +202,8 @@ def twr(points: Sequence[Point], *, opening_value: Decimal, opening_missing: int
         base = previous + point.flow + point.unvalued_flow
         if point.missing != previous_missing:
             skipped += 1
+        elif (point.flow + point.unvalued_flow) < ZERO and base < previous * EMPTIED_SHARE:
+            pass
         elif base > ZERO:
             factor *= point.value / base
             used += 1

backend/tests/analytics/test_returns.py

@@ -131,3 +131,26 @@ def test_a_large_but_plausible_rate_is_kept():
     rate = xirr([day(0), day(30)], [D("-1000"), D("1100")])
     assert rate is not None
     assert D("2.0") < rate < D("2.3")
+
+
+def test_emptying_the_portfolio_does_not_chain_the_residue():
+    """1 302 ₽ out of 1 302 ₽ minus a 3 ₽ residue that then loses 3 ₽ is not a -94 % day."""
+    points = [
+        Point(d=day(1), value=D("1302.27"), flow=D(0)),
+        Point(d=day(2), value=D("0.20"), flow=D("-1299.00")),
+        Point(d=day(3), value=D("5000.20"), flow=D("5000")),
+        Point(d=day(4), value=D("5500.22"), flow=D(0)),
+    ]
+    chain = twr(points, opening_value=D("1300"))
+    # day 1: +0.17 %; day 2 is a residue, not chained; day 3: flat; day 4: +10 %
+    assert chain.days_skipped == 0
+    assert chain.days_used == 3
+    assert chain.value is not None
+    assert D("0.1019") < chain.value < D("0.1020")
+
+
+def test_a_partial_withdrawal_is_still_chained():
+    points = [Point(d=day(1), value=D(55), flow=D(-50))]
+    chain = twr(points, opening_value=D(100))
+    assert chain.days_used == 1
+    assert chain.value == D("0.100000")