test(differential): stress corpus — @@ prices, many lots, pad+balance (Layer 4) (#213)

* test(differential): stress corpus — @@ prices, many lots, pad+balance (Layer 4)

Adds hand-authored fixtures for constructs absent from example/long-example/
query-example/off-by-one, and feeds them through the beancount differential
oracle plus explicit hand-verified assertions:

- stress-total-price: `@@` total-price posting (per-unit = 10/7); no fixture
  had `@@` before. rustfava matches beancount; price hand-checked to 1.4286.
- stress-many-lots: two lots of one commodity + a partial reduction of the
  cheaper lot -> 5 HOOL {50} + 10 HOOL {60}, booking matches beancount and the
  exact lots are asserted.
- stress-pad-balance: pad->balance fill then a passing balance; both engines
  agree and the assertions pass (diff_amount None).

All three currently book identically to beancount (0 mismatches), so this locks
in that parity against future engine bumps.

* test(differential): move stress fixtures out of tests/data (fix CI)

Putting stress-*.beancount in tests/data/ made the ingest directory walk pick
them up, churning the test_api_imports snapshot (and, because that test then
failed before exercising the extract endpoint, dropping coverage below 100%).
Relocate them to tests/ledgers/ (not an import dir) and resolve fixture paths
via a helper. No snapshot change; behaviour identical.

Author: robcohen

tests/ledgers/stress-many-lots.beancount

@@ -0,0 +1,17 @@
+; Multiple held-at-cost lots of one commodity + a partial FIFO-style reduction
+2020-01-01 open Assets:Stock
+2020-01-01 open Assets:Cash
+2020-01-01 open Income:Gains
+
+2020-02-01 * "buy lot at 50"
+  Assets:Stock   10 HOOL {50 USD}
+  Assets:Cash  -500 USD
+
+2020-03-01 * "buy lot at 60"
+  Assets:Stock   10 HOOL {60 USD}
+  Assets:Cash  -600 USD
+
+2020-04-01 * "sell 5 from the 50 lot at 60, realizing a gain"
+  Assets:Stock   -5 HOOL {50 USD}
+  Assets:Cash   300 USD
+  Income:Gains

tests/ledgers/stress-pad-balance.beancount

@@ -0,0 +1,12 @@
+; pad -> balance resolution, then a later passing balance
+2020-01-01 open Assets:Cash USD
+2020-01-01 open Equity:Opening-Balances
+
+2020-01-05 pad Assets:Cash Equity:Opening-Balances
+2020-01-10 balance Assets:Cash  500 USD
+
+2020-02-01 * "spend"
+  Assets:Cash  -100 USD
+  Equity:Opening-Balances
+
+2020-02-10 balance Assets:Cash  400 USD

tests/ledgers/stress-total-price.beancount

@@ -0,0 +1,11 @@
+; @@ total-price postings (per-unit = total / units); absent from other fixtures
+option "operating_currency" "USD"
+
+2020-01-01 open Assets:USD    USD
+2020-01-01 open Assets:EUR    EUR
+2020-01-01 open Income:Trade
+
+2020-02-01 * "convert 7 USD at a total price of 10 EUR"
+  Assets:USD   -7 USD @@ 10 EUR
+  Assets:EUR   10 EUR
+  Income:Trade

tests/test_differential_beancount.py

@@ -36,11 +36,30 @@
     from collections.abc import Iterable
 
 DATA = Path(__file__).parent / "data"
+# Hand-authored differential fixtures live outside tests/data/ so they are not
+# picked up by the ingest directory walk (test_api_imports snapshots that dir).
+LEDGERS_DIR = Path(__file__).parent / "ledgers"
+
+
+def _ledger_path(name: str) -> str:
+    """Resolve a fixture to tests/ledgers/ if present, else tests/data/."""
+    stress = LEDGERS_DIR / f"{name}.beancount"
+    return str(stress if stress.exists() else DATA / f"{name}.beancount")
 
 # Fixtures whose booking rustfava must reproduce exactly. These span
 # held-at-cost lots, prices, multiple currencies, a pad/balance pair (example)
 # and a date-boundary case (off-by-one); long-example alone has ~193 cost lots.
-LEDGERS = ["example", "long-example", "query-example", "off-by-one"]
+# The stress-* fixtures add constructs the others lack: `@@` total prices,
+# multiple lots of one commodity with a partial reduction, and pad->balance.
+LEDGERS = [
+    "example",
+    "long-example",
+    "query-example",
+    "off-by-one",
+    "stress-total-price",
+    "stress-many-lots",
+    "stress-pad-balance",
+]
 
 # Type of `cost` normalized to beancount's 4-tuple identity — deliberately
 # dropping rustfava's extra ``number_total`` field so two engines' economically
@@ -92,7 +111,7 @@ def _account_inventories(
 @pytest.mark.parametrize("ledger", LEDGERS)
 def test_booked_inventories_match_beancount(ledger: str) -> None:
     """Per-account booked inventories must equal beancount's, to the cent."""
-    path = str(DATA / f"{ledger}.beancount")
+    path = _ledger_path(ledger)
     bc_entries, _bc_errors, _ = beancount_loader.load_file(path)
     rf_entries, _rf_errors, _ = rf_loader.load_uncached(path)
 
@@ -208,3 +227,57 @@ def test_clamped_totals_match_beancount_balance_at_cutoff() -> None:
             f"clamped total mismatch for {account}: "
             f"rustfava={rf_inv[account]} beancount={bc_inv[account]}"
         )
+
+
+def test_stress_fixtures_hand_verified() -> None:
+    """Explicit expected numbers for the stress fixtures.
+
+    The differential tests above already cross-check these against beancount;
+    this pins the values by hand as a second, oracle-independent check of the
+    constructs the fixtures were added for.
+    """
+    d = datetime.date
+
+    # @@ total price: A drains 7 USD, B gains 10 EUR; the sold posting's price
+    # is the per-unit 10/7, and no cost is created.
+    entries, errors, _ = rf_loader.load_uncached(
+        str(LEDGERS_DIR / "stress-total-price.beancount")
+    )
+    assert not errors
+    inv = _account_inventories(entries)
+    assert inv["Assets:USD"] == {("USD", None): Decimal(-7)}
+    assert inv["Assets:EUR"] == {("EUR", None): Decimal(10)}
+    (usd_posting,) = [
+        p
+        for e in entries
+        for p in getattr(e, "postings", [])
+        if getattr(p, "account", "") == "Assets:USD"
+    ]
+    assert usd_posting.cost is None
+    assert usd_posting.price is not None
+    # per-unit = 10 EUR / 7 USD = 1.4286 (to 4dp); the engine keeps full
+    # precision, so compare at a sane scale rather than bit-for-bit.
+    assert round(usd_posting.price.number, 4) == Decimal("1.4286")
+
+    # Many lots: after selling 5 of the 50-lot, 5 HOOL @ {50} + 10 HOOL @ {60}.
+    entries, errors, _ = rf_loader.load_uncached(
+        str(LEDGERS_DIR / "stress-many-lots.beancount")
+    )
+    assert not errors
+    stock = _account_inventories(entries)["Assets:Stock"]
+    assert stock == {
+        ("HOOL", (Decimal(50), "USD", d(2020, 2, 1), None)): Decimal(5),
+        ("HOOL", (Decimal(60), "USD", d(2020, 3, 1), None)): Decimal(10),
+    }
+
+    # pad -> balance: the pad fills Cash to the asserted 500, spend leaves 400,
+    # and both balance assertions pass (no errors, diff_amount None).
+    entries, errors, _ = rf_loader.load_uncached(
+        str(LEDGERS_DIR / "stress-pad-balance.beancount")
+    )
+    assert not errors
+    assert _account_inventories(entries)["Assets:Cash"] == {
+        ("USD", None): Decimal(400)
+    }
+    for bal in _balance_dirs(entries):
+        assert bal.diff_amount is None