reports: Add RelatedPostings.balance_at_cost_by_date() method.

conservancy_beancount/reports/core.py

@@ -368,6 +368,14 @@ class RelatedPostings(Sequence[data.Posting]):
     def balance_at_cost(self) -> Balance:
         return Balance(post.at_cost() for post in self)
 
+    def balance_at_cost_by_date(self, date: datetime.date) -> Balance:
+        for index, post in enumerate(self):
+            if post.meta.date >= date:
+                break
+        else:
+            index += 1
+        return Balance(post.at_cost() for post in self._postings[:index])
+
     def meta_values(self,
                     key: MetaKey,
                     default: Optional[MetaValue]=None,

tests/test_reports_related_postings.py

@@ -197,6 +197,38 @@ def test_balance_at_cost_empty():
     balance = related.balance_at_cost()
     assert balance.is_zero()
 
+@pytest.mark.parametrize('date,expected', [
+    (testutil.FY_MID_DATE - datetime.timedelta(days=1), 0),
+    (testutil.FY_MID_DATE, 0),
+    (testutil.FY_MID_DATE + datetime.timedelta(days=1), 25),
+    (testutil.FY_MID_DATE + datetime.timedelta(days=2), 70),
+    (testutil.FY_MID_DATE + datetime.timedelta(days=3), 135),
+    (testutil.FY_MID_DATE + datetime.timedelta(days=4), 135),
+])
+def test_balance_at_cost_by_date(date, expected):
+    dates = testutil.date_seq()
+    jpy_cost = ('0.01', 'USD')
+    entries = [
+        testutil.Transaction(date=next(dates), postings=[
+            ('Assets:Cash', 1000, 'JPY', jpy_cost),
+            ('Assets:Cash', 15),
+        ]),
+        testutil.Transaction(date=next(dates), postings=[
+            ('Assets:Cash', 2000, 'JPY', jpy_cost),
+            ('Assets:Cash', 25),
+        ]),
+        testutil.Transaction(date=next(dates), postings=[
+            ('Assets:Cash', 3000, 'JPY', jpy_cost),
+            ('Assets:Cash', 35),
+        ]),
+    ]
+    related = core.RelatedPostings(data.Posting.from_entries(entries))
+    actual = related.balance_at_cost_by_date(date)
+    if not expected:
+        assert actual.is_zero()
+    else:
+        assert actual == {'USD': testutil.Amount(expected)}
+
 def test_meta_values_empty():
     related = core.RelatedPostings()
     assert related.meta_values('key') == set()