attempt to resolve the split issue

cgt_calc/main.py

@@ -42,7 +42,7 @@
 )
 from .parsers import read_broker_transactions, read_initial_prices
 from .spin_off_handler import SpinOffHandler
-from .transaction_log import add_to_list, has_key
+from .transaction_log import add_to_list, has_key, multiply_entries
 from .util import round_decimal
 
 LOGGER = logging.getLogger(__name__)
@@ -123,7 +123,16 @@ def add_acquisition(
             price, amount = self.handle_spin_off(transaction)
         elif transaction.action is ActionType.STOCK_SPLIT:
             price = Decimal(0)
-            amount = Decimal(0)
+            original = self.portfolio[symbol].quantity
+            multiple = Decimal((original + quantity) / original)
+            # Retroactively alter the acquisition / disposal list, instead of
+            # adding the split as a separate acquisition event.
+            multiply_entries(
+                self.acquisition_list, symbol, multiple, transaction.date)
+            multiply_entries(
+                self.disposal_list, symbol, multiple, transaction.date)
+            self.portfolio[symbol] += Position(quantity, Decimal(0))
+            return
         else:
             if price is None:
                 raise PriceMissingError(transaction)

cgt_calc/transaction_log.py

@@ -31,3 +31,15 @@ def add_to_list(
         amount=amount,
         fees=fees,
     )
+
+def multiply_entries(
+    current_list: HmrcTransactionLog,
+    symbol: str,
+    multiple: Decimal,
+    multiply_date: datetime.date | None = None
+) -> None:
+    for date, entries in current_list.items():
+        if multiply_date is not None:
+            assert multiply_date >= date
+        entries[symbol].quantity *= multiple
+

tests/test_data/calc_test_data.py

@@ -1055,10 +1055,10 @@ def split_transaction(
                 "buy$FOO": [
                     CalculationEntry(
                         RuleType.SECTION_104,
-                        quantity=Decimal(12),
+                        quantity=Decimal(24),
                         amount=Decimal(-60),
                         allowable_cost=Decimal(60),
-                        new_quantity=Decimal(12),
+                        new_quantity=Decimal(24),
                         fees=Decimal(0),
                         new_pool_cost=Decimal(60),
                     ),
@@ -1068,25 +1068,11 @@ def split_transaction(
                 "sell$FOO": [
                     CalculationEntry(
                         RuleType.SECTION_104,
-                        quantity=Decimal(2),
+                        quantity=Decimal(4),
                         amount=Decimal(12),
                         gain=Decimal(2),
                         allowable_cost=Decimal(10),
                         fees=Decimal(0),
-                        new_quantity=Decimal(10),
-                        new_pool_cost=Decimal(50),
-                    ),
-                ],
-            },
-            datetime.date(day=15, month=5, year=2023): {
-                "split$FOO": [
-                    CalculationEntry(
-                        RuleType.SECTION_104,
-                        quantity=Decimal(10),
-                        amount=Decimal(0),
-                        gain=Decimal(0),
-                        fees=Decimal(0),
-                        allowable_cost=Decimal(0),
                         new_quantity=Decimal(20),
                         new_pool_cost=Decimal(50),
                     ),