fix: roi: make sure empty cashflows are skipped when determining first cashflow

Empty cashflows are added when the begin date of the report is before the first
transaction.

hledger/Hledger/Cli/Commands/Roi.hs

@@ -162,7 +162,9 @@ timeWeightedReturn showCashFlow prettyTables investmentsQuery trans mixedAmountV
         -- PnL and CashFlow, we would not be able to apply pnl change to 0 unit,
         -- which would lead to an error. We make sure that we have at least one
         -- cashflow entry at the front, and we know that there would be at most
-        -- one for the given date, by construction.
+        -- one for the given date, by construction. Empty CashFlows added
+        -- because of a begin date before the first transaction are not seen as
+        -- a valid cashflow entry at the front.
         zeroUnitsNeedsCashflowAtTheFront
         $ sort
         $ datedCashflows ++ datedPnls
@@ -170,9 +172,14 @@ timeWeightedReturn showCashFlow prettyTables investmentsQuery trans mixedAmountV
           zeroUnitsNeedsCashflowAtTheFront changes =
             if initialUnits > 0 then changes
             else 
-              let (leadingPnls, rest) = span (isLeft . snd) changes
-                  (firstCashflow, rest') = splitAt 1 rest
-              in firstCashflow ++ leadingPnls ++ rest'
+              let (leadingEmptyCashFlows, rest) = span isEmptyCashflow changes
+                  (leadingPnls, rest') = span (isLeft . snd) rest
+                  (firstCashflow, rest'') = splitAt 1 rest'
+              in leadingEmptyCashFlows ++ firstCashflow ++ leadingPnls ++ rest''
+
+          isEmptyCashflow (_date, amt) = case amt of
+            Right amt -> mixedAmountIsZero amt
+            Left _ -> False
 
           datedPnls = map (second Left) $ aggregateByDate pnl
  

hledger/test/roi.test

@@ -345,3 +345,24 @@ hledger -f - roi --inv saving --pnl dividend
 +---++------------+------------++---------------+----------+-------------+---------++-------+-------+
 
 >>>=0
+
+# 14. Should support begin date before first transaction where first transaction has pnl
+hledger -f - roi --inv stocks --pnl expenses --value=then,€ -Y
+<<<
+P 2022-07-31 A € 1
+
+2022-08-01 Purchase
+    checking   € -101
+    stocks  100 A @ € 1
+    expenses   € 1
+
+P 2022-08-02 A € 2
+
+>>>
++---++------------+------------++---------------+----------+-------------+------++---------+--------+
+|   ||      Begin |        End || Value (begin) | Cashflow | Value (end) |  PnL ||     IRR |    TWR |
++===++============+============++===============+==========+=============+======++=========+========+
+| 1 || 2022-01-01 | 2022-12-31 ||             0 |    € 101 |       € 200 | € 99 || 410.31% | 98.02% |
++---++------------+------------++---------------+----------+-------------+------++---------+--------+
+
+>>>=0