fix: bal: Allow cumulative gain and valuechange reports

Previously, --cumulative with --gain or --valuechange would produce an
empty report. This fixes this issue to produce a reasonable report.

hledger-lib/Hledger/Reports/MultiBalanceReport.hs

@@ -315,20 +315,22 @@ calculateReportMatrix rspec@ReportSpec{_rsReportOpts=ropts} j priceoracle startb
     -- The valued row amounts to be displayed: per-period changes,
     -- zero-based cumulative totals, or
     -- starting-balance-based historical balances.
-    rowbals name changes = dbg5 "rowbals" $ case balanceaccum_ ropts of
-        PerPeriod  -> changeamts
+    rowbals name unvaluedChanges = dbg5 "rowbals" $ case balanceaccum_ ropts of
+        PerPeriod  -> changes
         Cumulative -> cumulative
         Historical -> historical
       where
-        -- changes to report on: usually just the changes itself, but use the
-        -- differences in the historical amount for ValueChangeReports.
-        changeamts = case balancecalc_ ropts of
-            CalcChange      -> M.mapWithKey avalue changes
-            CalcBudget      -> M.mapWithKey avalue changes
+        -- changes to report on: usually just the valued changes themselves, but use the
+        -- differences in the valued historical amount for CalcValueChange and CalcGain.
+        changes = case balancecalc_ ropts of
+            CalcChange      -> M.mapWithKey avalue unvaluedChanges
+            CalcBudget      -> M.mapWithKey avalue unvaluedChanges
             CalcValueChange -> periodChanges valuedStart historical
             CalcGain        -> periodChanges valuedStart historical
-        cumulative = cumulativeSum avalue nullacct changeamts
-        historical = cumulativeSum avalue startingBalance changes
+        -- the historical balance is the valued cumulative sum of all unvalued changes
+        historical = M.mapWithKey avalue $ cumulativeSum startingBalance unvaluedChanges
+        -- since this is a cumulative sum of valued amounts, it should not be valued again
+        cumulative = cumulativeSum nullacct changes
         startingBalance = HM.lookupDefault nullacct name startbals
         valuedStart = avalue (DateSpan Nothing historicalDate) startingBalance
 
@@ -580,11 +582,9 @@ periodChanges start amtmap =
     M.fromDistinctAscList . zip dates $ zipWith subtractAcct amts (start:amts)
   where (dates, amts) = unzip $ M.toAscList amtmap
 
--- | Calculate a cumulative sum from a list of period changes and a valuation
--- function.
-cumulativeSum :: (DateSpan -> Account -> Account) -> Account -> Map DateSpan Account -> Map DateSpan Account
-cumulativeSum value start = snd . M.mapAccumWithKey accumValued start
-  where accumValued startAmt date newAmt = let s = sumAcct startAmt newAmt in (s, value date s)
+-- | Calculate a cumulative sum from a list of period changes.
+cumulativeSum :: Account -> Map DateSpan Account -> Map DateSpan Account
+cumulativeSum start = snd . M.mapAccum (\a b -> let s = sumAcct a b in (s, s)) start
 
 -- | Given a table representing a multi-column balance report (for example,
 -- made using 'balanceReportAsTable'), render it in a format suitable for

hledger/test/journal/gain.test

@@ -49,7 +49,17 @@ Historical gain in 2000-01-01..2000-02-29, valued at period ends:
  assets:new ||       -1 A           0 
  assets:old ||          0         2 A 
 
-# 4. use a different valuation strategy
+# 4. cumulative gain report
+$ hledger -f- bal -M --gain -b 2000 --no-total --cumulative
+Cumulative gain in 2000-01-01..2000-02-29, valued at period ends:
+
+            || 2000-01-31  2000-02-29 
+============++========================
+ assets:b   ||      -13 A       -15 A 
+ assets:new ||       -1 A           0 
+ assets:old ||        2 A         4 A 
+
+# 5. use a different valuation strategy
 $ hledger -f- bal -M --gain --no-total --value=2000-02-01
 Incremental gain in 1999-12-01..2000-01-31, valued at 2000-02-01:
 
@@ -58,7 +68,7 @@ Incremental gain in 1999-12-01..2000-01-31, valued at 2000-02-01:
  assets:b   ||       0    -15 A 
  assets:old ||     2 A        0 
 
-# 5. use a different valuation strategy for historical
+# 6. use a different valuation strategy for historical
 $ hledger -f- bal -M --gain --no-total --value=2000-02-01 -b 2000 --historical
 Historical gain in 2000-01, valued at 2000-02-01:
 
@@ -67,7 +77,7 @@ Historical gain in 2000-01, valued at 2000-02-01:
  assets:b   ||      -15 A 
  assets:old ||        2 A 
 
-# 6. also works in balancesheet
+# 7. also works in balancesheet
 $ hledger -f- bs -M --gain --no-total
 Balance Sheet 1999-12-31..2000-02-29 (Historical Gain), valued at period ends