journal: fix txn modifier multipliers with total-priced amounts (#928)

Transaction modifier multipliers have never multiplied total-priced amounts
correctly (and prior to hledger 1.10, this could generate unbalanced
transactions).

Now, the generated postings in this situation will have unit prices,
and an extra digit of display precision. This helps ensure that
the modified transaction will remain balanced. I'm not sure yet if
it's guaranteed.

hledger-lib/Hledger/Data/Amount.hs

@@ -60,6 +60,7 @@ module Hledger.Data.Amount (
   divideAmount,
   multiplyAmount,
   amountValue,
+  amountTotalPriceToUnitPrice,
   -- ** rendering
   amountstyle,
   styleAmount,
@@ -99,6 +100,7 @@ module Hledger.Data.Amount (
   isReallyZeroMixedAmount,
   isReallyZeroMixedAmountCost,
   mixedAmountValue,
+  mixedAmountTotalPriceToUnitPrice,
   -- ** rendering
   styleMixedAmount,
   showMixedAmount,
@@ -209,6 +211,17 @@ costOfAmount a@Amount{aquantity=q, aprice=price} =
       UnitPrice  p@Amount{aquantity=pq} -> p{aquantity=pq * q}
       TotalPrice p@Amount{aquantity=pq} -> p{aquantity=pq * signum q}
 
+-- | Replace an amount's TotalPrice, if it has one, with an equivalent UnitPrice.
+-- Has no effect on amounts without one.
+-- Also increases the unit price's display precision to show one extra decimal place,
+-- to help the unit-priced amounts to still balance. 
+-- Does Decimal division, might be some rounding/irrational number issues.
+amountTotalPriceToUnitPrice :: Amount -> Amount
+amountTotalPriceToUnitPrice 
+  a@Amount{aquantity=q, aprice=TotalPrice pa@Amount{aquantity=pq, astyle=ps@AmountStyle{asprecision=pp}}}
+  = a{aprice = UnitPrice pa{aquantity=abs (pq/q), astyle=ps{asprecision=pp+1}}}
+amountTotalPriceToUnitPrice a = a
+
 -- | Divide an amount's quantity by a constant.
 divideAmount :: Amount -> Quantity -> Amount
 divideAmount a@Amount{aquantity=q} d = a{aquantity=q/d}
@@ -665,6 +678,12 @@ canonicaliseMixedAmount styles (Mixed as) = Mixed $ map (canonicaliseAmount styl
 mixedAmountValue :: Journal -> Day -> MixedAmount -> MixedAmount
 mixedAmountValue j d (Mixed as) = Mixed $ map (amountValue j d) as
 
+-- | Replace each component amount's TotalPrice, if it has one, with an equivalent UnitPrice.
+-- Has no effect on amounts without one. 
+-- Does Decimal division, might be some rounding/irrational number issues.
+mixedAmountTotalPriceToUnitPrice :: MixedAmount -> MixedAmount
+mixedAmountTotalPriceToUnitPrice (Mixed as) = Mixed $ map amountTotalPriceToUnitPrice as
+
 
 -------------------------------------------------------------------------------
 -- tests

hledger-lib/Hledger/Data/TransactionModifier.hs

@@ -24,7 +24,7 @@ import Hledger.Data.Amount
 import Hledger.Data.Transaction
 import Hledger.Query
 import Hledger.Utils.UTF8IOCompat (error')
--- import Hledger.Utils.Debug
+import Hledger.Utils.Debug
 
 -- $setup
 -- >>> :set -XOverloadedStrings
@@ -78,6 +78,8 @@ tmParseQuery mt = fst . flip parseQuery (tmquerytxt mt)
 
 -- | Converts a 'TransactionModifier''s posting rule to a 'Posting'-generating function,
 -- which will be used to make a new posting based on the old one (an "automated posting").
+-- The new posting's amount can optionally be the old posting's amount multiplied by a constant.
+-- If the old posting had a total-priced amount, the new posting's multiplied amount will be unit-priced. 
 tmPostingRuleToFunction :: TMPostingRule -> (Posting -> Posting)
 tmPostingRuleToFunction pr = 
   \p -> renderPostingCommentDates $ pr
@@ -88,10 +90,21 @@ tmPostingRuleToFunction pr =
   where
     amount' = case postingRuleMultiplier pr of
         Nothing -> const $ pamount pr
-        Just n  -> \p -> withAmountType (head $ amounts $ pamount pr) $ pamount p `multiplyMixedAmount` n
-    withAmountType pramount (Mixed as) = case acommodity pramount of
-        "" -> Mixed as
-        c  -> Mixed [a{acommodity = c, astyle = astyle pramount, aprice = aprice pramount} | a <- as]
+        Just n  -> \p ->
+          -- Multiply the old posting's amount by the posting rule's multiplier.
+          -- Its display precision will be increased if needed to show all digits.
+          let
+            pramount = dbg6 "pramount" $ head $ amounts $ pamount pr
+            matchedamount = dbg6 "matchedamount" $ pamount p
+            unitpricedmatchedamount = dbg6 "unitpricedmatchedamount" $ mixedAmountTotalPriceToUnitPrice matchedamount
+            Mixed as = dbg6 "scaledmatchedamount" $ unitpricedmatchedamount `multiplyMixedAmount` n
+          in
+            case acommodity pramount of
+              "" -> Mixed as
+              -- TODO multipliers with commodity symbols are not yet a documented feature.
+              -- For now: in addition to multiplying the quantity, it also replaces the 
+              -- matched amount's commodity, display style, and price with those of the posting rule.   
+              c  -> Mixed [a{acommodity = c, astyle = astyle pramount, aprice = aprice pramount} | a <- as]
 
 postingRuleMultiplier :: TMPostingRule -> Maybe Quantity
 postingRuleMultiplier p =

tests/journal/modifiers-928.test

@@ -0,0 +1,64 @@
+# Issue #928
+
+# Generating auto postings from a unit-priced amount.
+<
+= ^Expenses:Joint
+    Expenses:Joint                                               *-1
+    Liabilities:Joint:Bob                                       *0.5
+    Liabilities:Joint:Bill                                      *0.5
+    
+2018/01/01
+    Expenses:Joint:Widgets                                   $100.00 @ £0.50
+    Assets:Joint:Bank                                        -£50.00
+
+$ hledger -f- print --auto
+2018/01/01
+    Expenses:Joint:Widgets     $100.00 @ £0.50
+    Expenses:Joint            $-100.00 @ £0.50
+    Liabilities:Joint:Bob       $50.00 @ £0.50
+    Liabilities:Joint:Bill      $50.00 @ £0.50
+    Assets:Joint:Bank                  £-50.00
+
+>=0
+
+# Generating auto postings from a total-priced amount.
+<
+= ^Expenses:Joint
+    Expenses:Joint                                               *-1
+    Liabilities:Joint:Bob                                       *0.5
+    Liabilities:Joint:Bill                                      *0.5
+    
+2018/01/01
+    Expenses:Joint:Widgets                                   $100.00 @@ £50
+    Assets:Joint:Bank                                        -£50.00
+
+$ hledger -f- print --auto
+2018/01/01
+    Expenses:Joint:Widgets     $100.00 @@ £50
+    Expenses:Joint            $-100.00 @ £0.5
+    Liabilities:Joint:Bob       $50.00 @ £0.5
+    Liabilities:Joint:Bill      $50.00 @ £0.5
+    Assets:Joint:Bank                 £-50.00
+
+>=0
+
+# Generating auto postings from an implicitly-priced amount. Same as above.
+<
+= ^Expenses:Joint
+    Expenses:Joint                                               *-1
+    Liabilities:Joint:Bob                                       *0.5
+    Liabilities:Joint:Bill                                      *0.5
+
+2018/01/01
+    Expenses:Joint:Widgets                                   $100.00
+    Assets:Joint:Bank                                        -£50.00
+
+$ hledger -f- print --auto
+2018/01/01
+    Expenses:Joint:Widgets            $100.00
+    Expenses:Joint            $-100.00 @ £0.5
+    Liabilities:Joint:Bob       $50.00 @ £0.5
+    Liabilities:Joint:Bill      $50.00 @ £0.5
+    Assets:Joint:Bank                 £-50.00
+
+>=0