From b00aa32d7b1078e14308d050eeef45049977255c Mon Sep 17 00:00:00 2001
From: Simon Michael <simon@joyful.com>
Date: Tue, 13 Nov 2018 17:50:30 -0800
Subject: [PATCH] journal: txn modifier multipliers multiply total-priced
 amounts (#928)

A different approach: instead of converting to unit prices and fiddling
with the display precision, just multiply the total prices by the same
multiplier (and keep them positive).

This seems a little more natural. I'm not sure if one of these will be
more robust than the other.
---
 .../Hledger/Data/TransactionModifier.hs       |  8 ++++---
 tests/journal/modifiers-928.test              | 21 ++++++++++++-------
 2 files changed, 19 insertions(+), 10 deletions(-)

diff --git a/hledger-lib/Hledger/Data/TransactionModifier.hs b/hledger-lib/Hledger/Data/TransactionModifier.hs
index 1b071bec6..6eb956338 100644
--- a/hledger-lib/Hledger/Data/TransactionModifier.hs
+++ b/hledger-lib/Hledger/Data/TransactionModifier.hs
@@ -92,12 +92,14 @@ tmPostingRuleToFunction pr =
         Nothing -> const $ pamount pr
         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" $ n `multiplyMixedAmount` unitpricedmatchedamount 
+            -- Handle a matched amount with a total price carefully so as to keep the transaction balanced (#928).
+            -- Approach 1: convert to a unit price and increase the display precision slightly
+            -- Mixed as = dbg6 "multipliedamount" $ n `multiplyMixedAmount` mixedAmountTotalPriceToUnitPrice matchedamount 
+            -- Approach 2: multiply the total price (keeping it positive) as well as the quantity 
+            Mixed as = dbg6 "multipliedamount" $ n `multiplyMixedAmountAndPrice` matchedamount 
           in
             case acommodity pramount of
               "" -> Mixed as
diff --git a/tests/journal/modifiers-928.test b/tests/journal/modifiers-928.test
index d58e0d7af..5d4dc11a1 100644
--- a/tests/journal/modifiers-928.test
+++ b/tests/journal/modifiers-928.test
@@ -35,14 +35,21 @@ $ hledger -f- print --auto
 $ 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
+    Expenses:Joint            $-100.00 @@ £50
+    Liabilities:Joint:Bob       $50.00 @@ £25
+    Liabilities:Joint:Bill      $50.00 @@ £25
     Assets:Joint:Bank                 £-50.00
 
 >=0
+# alternate approach: convert to unit prices
+#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
 
-# Generating auto postings from an implicitly-priced amount. Same as above.
+# Generating auto postings from an implicitly-priced amount. Should work like the above.
 <
 = ^Expenses:Joint
     Expenses:Joint                                               *-1
@@ -56,9 +63,9 @@ $ hledger -f- print --auto
 $ 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
+    Expenses:Joint            $-100.00 @@ £50
+    Liabilities:Joint:Bob       $50.00 @@ £25
+    Liabilities:Joint:Bill      $50.00 @@ £25
     Assets:Joint:Bank                 £-50.00
 
 >=0