From a529207ae737d7f2aa1ca218307437a022e5ab5e Mon Sep 17 00:00:00 2001
From: Stephen Morgan <morgan.stephen99@gmail.com>
Date: Fri, 26 Mar 2021 15:22:47 +1100
Subject: [PATCH] lib: Do not round Decimal before applying JSON representation
 if existing representation is small enough.

Previously the JSON representation of Decimal was rounded to 10 points
of precision before serialising. This sometimes results in an
unnecessary increase of precision.
---
 hledger-lib/Hledger/Data/Json.hs | 16 ++++++++--------
 hledger/test/json.test           | 16 ++++++++--------
 2 files changed, 16 insertions(+), 16 deletions(-)

diff --git a/hledger-lib/Hledger/Data/Json.hs b/hledger-lib/Hledger/Data/Json.hs
index 90c0f43a1..6478d5dfb 100644
--- a/hledger-lib/Hledger/Data/Json.hs
+++ b/hledger-lib/Hledger/Data/Json.hs
@@ -41,8 +41,8 @@ import           Data.Aeson
 import           Data.Aeson.Encode.Pretty (encodePrettyToTextBuilder)
 --import           Data.Aeson.TH
 import qualified Data.ByteString.Lazy as BL
-import           Data.Decimal
-import           Data.Maybe
+import           Data.Decimal (DecimalRaw(..), roundTo)
+import           Data.Maybe (fromMaybe)
 import qualified Data.Text.Lazy    as TL
 import qualified Data.Text.Lazy.IO as TL
 import qualified Data.Text.Lazy.Builder as TB
@@ -80,13 +80,13 @@ instance ToJSON GenericSourcePos
 -- and that the overall number of significant digits in the floating point
 -- remains manageable in practice. (I'm not sure how to limit the number
 -- of significant digits in a Decimal right now.)
-instance ToJSON Decimal where
+instance (Integral a, ToJSON a) => ToJSON (DecimalRaw a) where
   toJSON d = object
-    ["decimalPlaces"   .= toJSON decimalPlaces
-    ,"decimalMantissa" .= toJSON decimalMantissa
-    ,"floatingPoint"   .= toJSON (fromRational $ toRational d' :: Double)
-    ]
-    where d'@Decimal{..} = roundTo 10 d
+      [ "decimalPlaces"   .= toJSON (decimalPlaces d')
+      , "decimalMantissa" .= toJSON (decimalMantissa d')
+      , "floatingPoint"   .= toJSON (realToFrac d' :: Double)
+      ]
+    where d' = if decimalPlaces d <= 10 then d else roundTo 10 d
 
 instance ToJSON Amount
 instance ToJSON AmountStyle
diff --git a/hledger/test/json.test b/hledger/test/json.test
index 229378e77..1a970981f 100644
--- a/hledger/test/json.test
+++ b/hledger/test/json.test
@@ -18,8 +18,8 @@ $ hledger -f- reg --output-format=json
                     "acommodity": "AAA",
                     "aquantity": {
                         "floatingPoint": 1,
-                        "decimalPlaces": 10,
-                        "decimalMantissa": 10000000000
+                        "decimalPlaces": 1,
+                        "decimalMantissa": 10
                     },
                     "aismultiplier": false,
                     "astyle": {
@@ -46,8 +46,8 @@ $ hledger -f- reg --output-format=json
                 "acommodity": "AAA",
                 "aquantity": {
                     "floatingPoint": 1,
-                    "decimalPlaces": 10,
-                    "decimalMantissa": 10000000000
+                    "decimalPlaces": 1,
+                    "decimalMantissa": 10
                 },
                 "aismultiplier": false,
                 "astyle": {
@@ -76,8 +76,8 @@ $ hledger -f- bal --output-format=json
                     "acommodity": "AAA",
                     "aquantity": {
                         "floatingPoint": 1,
-                        "decimalPlaces": 10,
-                        "decimalMantissa": 10000000000
+                        "decimalPlaces": 1,
+                        "decimalMantissa": 10
                     },
                     "aismultiplier": false,
                     "astyle": {
@@ -97,8 +97,8 @@ $ hledger -f- bal --output-format=json
             "acommodity": "AAA",
             "aquantity": {
                 "floatingPoint": 1,
-                "decimalPlaces": 10,
-                "decimalMantissa": 10000000000
+                "decimalPlaces": 1,
+                "decimalMantissa": 10
             },
             "aismultiplier": false,
             "astyle": {