fix: calculate value correctly when P amounts have few decimal digits [#2254]

Valuation in another commoditay could sometimes be inaccurate if the P
price amounts did not have enough decimal places.

hledger-lib/Hledger/Data/Valuation.hs

@@ -48,7 +48,8 @@ import Hledger.Data.Types
 import Hledger.Data.Amount
 import Hledger.Data.Dates (nulldate)
 import Text.Printf (printf)
-import Data.Decimal (decimalPlaces, roundTo)
+import Data.Decimal (decimalPlaces, roundTo, Decimal)
+import Data.Word (Word8)
 
 
 ------------------------------------------------------------------------------
@@ -290,12 +291,17 @@ priceLookup makepricegraph d from mto =
                 -- aggregate all the prices into one
                 product rates
                 -- product (Decimal's Num instance) normalises, stripping trailing zeros.
-                -- Here we undo that (by restoring the old max precision with roundTo), 
-                -- so that amountValueAtDate can see the original internal precision,
-                -- to use as the display precision of calculated value amounts.
-                -- (This can add more than the original number of trailing zeros to some prices,
-                -- making them seem more precise than they were, but it seems harmless here.)
-                & roundTo (maximum $ map decimalPlaces rates)
+                -- But we want to preserve even those, since the number of decimal digits
+                -- here will guide amountValueAtDate in setting the Amount display precision later.
+                -- So we restore them. Or rather, we ensure as many decimal digits as the maximum seen among rates.
+                -- (Some prices might end up more precise than they were, but that seems harmless here.)
+                & setMinDecimalPlaces (maximum $ map decimalPlaces rates)
+
+-- Ensure this Decimal has at least this many decimal places, adding trailing zeros if necessary.
+setMinDecimalPlaces :: Word8 -> Decimal -> Decimal
+setMinDecimalPlaces n d
+  | decimalPlaces d < n = roundTo n d  -- too few, add some zeros
+  | otherwise           = d            -- more than enough, keep as-is
 
 tests_priceLookup =
   let

hledger/test/journal/valuation2.test

@@ -341,3 +341,18 @@ Balance changes in 2019, valued at period ends:
 ==========++========
  Assets:A ||  $2.00 
  Income:B || $-2.00 
+
+# ** 33. #2254 Conversion rates should not be display-rounded during value calculation.
+# 100 * 10.5 * 100.5 = 105525
+<
+P 2000-01-01 A   10.5 B
+P 2000-01-01 B  100.5 C
+
+2000-01-01
+    (a)    100 A
+
+$ hledger -f- print -X C
+2000-01-01
+    (a)        105525 C
+
+>=