apacheGH-38395: [Go] fix rounding errors in decimal256 string functions

zeroshade · Oct 23, 2023 · 531d60d · 531d60d
1 parent 0428c5e
commit 531d60d
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Showing 2 changed files with 34 additions and 2 deletions.
diff --git a/go/arrow/decimal256/decimal256.go b/go/arrow/decimal256/decimal256.go
@@ -154,7 +154,7 @@ func FromString(v string, prec, scale int32) (n Num, err error) {
 		return
 	}
 
-	out.Mul(out, big.NewFloat(math.Pow10(int(scale)))).SetPrec(precInBits)
+	out.Mul(out, (&big.Float{}).SetInt(scaleMultipliers[scale].BigInt())).SetPrec(precInBits)
 	// Since we're going to truncate this to get an integer, we need to round
 	// the value instead because of edge cases so that we match how other implementations
 	// (e.g. C++) handles Decimal values. So if we're negative we'll subtract 0.5 and if
@@ -506,7 +506,7 @@ func (n Num) FitsInPrecision(prec int32) bool {
 
 func (n Num) ToString(scale int32) string {
 	f := (&big.Float{}).SetInt(n.BigInt())
-	f.Quo(f, (&big.Float{}).SetInt(scaleMultipliers[scale].BigInt()))
+	f.SetPrec(256).Quo(f, (&big.Float{}).SetInt(scaleMultipliers[scale].BigInt()))
 	return f.Text('f', int(scale))
 }
 

diff --git a/go/arrow/decimal256/decimal256_test.go b/go/arrow/decimal256/decimal256_test.go
@@ -20,6 +20,7 @@ import (
 	"fmt"
 	"math"
 	"math/big"
+	"strings"
 	"testing"
 
 	"github.com/apache/arrow/go/v14/arrow/decimal256"
@@ -575,3 +576,34 @@ func TestFromString(t *testing.T) {
 		})
 	}
 }
+
+// Test issues from GH-38395
+func TestToString(t *testing.T) {
+	const decStr = "3379334159166193114608287418738414931564221155305735605033949613740461239999"
+
+	integer, _ := (&big.Int{}).SetString(decStr, 10)
+	dec := decimal256.FromBigInt(integer)
+
+	expected := "0." + decStr
+	actual := dec.ToString(76)
+
+	if expected != actual {
+		t.Errorf("expected: %s, actual: %s\n", expected, actual)
+	}
+}
+
+// Test issues from GH-38395
+func TestHexFromString(t *testing.T) {
+	const decStr = "11111111111111111111111111111111111111.00000000000000000000000000000000000000"
+
+	num, err := decimal256.FromString(decStr, 76, 38)
+	if err != nil {
+		t.Error(err)
+	} else if decStr != num.ToString(38) {
+		t.Errorf("expected: %s, actual: %s\n", decStr, num.ToString(38))
+
+		actualCoeff := num.BigInt()
+		expectedCoeff, _ := (&big.Int{}).SetString(strings.Replace(decStr, ".", "", -1), 10)
+		t.Errorf("expected(hex): %X, actual(hex): %X\n", expectedCoeff.Bytes(), actualCoeff.Bytes())
+	}
+}