[go] Fix bug with encoding negative decimal numbers

lib/debezium/decimal.go

@@ -3,6 +3,7 @@ package debezium
 import (
 	"fmt"
 	"math/big"
+	"slices"
 
 	"github.com/artie-labs/transfer/lib/typing/decimal"
 )
@@ -23,11 +24,14 @@ func EncodeDecimal(value string, scale uint16) ([]byte, error) {
 		return nil, fmt.Errorf("unable to use %q as a floating-point number", value)
 	}
 
-	data := bigIntValue.Bytes()
+	data := bigIntValue.Bytes() // [Bytes] returns the absolute value of the number.
 	if bigIntValue.Sign() < 0 {
 		// Convert to two's complement if the number is negative
-		bigIntValue = bigIntValue.Neg(bigIntValue)
-		data = bigIntValue.Bytes()
+
+		if data[0] > 127 {
+			// Prepend an empty byte if the first bit is already set.
+			data = slices.Concat([]byte{0}, data)
+		}
 
 		// Inverting bits for two's complement.
 		for i := range data {

lib/debezium/decimal_test.go

@@ -6,6 +6,23 @@ import (
 	"github.com/stretchr/testify/assert"
 )
 
+func encodeDecode(value string, scale uint16) (string, error) {
+	bytes, err := EncodeDecimal(value, scale)
+	if err != nil {
+		return "", err
+	}
+	return DecodeDecimal(bytes, nil, int(scale)).String(), nil
+}
+
+func mustEncodeDecode(value string, scale uint16) string {
+	out, err := encodeDecode(value, scale)
+	if err != nil {
+		panic(err)
+	}
+
+	return out
+}
+
 func TestEncodeDecimal(t *testing.T) {
 	testCases := []struct {
 		name  string
@@ -71,6 +88,16 @@ func TestEncodeDecimal(t *testing.T) {
 			value: "1.05",
 			scale: 2,
 		},
+		{
+			name:  "negative number: 2^16 - 255",
+			value: "-65281",
+			scale: 0,
+		},
+		{
+			name:  "negative number: 2^16 - 1",
+			value: "-65535",
+			scale: 0,
+		},
 		{
 			name:        "malformed - empty string",
 			value:       "",