Defensive Copy on Coefficient (#52)

2 files changed, 7 insertions, 1 deletions

diff --git a/decimal.go b/decimal.go
index 5fcb628..cf0f3ce 100644
--- a/decimal.go
+++ b/decimal.go
@@ -443,7 +443,9 @@ func (d Decimal) Exponent() int32 {
 
 // Coefficient returns the coefficient of the decimal.  It is scaled by 10^Exponent()
 func (d Decimal) Coefficient() *big.Int {
-	return d.value
+	// we copy the coefficient so that mutating the result does not mutate the
+	// Decimal.
+	return big.NewInt(0).Set(d.value)
 }
 
 // IntPart returns the integer component of the decimal.
diff --git a/decimal_test.go b/decimal_test.go
index a0ad37c..96f7340 100644
--- a/decimal_test.go
+++ b/decimal_test.go
@@ -1421,6 +1421,10 @@ func TestDecimal_Coefficient(t *testing.T) {
 	if co.Int64() != 123 {
 		t.Error("Coefficient should be 123; Got:", co)
 	}
+	co.Set(big.NewInt(0))
+	if d.IntPart() != 123 {
+		t.Error("Modifying coefficient modified Decimal; Got:", d)
+	}
 }
 
 type DecimalSlice []Decimal