fixing scale of + and - (#61)

Author: pascallouisperez

values.go

@@ -181,14 +181,20 @@ func (value *Number) scaleUp(scale int) int64 {
 }
 
 func (left *Number) Plus(right *Number) *Number {
-	scale := left.typ.scale + right.typ.scale
+	scale := left.typ.scale
+	if scale < right.typ.scale {
+		scale = right.typ.scale
+	}
 	lv, rv := left.scaleUp(scale), right.scaleUp(scale)
 
 	return &Number{lv + rv, &NumberType{scale}}
 }
 
 func (left *Number) Minus(right *Number) *Number {
-	scale := left.typ.scale + right.typ.scale
+	scale := left.typ.scale
+	if scale < right.typ.scale {
+		scale = right.typ.scale
+	}
 	lv, rv := left.scaleUp(scale), right.scaleUp(scale)
 
 	return &Number{lv - rv, &NumberType{scale}}

values_test.go

@@ -125,6 +125,11 @@ func (s *Zuite) TestNumber_Plus() {
 			right:    MustNewValue("3").(*Number),
 			expected: MustNewValue("5.0").(*Number),
 		},
+		{
+			left:     MustNewValue("2.0").(*Number),
+			right:    MustNewValue("3.0").(*Number),
+			expected: MustNewValue("5.0").(*Number),
+		},
 	}
 	for _, ex := range cases {
 		actual := ex.left.Plus(ex.right)
@@ -149,6 +154,11 @@ func (s *Zuite) TestNumber_Minus() {
 			right:    MustNewValue("3").(*Number),
 			expected: MustNewValue("-1.0").(*Number),
 		},
+		{
+			left:     MustNewValue("2.0").(*Number),
+			right:    MustNewValue("3.0").(*Number),
+			expected: MustNewValue("-1.0").(*Number),
+		},
 	}
 	for _, ex := range cases {
 		actual := ex.left.Minus(ex.right)