roman: check number range

roman/roman.go

@@ -1,6 +1,9 @@
 package roman
 
-import "strings"
+import (
+	"errors"
+	"strings"
+)
 
 type RomanNumeral struct {
 	Value  uint16
@@ -24,7 +27,12 @@ var allRomanNumerals = []RomanNumeral{
 	{1, "I"},
 }
 
-func ConvertToRoman(arabic uint16) string {
+var ErrNumberOutOfRange = errors.New("numbers should not be bigger than 3999")
+
+func ConvertToRoman(arabic uint16) (string, error) {
+	if arabic > 3999 {
+		return "", ErrNumberOutOfRange
+	}
 	var converted strings.Builder
 	for _, numeral := range allRomanNumerals {
 		for arabic >= numeral.Value {
@@ -33,9 +41,10 @@ func ConvertToRoman(arabic uint16) string {
 		}
 	}
 
-	return converted.String()
+	return converted.String(), nil
 }
 
+// XXX: This convert function doesn't check the validity of the roman numerals string
 func ConvertToArabic(roman string) uint16 {
 	var converted uint16
 	for _, numeral := range allRomanNumerals {

roman/roman_test.go

@@ -44,7 +44,7 @@ var cases = []struct {
 func TestRomanNemerals(t *testing.T) {
 	for _, test := range cases {
 		t.Run(fmt.Sprintf("%d gets converted to %q", test.Arabic, test.Roman), func(t *testing.T) {
-			got := ConvertToRoman(test.Arabic)
+			got, _ := ConvertToRoman(test.Arabic)
 			want := test.Roman
 
 			if got != want {
@@ -69,10 +69,10 @@ func TestConvertingToArabic(t *testing.T) {
 
 func TestPropertiesOfConversion(t *testing.T) {
 	assertion := func(arabic uint16) bool {
+		roman, err := ConvertToRoman(arabic)
 		if arabic > 3999 {
-			return true
+			return err == ErrNumberOutOfRange
 		}
-		roman := ConvertToRoman(arabic)
 		fromRoman := ConvertToArabic(roman)
 		return fromRoman == arabic
 	}