check for an invalid id with a repeating reserved character

CHANGELOG.md

@@ -1,3 +1,7 @@
 # CHANGELOG
 
-@todo
+**v0.2.0:**
+- Bug fix: test for decoding an invalid ID with a repeating reserved character
+
+**v0.1.0:**
+- Initial implementation of [the spec](https://github.com/sqids/sqids-spec)

encoding_test.go

@@ -213,5 +213,16 @@ func TestEncodingInvalidCharacter(t *testing.T) {
 	}
 }
 
+func TestDecodeInvalidIDWithRepeatingReservedCharacter(t *testing.T) {
+	s, err := New()
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	if !reflect.DeepEqual(s.Decode("fff"), []uint64{}) {
+		t.Errorf("Could not decode invalid ID with repeating reserved character")
+	}
+}
+
 // TestEncodingOutOfRange - no need since `[]uint64` handles ranges
 // func TestEncodingOutOfRange(t *testing.T) {}

sqids.go

@@ -221,6 +221,15 @@ func (s *Sqids) Decode(id string) []uint64 {
 
 		if len(chunks) > 0 {
 			alphabetWithoutSeparator := alphabet[:len(alphabet)-1]
+			charSet := make(map[rune]bool)
+			for _, c := range alphabetWithoutSeparator {
+				charSet[c] = true
+			}
+			for _, c := range chunks[0] {
+				if _, exists := charSet[c]; !exists {
+					return []uint64{}
+				}
+			}
 			ret = append(ret, toNumber(chunks[0], alphabetWithoutSeparator))
 
 			if len(chunks) > 1 {