Parse: do prefix check before len check

prettyuuid.go

@@ -80,14 +80,14 @@ func (f *Format) Format(uuid [16]byte) string {
 
 // Parse converts a pretty string to a UUID.
 func (f *Format) Parse(s string) ([16]byte, error) {
-	if len(s) != f.len() {
-		return [16]byte{}, fmt.Errorf("%q does not have expected length %v", s, f.len())
-	}
-
 	if !strings.HasPrefix(s, f.prefix) {
 		return [16]byte{}, fmt.Errorf("%q does not have expected prefix %q", s, f.prefix)
 	}
 
+	if len(s) != f.len() {
+		return [16]byte{}, fmt.Errorf("%q does not have expected length %v", s, f.len())
+	}
+
 	var n big.Int
 	for i := len(f.prefix); i < len(s); i++ {
 		d := strings.IndexByte(f.alphabet, s[i])

prettyuuid_test.go

@@ -75,6 +75,12 @@ func TestFormat_Parse(t *testing.T) {
 			ID:      "0000000000000000000000000000000x",
 			WantErr: `"0000000000000000000000000000000x" contains illegal char at position 31`,
 		},
+		{
+			Name:    "bad length and bad prefix",
+			Format:  prettyuuid.MustNewFormat("prefix_", "0123456789abcdef"),
+			ID:      "notprefix_00000000000000000000000000000000x",
+			WantErr: `"notprefix_00000000000000000000000000000000x" does not have expected prefix "prefix_"`,
+		},
 	}
 
 	for _, tt := range testCases {