Use generics for ddd escape parsing (#1435)

* Use generics for dddToByte and dddStringToByte

* Introduce generic isDDD helper

Almost all uses of isDigit look exactly the same, turn them into a
matching helper.

defaults.go

@@ -208,7 +208,7 @@ func IsDomainName(s string) (labels int, ok bool) {
  }
 
  // check for \DDD
- if i+3 < len(s) && isDigit(s[i+1]) && isDigit(s[i+2]) && isDigit(s[i+3]) {
+ if isDDD(s[i+1:]) {
  i += 3
  begin += 3
  } else {

msg.go

@@ -252,7 +252,7 @@ loop:
  }
 
  // check for \DDD
- if i+3 < ls && isDigit(bs[i+1]) && isDigit(bs[i+2]) && isDigit(bs[i+3]) {
+ if isDDD(bs[i+1:]) {
  bs[i] = dddToByte(bs[i+1:])
  copy(bs[i+1:ls-3], bs[i+4:])
  ls -= 3
@@ -482,8 +482,8 @@ func packTxtString(s string, msg []byte, offset int) (int, error) {
  break
  }
  // check for \DDD
- if i+2 < len(s) && isDigit(s[i]) && isDigit(s[i+1]) && isDigit(s[i+2]) {
- msg[offset] = dddStringToByte(s[i:])
+ if isDDD(s[i:]) {
+ msg[offset] = dddToByte(s[i:])
  i += 2
  } else {
  msg[offset] = s[i]
@@ -517,7 +517,7 @@ func packOctetString(s string, msg []byte, offset int, tmp []byte) (int, error)
  break
  }
  // check for \DDD
- if i+2 < len(bs) && isDigit(bs[i]) && isDigit(bs[i+1]) && isDigit(bs[i+2]) {
+ if isDDD(bs[i:]) {
  msg[offset] = dddToByte(bs[i:])
  i += 2
  } else {
@@ -546,12 +546,11 @@ func unpackTxt(msg []byte, off0 int) (ss []string, off int, err error) {
 // Helpers for dealing with escaped bytes
 func isDigit(b byte) bool { return b >= '0' && b <= '9' }
 
-func dddToByte(s []byte) byte {
- _ = s[2] // bounds check hint to compiler; see golang.org/issue/14808
- return byte((s[0]-'0')*100 + (s[1]-'0')*10 + (s[2] - '0'))
+func isDDD[T ~[]byte | ~string](s T) bool {
+ return len(s) >= 3 && isDigit(s[0]) && isDigit(s[1]) && isDigit(s[2])
 }
 
-func dddStringToByte(s string) byte {
+func dddToByte[T ~[]byte | ~string](s T) byte {
  _ = s[2] // bounds check hint to compiler; see golang.org/issue/14808
  return byte((s[0]-'0')*100 + (s[1]-'0')*10 + (s[2] - '0'))
 }
@@ -1019,7 +1018,7 @@ func escapedNameLen(s string) int {
  continue
  }
 
- if i+3 < len(s) && isDigit(s[i+1]) && isDigit(s[i+2]) && isDigit(s[i+3]) {
+ if isDDD(s[i+1:]) {
  nameLen -= 3
  i += 3
  } else {

types.go

@@ -631,8 +631,8 @@ func nextByte(s string, offset int) (byte, int) {
  return 0, 0
  case 2, 3: // too short to be \ddd
  default: // maybe \ddd
- if isDigit(s[offset+1]) && isDigit(s[offset+2]) && isDigit(s[offset+3]) {
- return dddStringToByte(s[offset+1:]), 4
+ if isDDD(s[offset+1:]) {
+ return dddToByte(s[offset+1:]), 4
  }
  }
  // not \ddd, just an RFC 1035 "quoted" character