Avoid using strings.Split (#1501)

* Avoid using strings.Split

strings.Split has to allocate for the return slice. This allocation
was wasteful in ever case it was used in this library.

Instead we use the new strings.Cut and other string manipulation where
appropriate. This tends to lead to cleaner and more readable code in
addition to the benefits this has on the garbage collector.

* Further simplify structTag in the msg_generate.go

This doesn't need to call strings.TrimPrefix twice.

dnssec_keyscan.go

@@ -37,7 +37,8 @@ func (k *DNSKEY) ReadPrivateKey(q io.Reader, file string) (crypto.PrivateKey, er
 		return nil, ErrPrivKey
 	}
 	// TODO(mg): check if the pubkey matches the private key
-	algo, err := strconv.ParseUint(strings.SplitN(m["algorithm"], " ", 2)[0], 10, 8)
+	algoStr, _, _ := strings.Cut(m["algorithm"], " ")
+	algo, err := strconv.ParseUint(algoStr, 10, 8)
 	if err != nil {
 		return nil, ErrPrivKey
 	}

generate.go

@@ -35,17 +35,17 @@ func (zp *ZoneParser) generate(l lex) (RR, bool) {
 		token = token[:i]
 	}
 
-	sx := strings.SplitN(token, "-", 2)
-	if len(sx) != 2 {
+	startStr, endStr, ok := strings.Cut(token, "-")
+	if !ok {
 		return zp.setParseError("bad start-stop in $GENERATE range", l)
 	}
 
-	start, err := strconv.ParseInt(sx[0], 10, 64)
+	start, err := strconv.ParseInt(startStr, 10, 64)
 	if err != nil {
 		return zp.setParseError("bad start in $GENERATE range", l)
 	}
 
-	end, err := strconv.ParseInt(sx[1], 10, 64)
+	end, err := strconv.ParseInt(endStr, 10, 64)
 	if err != nil {
 		return zp.setParseError("bad stop in $GENERATE range", l)
 	}
@@ -54,7 +54,7 @@ func (zp *ZoneParser) generate(l lex) (RR, bool) {
 	}
 
 	// _BLANK
-	l, ok := zp.c.Next()
+	l, ok = zp.c.Next()
 	if !ok || l.value != zBlank {
 		return zp.setParseError("garbage after $GENERATE range", l)
 	}
@@ -211,15 +211,16 @@ func (r *generateReader) ReadByte() (byte, error) {
 func modToPrintf(s string) (string, int64, string) {
 	// Modifier is { offset [ ,width [ ,base ] ] } - provide default
 	// values for optional width and type, if necessary.
-	var offStr, widthStr, base string
-	switch xs := strings.Split(s, ","); len(xs) {
-	case 1:
-		offStr, widthStr, base = xs[0], "0", "d"
-	case 2:
-		offStr, widthStr, base = xs[0], xs[1], "d"
-	case 3:
-		offStr, widthStr, base = xs[0], xs[1], xs[2]
-	default:
+	offStr, s, ok0 := strings.Cut(s, ",")
+	widthStr, s, ok1 := strings.Cut(s, ",")
+	base, _, ok2 := strings.Cut(s, ",")
+	if !ok0 {
+		widthStr = "0"
+	}
+	if !ok1 {
+		base = "d"
+	}
+	if ok2 {
 		return "", 0, "bad modifier in $GENERATE"
 	}
 
@@ -234,8 +235,8 @@ func modToPrintf(s string) (string, int64, string) {
 		return "", 0, "bad offset in $GENERATE"
 	}
 
-	width, err := strconv.ParseInt(widthStr, 10, 64)
-	if err != nil || width < 0 || width > 255 {
+	width, err := strconv.ParseUint(widthStr, 10, 8)
+	if err != nil {
 		return "", 0, "bad width in $GENERATE"
 	}
 

scan.go

@@ -1216,42 +1216,34 @@ func stringToCm(token string) (e, m uint8, ok bool) {
 	if token[len(token)-1] == 'M' || token[len(token)-1] == 'm' {
 		token = token[0 : len(token)-1]
 	}
-	s := strings.SplitN(token, ".", 2)
-	var meters, cmeters, val int
-	var err error
-	switch len(s) {
-	case 2:
-		if cmeters, err = strconv.Atoi(s[1]); err != nil {
-			return
-		}
+
+	var (
+		meters, cmeters, val int
+		err                  error
+	)
+	mStr, cmStr, hasCM := strings.Cut(token, ".")
+	if hasCM {
 		// There's no point in having more than 2 digits in this part, and would rather make the implementation complicated ('123' should be treated as '12').
 		// So we simply reject it.
 		// We also make sure the first character is a digit to reject '+-' signs.
-		if len(s[1]) > 2 || s[1][0] < '0' || s[1][0] > '9' {
+		cmeters, err = strconv.Atoi(cmStr)
+		if err != nil || len(cmStr) > 2 || cmStr[0] < '0' || cmStr[0] > '9' {
 			return
 		}
-		if len(s[1]) == 1 {
+		if len(cmStr) == 1 {
 			// 'nn.1' must be treated as 'nn-meters and 10cm, not 1cm.
 			cmeters *= 10
 		}
-		if s[0] == "" {
-			// This will allow omitting the 'meter' part, like .01 (meaning 0.01m = 1cm).
-			break
-		}
-		fallthrough
-	case 1:
-		if meters, err = strconv.Atoi(s[0]); err != nil {
-			return
-		}
+	}
+	// This slighly ugly condition will allow omitting the 'meter' part, like .01 (meaning 0.01m = 1cm).
+	if !hasCM || mStr != "" {
+		meters, err = strconv.Atoi(mStr)
 		// RFC1876 states the max value is 90000000.00.  The latter two conditions enforce it.
-		if s[0][0] < '0' || s[0][0] > '9' || meters > 90000000 || (meters == 90000000 && cmeters != 0) {
+		if err != nil || mStr[0] < '0' || mStr[0] > '9' || meters > 90000000 || (meters == 90000000 && cmeters != 0) {
 			return
 		}
-	case 0:
-		// huh?
-		return 0, 0, false
 	}
-	ok = true
+
 	if meters > 0 {
 		e = 2
 		val = meters
@@ -1263,8 +1255,7 @@ func stringToCm(token string) (e, m uint8, ok bool) {
 		e++
 		val /= 10
 	}
-	m = uint8(val)
-	return
+	return e, uint8(val), true
 }
 
 func toAbsoluteName(name, origin string) (absolute string, ok bool) {

svcb.go

@@ -314,10 +314,11 @@ func (s *SVCBMandatory) unpack(b []byte) error {
 }
 
 func (s *SVCBMandatory) parse(b string) error {
-	str := strings.Split(b, ",")
-	codes := make([]SVCBKey, 0, len(str))
-	for _, e := range str {
-		codes = append(codes, svcbStringToKey(e))
+	codes := make([]SVCBKey, 0, strings.Count(b, ",")+1)
+	for len(b) > 0 {
+		var key string
+		key, b, _ = strings.Cut(b, ",")
+		codes = append(codes, svcbStringToKey(key))
 	}
 	s.Code = codes
 	return nil
@@ -613,19 +614,24 @@ func (s *SVCBIPv4Hint) String() string {
 }
 
 func (s *SVCBIPv4Hint) parse(b string) error {
+	if b == "" {
+		return errors.New("dns: svcbipv4hint: empty hint")
+	}
 	if strings.Contains(b, ":") {
 		return errors.New("dns: svcbipv4hint: expected ipv4, got ipv6")
 	}
-	str := strings.Split(b, ",")
-	dst := make([]net.IP, len(str))
-	for i, e := range str {
+
+	hint := make([]net.IP, 0, strings.Count(b, ",")+1)
+	for len(b) > 0 {
+		var e string
+		e, b, _ = strings.Cut(b, ",")
 		ip := net.ParseIP(e).To4()
 		if ip == nil {
 			return errors.New("dns: svcbipv4hint: bad ip")
 		}
-		dst[i] = ip
+		hint = append(hint, ip)
 	}
-	s.Hint = dst
+	s.Hint = hint
 	return nil
 }
 
@@ -733,19 +739,24 @@ func (s *SVCBIPv6Hint) String() string {
 }
 
 func (s *SVCBIPv6Hint) parse(b string) error {
-	str := strings.Split(b, ",")
-	dst := make([]net.IP, len(str))
-	for i, e := range str {
+	if b == "" {
+		return errors.New("dns: svcbipv6hint: empty hint")
+	}
+
+	hint := make([]net.IP, 0, strings.Count(b, ",")+1)
+	for len(b) > 0 {
+		var e string
+		e, b, _ = strings.Cut(b, ",")
 		ip := net.ParseIP(e)
 		if ip == nil {
 			return errors.New("dns: svcbipv6hint: bad ip")
 		}
 		if ip.To4() != nil {
 			return errors.New("dns: svcbipv6hint: expected ipv6, got ipv4-mapped-ipv6")
 		}
-		dst[i] = ip
+		hint = append(hint, ip)
 	}
-	s.Hint = dst
+	s.Hint = hint
 	return nil
 }