Some code improvements

* Rewrite empty string checks more idiomatically.
* Change strings.ToLower comparisons to strings.EqualFold.
* Rewrite switch statement with only one case as if.

app/dns/server.go

@@ -42,7 +42,7 @@ func New(ctx context.Context, config *Config) (*Server, error) {
 		clients: make([]Client, 0, len(config.NameServers)+len(config.NameServer)),
 		tag:     config.Tag,
 	}
-	if len(server.tag) == 0 {
+	if server.tag == "" {
 		server.tag = generateRandomTag()
 	}
 	if len(config.ClientIp) > 0 {
@@ -196,7 +196,7 @@ func toNetIP(ips []net.Address) []net.IP {
 }
 
 func (s *Server) lookupIPInternal(domain string, option IPOption) ([]net.IP, error) {
-	if len(domain) == 0 {
+	if domain == "" {
 		return nil, newError("empty domain name")
 	}
 

app/proxyman/inbound/inbound.go

@@ -68,7 +68,7 @@ func (m *Manager) GetHandler(ctx context.Context, tag string) (inbound.Handler,
 
 // RemoveHandler implements inbound.Manager.
 func (m *Manager) RemoveHandler(ctx context.Context, tag string) error {
-	if len(tag) == 0 {
+	if tag == "" {
 		return common.ErrNoClue
 	}
 

app/proxyman/outbound/outbound.go

@@ -123,7 +123,7 @@ func (m *Manager) AddHandler(ctx context.Context, handler outbound.Handler) erro
 
 // RemoveHandler implements outbound.Manager.
 func (m *Manager) RemoveHandler(ctx context.Context, tag string) error {
-	if len(tag) == 0 {
+	if tag == "" {
 		return common.ErrNoClue
 	}
 	m.access.Lock()

app/reverse/bridge.go

@@ -27,10 +27,10 @@ type Bridge struct {
 
 // NewBridge creates a new Bridge instance.
 func NewBridge(config *BridgeConfig, dispatcher routing.Dispatcher) (*Bridge, error) {
-	if len(config.Tag) == 0 {
+	if config.Tag == "" {
 		return nil, newError("bridge tag is empty")
 	}
-	if len(config.Domain) == 0 {
+	if config.Domain == "" {
 		return nil, newError("bridge domain is empty")
 	}
 

app/reverse/portal.go

@@ -28,11 +28,11 @@ type Portal struct {
 }
 
 func NewPortal(config *PortalConfig, ohm outbound.Manager) (*Portal, error) {
-	if len(config.Tag) == 0 {
+	if config.Tag == "" {
 		return nil, newError("portal tag is empty")
 	}
 
-	if len(config.Domain) == 0 {
+	if config.Domain == "" {
 		return nil, newError("portal domain is empty")
 	}
 

app/router/balancing.go

@@ -39,7 +39,7 @@ func (b *Balancer) PickOutbound() (string, error) {
 		return "", newError("no available outbounds selected")
 	}
 	tag := b.strategy.PickOutbound(tags)
-	if len(tag) == 0 {
+	if tag == "" {
 		return "", newError("balancing strategy returns empty tag")
 	}
 	return tag, nil

common/protocol/http/headers.go

@@ -11,7 +11,7 @@ import (
 // ParseXForwardedFor parses X-Forwarded-For header in http headers, and return the IP list in it.
 func ParseXForwardedFor(header http.Header) []net.Address {
 	xff := header.Get("X-Forwarded-For")
-	if len(xff) == 0 {
+	if xff == "" {
 		return nil
 	}
 	list := strings.Split(xff, ",")
@@ -38,7 +38,7 @@ func RemoveHopByHopHeaders(header http.Header) {
 
 	connections := header.Get("Connection")
 	header.Del("Connection")
-	if len(connections) == 0 {
+	if connections == "" {
 		return
 	}
 	for _, h := range strings.Split(connections, ",") {

common/protocol/http/sniff.go

@@ -44,7 +44,7 @@ var (
 
 func beginWithHTTPMethod(b []byte) error {
 	for _, m := range &methods {
-		if len(b) >= len(m) && strings.ToLower(string(b[:len(m)])) == m {
+		if len(b) >= len(m) && strings.EqualFold(string(b[:len(m)]), m) {
 			return nil
 		}
 

common/protocol/tls/sniff.go

@@ -81,8 +81,7 @@ func ReadClientHello(data []byte, h *SniffHeader) error {
 			return errNotClientHello
 		}
 
-		switch extension {
+		if extension == 0x00 { /* extensionServerName */
-		case 0x00: /* extensionServerName */
 			d := data[:length]
 			if len(d) < 2 {
 				return errNotClientHello

common/strmatcher/domain_matcher.go

@@ -51,7 +51,7 @@ func (g *DomainMatcherGroup) addMatcher(m domainMatcher, value uint32) {
 }
 
 func (g *DomainMatcherGroup) Match(domain string) uint32 {
-	if len(domain) == 0 {
+	if domain == "" {
 		return 0
 	}
 

infra/conf/api.go

@@ -16,7 +16,7 @@ type ApiConfig struct {
 }
 
 func (c *ApiConfig) Build() (*commander.Config, error) {
-	if len(c.Tag) == 0 {
+	if c.Tag == "" {
 		return nil, newError("Api tag can't be empty.")
 	}
 

infra/conf/router.go

@@ -23,7 +23,7 @@ type BalancingRule struct {
 }
 
 func (r *BalancingRule) Build() (*router.BalancingRule, error) {
-	if len(r.Tag) == 0 {
+	if r.Tag == "" {
 		return nil, newError("empty balancer tag")
 	}
 	if len(r.Selectors) == 0 {

infra/conf/shadowsocks.go

@@ -46,7 +46,7 @@ func (v *ShadowsocksServerConfig) Build() (proto.Message, error) {
 	config.UdpEnabled = v.UDP
 	config.Network = v.NetworkList.Build()
 
-	if len(v.Password) == 0 {
+	if v.Password == "" {
 		return nil, newError("Shadowsocks password is not specified.")
 	}
 	account := &shadowsocks.Account{
@@ -103,7 +103,7 @@ func (v *ShadowsocksClientConfig) Build() (proto.Message, error) {
 		if server.Port == 0 {
 			return nil, newError("Invalid Shadowsocks port.")
 		}
-		if len(server.Password) == 0 {
+		if server.Password == "" {
 			return nil, newError("Shadowsocks password is not specified.")
 		}
 		account := &shadowsocks.Account{

infra/conf/transport_internet.go

@@ -134,7 +134,7 @@ type WebSocketConfig struct {
 // Build implements Buildable.
 func (c *WebSocketConfig) Build() (proto.Message, error) {
 	path := c.Path
-	if len(path) == 0 && len(c.Path2) > 0 {
+	if path == "" && c.Path2 != "" {
 		path = c.Path2
 	}
 	header := make([]*websocket.Header, 0, 32)
@@ -380,7 +380,7 @@ func (c *StreamConfig) Build() (*internet.StreamConfig, error) {
 		}
 		config.ProtocolName = protocol
 	}
-	if strings.ToLower(c.Security) == "tls" {
+	if strings.EqualFold(c.Security, "tls") {
 		tlsSettings := c.TLSSettings
 		if tlsSettings == nil {
 			tlsSettings = &TLSConfig{}
@@ -469,7 +469,7 @@ type ProxyConfig struct {
 
 // Build implements Buildable.
 func (v *ProxyConfig) Build() (*internet.ProxyConfig, error) {
-	if len(v.Tag) == 0 {
+	if v.Tag == "" {
 		return nil, newError("Proxy tag is not set.")
 	}
 	return &internet.ProxyConfig{

infra/control/cert.go

@@ -21,7 +21,7 @@ func (l *stringList) String() string {
 }
 
 func (l *stringList) Set(v string) error {
-	if len(v) == 0 {
+	if v == "" {
 		return newError("empty value")
 	}
 	*l = append(*l, v)

infra/control/command.go

@@ -22,7 +22,7 @@ var (
 
 func RegisterCommand(cmd Command) error {
 	entry := strings.ToLower(cmd.Name())
-	if len(entry) == 0 {
+	if entry == "" {
 		return newError("empty command name")
 	}
 	commandRegistry[entry] = cmd

infra/control/verify.go

@@ -128,11 +128,11 @@ func (c *VerifyCommand) Execute(args []string) error {
 	}
 
 	target := fs.Arg(0)
-	if len(target) == 0 {
+	if target == "" {
 		return newError("empty file path.")
 	}
 
-	if len(*sigFile) == 0 {
+	if *sigFile == "" {
 		*sigFile = target + ".sig"
 	}
 

proxy/http/server.go

@@ -120,11 +120,11 @@ Start:
 	}
 
 	defaultPort := net.Port(80)
-	if strings.ToLower(request.URL.Scheme) == "https" {
+	if strings.EqualFold(request.URL.Scheme, "https") {
 		defaultPort = net.Port(443)
 	}
 	host := request.Host
-	if len(host) == 0 {
+	if host == "" {
 		host = request.URL.Host
 	}
 	dest, err := http_proto.ParseHost(host, defaultPort)
@@ -137,7 +137,7 @@ Start:
 		Status: log.AccessAccepted,
 	})
 
-	if strings.ToUpper(request.Method) == "CONNECT" {
+	if strings.EqualFold(request.Method, "CONNECT") {
 		return s.handleConnect(ctx, request, reader, conn, dest, dispatcher)
 	}
 
@@ -211,7 +211,7 @@ func (s *Server) handleConnect(ctx context.Context, request *http.Request, reade
 var errWaitAnother = newError("keep alive")
 
 func (s *Server) handlePlainHTTP(ctx context.Context, request *http.Request, writer io.Writer, dest net.Destination, dispatcher routing.Dispatcher) error {
-	if !s.config.AllowTransparent && len(request.URL.Host) <= 0 {
+	if !s.config.AllowTransparent && request.URL.Host == "" {
 		// RFC 2068 (HTTP/1.1) requires URL to be absolute URL in HTTP proxy.
 		response := &http.Response{
 			Status:        "Bad Request",
@@ -235,7 +235,7 @@ func (s *Server) handlePlainHTTP(ctx context.Context, request *http.Request, wri
 	http_proto.RemoveHopByHopHeaders(request.Header)
 
 	// Prevent UA from being set to golang's default ones
-	if len(request.Header.Get("User-Agent")) == 0 {
+	if request.Header.Get("User-Agent") == "" {
 		request.Header.Set("User-Agent", "")
 	}
 

proxy/vmess/inbound/inbound.go

@@ -167,7 +167,7 @@ func (h *Handler) AddUser(ctx context.Context, user *protocol.MemoryUser) error
 }
 
 func (h *Handler) RemoveUser(ctx context.Context, email string) error {
-	if len(email) == 0 {
+	if email == "" {
 		return newError("Email must not be empty.")
 	}
 	if !h.usersByEmail.Remove(email) {

proxy/vmess/validator.go

@@ -149,7 +149,7 @@ func (v *TimedUserValidator) Remove(email string) bool {
 	email = strings.ToLower(email)
 	idx := -1
 	for i, u := range v.users {
-		if strings.ToLower(u.user.Email) == email {
+		if strings.EqualFold(u.user.Email, email) {
 			idx = i
 			break
 		}

transport/internet/domainsocket/config.go

@@ -12,7 +12,7 @@ const protocolName = "domainsocket"
 
 func (c *Config) GetUnixAddr() (*net.UnixAddr, error) {
 	path := c.Path
-	if len(path) == 0 {
+	if path == "" {
 		return nil, newError("empty domain socket path")
 	}
 	if c.Abstract && path[0] != '\x00' {

transport/internet/headers/http/config.go

@@ -57,7 +57,7 @@ func (v *RequestConfig) GetFullVersion() string {
 func (v *ResponseConfig) HasHeader(header string) bool {
 	cHeader := strings.ToLower(header)
 	for _, tHeader := range v.Header {
-		if strings.ToLower(tHeader.Name) == cHeader {
+		if strings.EqualFold(tHeader.Name, cHeader) {
 			return true
 		}
 	}

transport/internet/http/config.go

@@ -33,7 +33,7 @@ func (c *Config) getRandomHost() string {
 }
 
 func (c *Config) getNormalizedPath() string {
-	if len(c.Path) == 0 {
+	if c.Path == "" {
 		return "/"
 	}
 	if c.Path[0] != '/' {

transport/internet/tls/config.go

@@ -236,7 +236,7 @@ type Option func(*tls.Config)
 // WithDestination sets the server name in TLS config.
 func WithDestination(dest net.Destination) Option {
 	return func(config *tls.Config) {
-		if dest.Address.Family().IsDomain() && len(config.ServerName) == 0 {
+		if dest.Address.Family().IsDomain() && config.ServerName == "" {
 			config.ServerName = dest.Address.Domain()
 		}
 	}

transport/internet/tls/tls.go

@@ -33,7 +33,7 @@ func (c *conn) HandshakeAddress() net.Address {
 		return nil
 	}
 	state := c.Conn.ConnectionState()
-	if len(state.ServerName) == 0 {
+	if state.ServerName == "" {
 		return nil
 	}
 	return net.ParseAddress(state.ServerName)

transport/internet/websocket/config.go

@@ -13,7 +13,7 @@ const protocolName = "websocket"
 
 func (c *Config) GetNormalizedPath() string {
 	path := c.Path
-	if len(path) == 0 {
+	if path == "" {
 		return "/"
 	}
 	if path[0] != '/' {