Optimize unmarshal type handling

Dispatch type as a byte, instead of allocating a string

Before
'''
14433 ns/op          2576 B/op      108 allocs/op
'''

After
'''
13616 ns/op          2512 B/op       83 allocs/op
'''

Author: Sean-Der

Diff for: base_lexer.go

@@ -178,45 +178,27 @@ func (l *baseLexer) readLine() (string, error) {
 	}
 }
 
-func (l *baseLexer) readString(until byte) (string, error) {
-	start := l.pos
+func (l *baseLexer) readType() (byte, error) {
 	for {
-		ch, err := l.readByte()
+		firstByte, err := l.readByte()
 		if err != nil {
-			return "", err
-		}
-		if ch == until {
-			return string(l.value[start:l.pos]), nil
+			return 0, err
 		}
-	}
-}
 
-func (l *baseLexer) readType() (string, error) {
-	for {
-		b, err := l.readByte()
-		if err != nil {
-			return "", err
-		}
-
-		if isNewline(b) {
+		if isNewline(firstByte) {
 			continue
 		}
 
-		err = l.unreadByte()
-		if err != nil {
-			return "", err
-		}
-
-		key, err := l.readString('=')
+		secondByte, err := l.readByte()
 		if err != nil {
-			return key, err
+			return 0, err
 		}
 
-		if len(key) == 2 {
-			return key, nil
+		if secondByte != '=' {
+			return firstByte, l.syntaxError()
 		}
 
-		return key, l.syntaxError()
+		return firstByte, nil
 	}
 }
 

Diff for: unmarshal.go

@@ -114,260 +114,260 @@ func (s *SessionDescription) Unmarshal(value []byte) error {
 }
 
 func s1(l *lexer) (stateFn, error) {
-	return l.handleType(func(key string) stateFn {
-		if key == "v=" {
+	return l.handleType(func(key byte) stateFn {
+		if key == 'v' {
 			return unmarshalProtocolVersion
 		}
 		return nil
 	})
 }
 
 func s2(l *lexer) (stateFn, error) {
-	return l.handleType(func(key string) stateFn {
-		if key == "o=" {
+	return l.handleType(func(key byte) stateFn {
+		if key == 'o' {
 			return unmarshalOrigin
 		}
 		return nil
 	})
 }
 
 func s3(l *lexer) (stateFn, error) {
-	return l.handleType(func(key string) stateFn {
-		if key == "s=" {
+	return l.handleType(func(key byte) stateFn {
+		if key == 's' {
 			return unmarshalSessionName
 		}
 		return nil
 	})
 }
 
 func s4(l *lexer) (stateFn, error) {
-	return l.handleType(func(key string) stateFn {
+	return l.handleType(func(key byte) stateFn {
 		switch key {
-		case "i=":
+		case 'i':
 			return unmarshalSessionInformation
-		case "u=":
+		case 'u':
 			return unmarshalURI
-		case "e=":
+		case 'e':
 			return unmarshalEmail
-		case "p=":
+		case 'p':
 			return unmarshalPhone
-		case "c=":
+		case 'c':
 			return unmarshalSessionConnectionInformation
-		case "b=":
+		case 'b':
 			return unmarshalSessionBandwidth
-		case "t=":
+		case 't':
 			return unmarshalTiming
 		}
 		return nil
 	})
 }
 
 func s5(l *lexer) (stateFn, error) {
-	return l.handleType(func(key string) stateFn {
+	return l.handleType(func(key byte) stateFn {
 		switch key {
-		case "b=":
+		case 'b':
 			return unmarshalSessionBandwidth
-		case "t=":
+		case 't':
 			return unmarshalTiming
 		}
 		return nil
 	})
 }
 
 func s6(l *lexer) (stateFn, error) {
-	return l.handleType(func(key string) stateFn {
+	return l.handleType(func(key byte) stateFn {
 		switch key {
-		case "p=":
+		case 'p':
 			return unmarshalPhone
-		case "c=":
+		case 'c':
 			return unmarshalSessionConnectionInformation
-		case "b=":
+		case 'b':
 			return unmarshalSessionBandwidth
-		case "t=":
+		case 't':
 			return unmarshalTiming
 		}
 		return nil
 	})
 }
 
 func s7(l *lexer) (stateFn, error) {
-	return l.handleType(func(key string) stateFn {
+	return l.handleType(func(key byte) stateFn {
 		switch key {
-		case "u=":
+		case 'u':
 			return unmarshalURI
-		case "e=":
+		case 'e':
 			return unmarshalEmail
-		case "p=":
+		case 'p':
 			return unmarshalPhone
-		case "c=":
+		case 'c':
 			return unmarshalSessionConnectionInformation
-		case "b=":
+		case 'b':
 			return unmarshalSessionBandwidth
-		case "t=":
+		case 't':
 			return unmarshalTiming
 		}
 		return nil
 	})
 }
 
 func s8(l *lexer) (stateFn, error) {
-	return l.handleType(func(key string) stateFn {
+	return l.handleType(func(key byte) stateFn {
 		switch key {
-		case "c=":
+		case 'c':
 			return unmarshalSessionConnectionInformation
-		case "b=":
+		case 'b':
 			return unmarshalSessionBandwidth
-		case "t=":
+		case 't':
 			return unmarshalTiming
 		}
 		return nil
 	})
 }
 
 func s9(l *lexer) (stateFn, error) {
-	return l.handleType(func(key string) stateFn {
+	return l.handleType(func(key byte) stateFn {
 		switch key {
-		case "z=":
+		case 'z':
 			return unmarshalTimeZones
-		case "k=":
+		case 'k':
 			return unmarshalSessionEncryptionKey
-		case "a=":
+		case 'a':
 			return unmarshalSessionAttribute
-		case "r=":
+		case 'r':
 			return unmarshalRepeatTimes
-		case "t=":
+		case 't':
 			return unmarshalTiming
-		case "m=":
+		case 'm':
 			return unmarshalMediaDescription
 		}
 		return nil
 	})
 }
 
 func s10(l *lexer) (stateFn, error) {
-	return l.handleType(func(key string) stateFn {
+	return l.handleType(func(key byte) stateFn {
 		switch key {
-		case "e=":
+		case 'e':
 			return unmarshalEmail
-		case "p=":
+		case 'p':
 			return unmarshalPhone
-		case "c=":
+		case 'c':
 			return unmarshalSessionConnectionInformation
-		case "b=":
+		case 'b':
 			return unmarshalSessionBandwidth
-		case "t=":
+		case 't':
 			return unmarshalTiming
 		}
 		return nil
 	})
 }
 
 func s11(l *lexer) (stateFn, error) {
-	return l.handleType(func(key string) stateFn {
+	return l.handleType(func(key byte) stateFn {
 		switch key {
-		case "a=":
+		case 'a':
 			return unmarshalSessionAttribute
-		case "m=":
+		case 'm':
 			return unmarshalMediaDescription
 		}
 		return nil
 	})
 }
 
 func s12(l *lexer) (stateFn, error) {
-	return l.handleType(func(key string) stateFn {
+	return l.handleType(func(key byte) stateFn {
 		switch key {
-		case "a=":
+		case 'a':
 			return unmarshalMediaAttribute
-		case "k=":
+		case 'k':
 			return unmarshalMediaEncryptionKey
-		case "b=":
+		case 'b':
 			return unmarshalMediaBandwidth
-		case "c=":
+		case 'c':
 			return unmarshalMediaConnectionInformation
-		case "i=":
+		case 'i':
 			return unmarshalMediaTitle
-		case "m=":
+		case 'm':
 			return unmarshalMediaDescription
 		}
 		return nil
 	})
 }
 
 func s13(l *lexer) (stateFn, error) {
-	return l.handleType(func(key string) stateFn {
+	return l.handleType(func(key byte) stateFn {
 		switch key {
-		case "a=":
+		case 'a':
 			return unmarshalSessionAttribute
-		case "k=":
+		case 'k':
 			return unmarshalSessionEncryptionKey
-		case "m=":
+		case 'm':
 			return unmarshalMediaDescription
 		}
 		return nil
 	})
 }
 
 func s14(l *lexer) (stateFn, error) {
-	return l.handleType(func(key string) stateFn {
+	return l.handleType(func(key byte) stateFn {
 		switch key {
-		case "a=":
+		case 'a':
 			return unmarshalMediaAttribute
-		case "k=":
+		case 'k':
 			// Non-spec ordering
 			return unmarshalMediaEncryptionKey
-		case "b=":
+		case 'b':
 			// Non-spec ordering
 			return unmarshalMediaBandwidth
-		case "c=":
+		case 'c':
 			// Non-spec ordering
 			return unmarshalMediaConnectionInformation
-		case "i=":
+		case 'i':
 			// Non-spec ordering
 			return unmarshalMediaTitle
-		case "m=":
+		case 'm':
 			return unmarshalMediaDescription
 		}
 		return nil
 	})
 }
 
 func s15(l *lexer) (stateFn, error) {
-	return l.handleType(func(key string) stateFn {
+	return l.handleType(func(key byte) stateFn {
 		switch key {
-		case "a=":
+		case 'a':
 			return unmarshalMediaAttribute
-		case "k=":
+		case 'k':
 			return unmarshalMediaEncryptionKey
-		case "b=":
+		case 'b':
 			return unmarshalMediaBandwidth
-		case "c=":
+		case 'c':
 			return unmarshalMediaConnectionInformation
-		case "i=":
+		case 'i':
 			// Non-spec ordering
 			return unmarshalMediaTitle
-		case "m=":
+		case 'm':
 			return unmarshalMediaDescription
 		}
 		return nil
 	})
 }
 
 func s16(l *lexer) (stateFn, error) {
-	return l.handleType(func(key string) stateFn {
+	return l.handleType(func(key byte) stateFn {
 		switch key {
-		case "a=":
+		case 'a':
 			return unmarshalMediaAttribute
-		case "k=":
+		case 'k':
 			return unmarshalMediaEncryptionKey
-		case "c=":
+		case 'c':
 			return unmarshalMediaConnectionInformation
-		case "b=":
+		case 'b':
 			return unmarshalMediaBandwidth
-		case "i=":
+		case 'i':
 			// Non-spec ordering
 			return unmarshalMediaTitle
-		case "m=":
+		case 'm':
 			return unmarshalMediaDescription
 		}
 		return nil

Diff for: util.go

@@ -315,11 +315,11 @@ type lexer struct {
 	baseLexer
 }
 
-type keyToState func(key string) stateFn
+type keyToState func(key byte) stateFn
 
 func (l *lexer) handleType(fn keyToState) (stateFn, error) {
 	key, err := l.readType()
-	if errors.Is(err, io.EOF) && key == "" {
+	if errors.Is(err, io.EOF) && key == 0 {
 		return nil, nil //nolint:nilnil
 	} else if err != nil {
 		return nil, err