Optimize JSON encoding even further

Last optimization was for JSON string with no character to encode. This
version focuses on strings with some chars to encode, trying to apply
the same trick for substrings that do not need encoding.

benchmark                old ns/op     new ns/op    delta
.../NoEncoding-8         60.2          51.3         -14.78%
.../EncodingFirst-8      140           116          -17.14%
.../EncodingMiddle-8     112           86.4         -22.86%
.../EncodingLast-8       62.8          61.1         -2.71%
.../MultiBytesFirst-8    164           129          -21.34%
.../MultiBytesMiddle-8   133           96.9         -27.14%
.../MultiBytesLast-8     81.9          73.5         -10.26%

README.md

@@ -255,11 +255,11 @@ Some settings can be changed and will by applied to all loggers:
 All operations are allocation free (those numbers *include* JSON encoding):
 
 ```
-BenchmarkLogEmpty-8        100000000	    19.1 ns/op	   0 B/op       0 allocs/op
+BenchmarkLogEmpty-8        100000000    19.1 ns/op	   0 B/op       0 allocs/op
-BenchmarkDisabled-8        500000000	     4.07 ns/op	   0 B/op       0 allocs/op
+BenchmarkDisabled-8        500000000     4.07 ns/op	   0 B/op       0 allocs/op
-BenchmarkInfo-8            30000000	    46.3 ns/op	   0 B/op       0 allocs/op
+BenchmarkInfo-8            30000000	    42.5 ns/op	   0 B/op       0 allocs/op
-BenchmarkContextFields-8   30000000	    47.1 ns/op	   0 B/op       0 allocs/op
+BenchmarkContextFields-8   30000000	    44.9 ns/op	   0 B/op       0 allocs/op
-BenchmarkLogFields-8       10000000	   186 ns/op	   0 B/op       0 allocs/op
+BenchmarkLogFields-8       10000000	   184 ns/op	   0 B/op       0 allocs/op
 ```
 
 Using Uber's zap [comparison benchmark](https://github.com/uber-go/zap#performance):

json.go

@@ -21,48 +21,10 @@ func appendJSONString(dst []byte, s string) []byte {
 		// Check if the character needs encoding. Control characters, slashes,
 		// and the double quote need json encoding. Bytes above the ascii
 		// boundary needs utf8 encoding.
-		if s[i] < ' ' || s[i] == '\\' || s[i] == '"' || s[i] > 126 {
+		if s[i] < 0x20 || s[i] > 0x7e || s[i] == '\\' || s[i] == '"' {
-			// We encountered a character that needs to be encoded. Let's
+			// We encountered a character that needs to be encoded. Switch
-			// append the previous simple characters to the byte slice
+			// to complex version of the algorithm.
-			// and switch our operation to read and encode the remainder
+			dst = appendJSONStringComplex(dst, s, i)
-			// characters byte-by-byte.
-			dst = append(dst, s[:i]...)
-			for i < len(s) {
-				if b := s[i]; b < utf8.RuneSelf {
-					switch b {
-					case '"', '\\':
-						dst = append(dst, '\\', b)
-					case '\b':
-						dst = append(dst, '\\', 'b')
-					case '\f':
-						dst = append(dst, '\\', 'f')
-					case '\n':
-						dst = append(dst, '\\', 'n')
-					case '\r':
-						dst = append(dst, '\\', 'r')
-					case '\t':
-						dst = append(dst, '\\', 't')
-					default:
-						if b >= 0x20 {
-							dst = append(dst, b)
-						} else {
-							dst = append(dst, '\\', 'u', '0', '0',
-								hex[b>>4], hex[b&0xF])
-						}
-					}
-					i++
-					continue
-				}
-				r, size := utf8.DecodeRuneInString(s[i:])
-				if r == utf8.RuneError && size == 1 {
-					dst = append(dst, `\ufffd`...)
-					i++
-					continue
-				}
-				dst = append(dst, s[i:i+size]...)
-				i += size
-			}
-			// End with a double quote
 			return append(dst, '"')
 		}
 	}
@@ -72,3 +34,63 @@ func appendJSONString(dst []byte, s string) []byte {
 	// End with a double quote
 	return append(dst, '"')
 }
+
+// appendJSONStringComplex is used by appendJSONString to take over an in
+// progress JSON string encoding that encountered a character that needs
+// to be encoded.
+func appendJSONStringComplex(dst []byte, s string, i int) []byte {
+	start := 0
+	for i < len(s) {
+		b := s[i]
+		if b >= utf8.RuneSelf {
+			r, size := utf8.DecodeRuneInString(s[i:])
+			if r == utf8.RuneError && size == 1 {
+				// In case of error, first append previous simple characters to
+				// the byte slice if any and append a remplacement character code
+				// in place of the invalid sequence.
+				if start < i {
+					dst = append(dst, s[start:i]...)
+				}
+				dst = append(dst, `\ufffd`...)
+				i += size
+				start = i
+				continue
+			}
+			i += size
+			continue
+		}
+		if b >= 0x20 && b <= 0x7e && b != '\\' && b != '"' {
+			i++
+			continue
+		}
+		// We encountered a character that needs to be encoded.
+		// Let's append the previous simple characters to the byte slice
+		// and switch our operation to read and encode the remainder
+		// characters byte-by-byte.
+		if start < i {
+			dst = append(dst, s[start:i]...)
+		}
+		switch b {
+		case '"', '\\':
+			dst = append(dst, '\\', b)
+		case '\b':
+			dst = append(dst, '\\', 'b')
+		case '\f':
+			dst = append(dst, '\\', 'f')
+		case '\n':
+			dst = append(dst, '\\', 'n')
+		case '\r':
+			dst = append(dst, '\\', 'r')
+		case '\t':
+			dst = append(dst, '\\', 't')
+		default:
+			dst = append(dst, '\\', 'u', '0', '0', hex[b>>4], hex[b&0xF])
+		}
+		i++
+		start = i
+	}
+	if start < len(s) {
+		dst = append(dst, s[start:]...)
+	}
+	return dst
+}

json_test.go

@@ -43,6 +43,8 @@ func TestAppendJSONString(t *testing.T) {
 		{"\x1d", `"\u001d"`},
 		{"\x1e", `"\u001e"`},
 		{"\x1f", `"\u001f"`},
+		{"✭", `"✭"`},
+		{"foo\xc2\x7fbar", `"foo\ufffd\u007fbar"`}, // invalid sequence
 		{"ascii", `"ascii"`},
 		{"\"a", `"\"a"`},
 		{"\x1fa", `"\u001fa"`},
@@ -61,13 +63,13 @@ func TestAppendJSONString(t *testing.T) {
 
 func BenchmarkAppendJSONString(b *testing.B) {
 	tests := map[string]string{
-		"NoEncoding":     `aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa`,
+		"NoEncoding":       `aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa`,
-		"EncodingFirst":  `"aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa`,
+		"EncodingFirst":    `"aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa`,
-		"EncodingMiddle": `aaaaaaaaaaaaaaaaaaaaaaaaa"aaaaaaaaaaaaaaaaaaaaaaaa`,
+		"EncodingMiddle":   `aaaaaaaaaaaaaaaaaaaaaaaaa"aaaaaaaaaaaaaaaaaaaaaaaa`,
-		"EncodingLast":   `aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa"`,
+		"EncodingLast":     `aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa"`,
-		"RuneFirst":      `❤️aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa`,
+		"MultiBytesFirst":  `❤️aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa`,
-		"RuneMiddle":     `aaaaaaaaaaaaaaaaaaaaaaaaa❤️aaaaaaaaaaaaaaaaaaaaaaaa`,
+		"MultiBytesMiddle": `aaaaaaaaaaaaaaaaaaaaaaaaa❤️aaaaaaaaaaaaaaaaaaaaaaaa`,
-		"RuneLast":       `aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa❤️`,
+		"MultiBytesLast":   `aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa❤️`,
 	}
 	for name, str := range tests {
 		b.Run(name, func(b *testing.B) {