Optimize encoding path for escaped string

internal/encoder/decode_rune.go

@@ -0,0 +1,142 @@
+package encoder
+
+import "unicode/utf8"
+
+// Code points in the surrogate range are not valid for UTF-8.
+const (
+	surrogateMin = 0xD800
+	surrogateMax = 0xDFFF
+)
+
+const (
+	maskx = 63 //0b00111111
+	mask2 = 31 //0b00011111
+	mask3 = 15 //0b00001111
+	mask4 = 7  //0b00000111
+
+	rune1Max = 1<<7 - 1
+	rune2Max = 1<<11 - 1
+	rune3Max = 1<<16 - 1
+
+	// The default lowest and highest continuation byte.
+	locb = 128 //0b10000000
+	hicb = 191 //0b10111111
+
+	// These names of these constants are chosen to give nice alignment in the
+	// table below. The first nibble is an index into acceptRanges or F for
+	// special one-byte cases. The second nibble is the Rune length or the
+	// Status for the special one-byte case.
+	xx = 0xF1 // invalid: size 1
+	as = 0xF0 // ASCII: size 1
+	s1 = 0x02 // accept 0, size 2
+	s2 = 0x13 // accept 1, size 3
+	s3 = 0x03 // accept 0, size 3
+	s4 = 0x23 // accept 2, size 3
+	s5 = 0x34 // accept 3, size 4
+	s6 = 0x04 // accept 0, size 4
+	s7 = 0x44 // accept 4, size 4
+)
+
+// first is information about the first byte in a UTF-8 sequence.
+var first = [256]uint8{
+	//   1   2   3   4   5   6   7   8   9   A   B   C   D   E   F
+	as, as, as, as, as, as, as, as, as, as, as, as, as, as, as, as, // 0x00-0x0F
+	as, as, as, as, as, as, as, as, as, as, as, as, as, as, as, as, // 0x10-0x1F
+	as, as, as, as, as, as, as, as, as, as, as, as, as, as, as, as, // 0x20-0x2F
+	as, as, as, as, as, as, as, as, as, as, as, as, as, as, as, as, // 0x30-0x3F
+	as, as, as, as, as, as, as, as, as, as, as, as, as, as, as, as, // 0x40-0x4F
+	as, as, as, as, as, as, as, as, as, as, as, as, as, as, as, as, // 0x50-0x5F
+	as, as, as, as, as, as, as, as, as, as, as, as, as, as, as, as, // 0x60-0x6F
+	as, as, as, as, as, as, as, as, as, as, as, as, as, as, as, as, // 0x70-0x7F
+	//   1   2   3   4   5   6   7   8   9   A   B   C   D   E   F
+	xx, xx, xx, xx, xx, xx, xx, xx, xx, xx, xx, xx, xx, xx, xx, xx, // 0x80-0x8F
+	xx, xx, xx, xx, xx, xx, xx, xx, xx, xx, xx, xx, xx, xx, xx, xx, // 0x90-0x9F
+	xx, xx, xx, xx, xx, xx, xx, xx, xx, xx, xx, xx, xx, xx, xx, xx, // 0xA0-0xAF
+	xx, xx, xx, xx, xx, xx, xx, xx, xx, xx, xx, xx, xx, xx, xx, xx, // 0xB0-0xBF
+	xx, xx, s1, s1, s1, s1, s1, s1, s1, s1, s1, s1, s1, s1, s1, s1, // 0xC0-0xCF
+	s1, s1, s1, s1, s1, s1, s1, s1, s1, s1, s1, s1, s1, s1, s1, s1, // 0xD0-0xDF
+	s2, s3, s3, s3, s3, s3, s3, s3, s3, s3, s3, s3, s3, s4, s3, s3, // 0xE0-0xEF
+	s5, s6, s6, s6, s7, xx, xx, xx, xx, xx, xx, xx, xx, xx, xx, xx, // 0xF0-0xFF
+}
+
+// acceptRange gives the range of valid values for the second byte in a UTF-8
+// sequence.
+type acceptRange struct {
+	lo uint8 // lowest value for second byte.
+	hi uint8 // highest value for second byte.
+}
+
+var (
+	sep          = [2]byte{226, 128}
+	lineSep      = byte(168) //'\u2028'
+	paragraphSep = byte(169) //'\u2029'
+)
+
+type decodeRuneState int
+
+const (
+	validUTF8State decodeRuneState = iota
+	runeErrorState
+	lineSepState
+	paragraphSepState
+)
+
+func decodeRuneInString(s string) (decodeRuneState, int) {
+	n := len(s)
+	s0 := s[0]
+	x := first[s0]
+	if x >= as {
+		// The following code simulates an additional check for x == xx and
+		// handling the ASCII and invalid cases accordingly. This mask-and-or
+		// approach prevents an additional branch.
+		mask := rune(x) << 31 >> 31 // Create 0x0000 or 0xFFFF.
+		if rune(s[0])&^mask|utf8.RuneError&mask == utf8.RuneError {
+			return runeErrorState, 1
+		}
+		return validUTF8State, 1
+	}
+	sz := int(x & 7)
+	var accept acceptRange
+	switch x >> 4 {
+	case 0:
+		accept = acceptRange{locb, hicb}
+	case 1:
+		accept = acceptRange{0xA0, hicb}
+	case 2:
+		accept = acceptRange{locb, 0x9F}
+	case 3:
+		accept = acceptRange{0x90, hicb}
+	case 4:
+		accept = acceptRange{locb, 0x8F}
+	}
+	if n < sz {
+		return runeErrorState, 1
+	}
+	s1 := s[1]
+	if s1 < accept.lo || accept.hi < s1 {
+		return runeErrorState, 1
+	}
+	if sz <= 2 {
+		return validUTF8State, 2
+	}
+	s2 := s[2]
+	if s2 < locb || hicb < s2 {
+		return runeErrorState, 1
+	}
+	if sz <= 3 {
+		if s[0] == sep[0] && s[1] == sep[1] {
+			switch s[2] {
+			case lineSep:
+				return lineSepState, 3
+			case paragraphSep:
+				return paragraphSepState, 3
+			}
+		}
+		return validUTF8State, 3
+	}
+	s3 := s[3]
+	if s3 < locb || hicb < s3 {
+		return runeErrorState, 1
+	}
+	return validUTF8State, 4
+}

internal/encoder/string.go

@@ -3,7 +3,6 @@ package encoder
 import (
 	"math/bits"
 	"reflect"
-	"unicode/utf8"
 	"unsafe"
 )
 
@@ -489,10 +488,9 @@ ESCAPE_END:
 			i = j + 1
 			j = j + 1
 			continue
-		}
 
-		// This encodes bytes < 0x20 except for \t, \n and \r.
+		case 0x00, 0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07, 0x08, 0x0B, 0x0C, 0x0E, 0x0F, // 0x00-0x0F
-		if c < 0x20 {
+			0x10, 0x11, 0x12, 0x13, 0x14, 0x15, 0x16, 0x17, 0x18, 0x19, 0x1A, 0x1B, 0x1C, 0x1D, 0x1E, 0x1F: // 0x10-0x1F
 			buf = append(buf, s[i:j]...)
 			buf = append(buf, `\u00`...)
 			buf = append(buf, hex[c>>4], hex[c&0xF])
@@ -501,18 +499,14 @@ ESCAPE_END:
 			continue
 		}
 
-		r, size := utf8.DecodeRuneInString(s[j:])
+		state, size := decodeRuneInString(s[j:])
-
+		switch state {
-		if r == utf8.RuneError && size == 1 {
+		case runeErrorState:
 			buf = append(buf, s[i:j]...)
 			buf = append(buf, `\ufffd`...)
-			i = j + size
+			i = j + 1
-			j = j + size
+			j = j + 1
 			continue
-		}
-
-		switch r {
-		case '\u2028', '\u2029':
 			// U+2028 is LINE SEPARATOR.
 			// U+2029 is PARAGRAPH SEPARATOR.
 			// They are both technically valid characters in JSON strings,
@@ -520,14 +514,19 @@ ESCAPE_END:
 			// and can lead to security holes there. It is valid JSON to
 			// escape them, so we do so unconditionally.
 			// See http://timelessrepo.com/json-isnt-a-javascript-subset for discussion.
+		case lineSepState:
 			buf = append(buf, s[i:j]...)
-			buf = append(buf, `\u202`...)
+			buf = append(buf, `\u2028`...)
-			buf = append(buf, hex[r&0xF])
+			i = j + 3
-			i = j + size
+			j = j + 3
-			j = j + size
+			continue
+		case paragraphSepState:
+			buf = append(buf, s[i:j]...)
+			buf = append(buf, `\u2029`...)
+			i = j + 3
+			j = j + 3
 			continue
 		}
-
 		j += size
 	}
 
@@ -594,10 +593,9 @@ func appendString(buf []byte, s string) []byte {
 			i = j + 1
 			j = j + 1
 			continue
-		}
 
-		// This encodes bytes < 0x20 except for \t, \n and \r.
+		case 0x00, 0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07, 0x08, 0x0B, 0x0C, 0x0E, 0x0F, // 0x00-0x0F
-		if c < 0x20 {
+			0x10, 0x11, 0x12, 0x13, 0x14, 0x15, 0x16, 0x17, 0x18, 0x19, 0x1A, 0x1B, 0x1C, 0x1D, 0x1E, 0x1F: // 0x10-0x1F
 			buf = append(buf, s[i:j]...)
 			buf = append(buf, `\u00`...)
 			buf = append(buf, hex[c>>4], hex[c&0xF])
@@ -606,18 +604,14 @@ func appendString(buf []byte, s string) []byte {
 			continue
 		}
 
-		r, size := utf8.DecodeRuneInString(s[j:])
+		state, size := decodeRuneInString(s[j:])
-
+		switch state {
-		if r == utf8.RuneError && size == 1 {
+		case runeErrorState:
 			buf = append(buf, s[i:j]...)
 			buf = append(buf, `\ufffd`...)
-			i = j + size
+			i = j + 1
-			j = j + size
+			j = j + 1
 			continue
-		}
-
-		switch r {
-		case '\u2028', '\u2029':
 			// U+2028 is LINE SEPARATOR.
 			// U+2029 is PARAGRAPH SEPARATOR.
 			// They are both technically valid characters in JSON strings,
@@ -625,14 +619,19 @@ func appendString(buf []byte, s string) []byte {
 			// and can lead to security holes there. It is valid JSON to
 			// escape them, so we do so unconditionally.
 			// See http://timelessrepo.com/json-isnt-a-javascript-subset for discussion.
+		case lineSepState:
 			buf = append(buf, s[i:j]...)
-			buf = append(buf, `\u202`...)
+			buf = append(buf, `\u2028`...)
-			buf = append(buf, hex[r&0xF])
+			i = j + 3
-			i = j + size
+			j = j + 3
-			j = j + size
+			continue
+		case paragraphSepState:
+			buf = append(buf, s[i:j]...)
+			buf = append(buf, `\u2029`...)
+			i = j + 3
+			j = j + 3
 			continue
 		}
-
 		j += size
 	}