unicode/utf8: add AppendRune

AppendRune appends the UTF-8 encoding of a rune to a []byte.
It is a generally more user friendly than EncodeRune.

    EncodeASCIIRune-4     2.35ns ± 2%
    EncodeJapaneseRune-4  4.60ns ± 2%
    AppendASCIIRune-4     0.30ns ± 3%
    AppendJapaneseRune-4  4.70ns ± 2%

The ASCII case is written to be inlineable.

Fixes #47609

Change-Id: If4f71eedffd2bd4ef0d7f960cb55b41c637eec54
Reviewed-on: https://go-review.googlesource.com/c/go/+/345571
Trust: Joe Tsai <joetsai@digital-static.net>
Reviewed-by: Rob Pike <r@golang.org>
Run-TryBot: Rob Pike <r@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>

src/cmd/compile/internal/test/inl_test.go

@@ -122,6 +122,7 @@ func TestIntendedInlining(t *testing.T) {
 			"FullRune",
 			"FullRuneInString",
 			"RuneLen",
+			"AppendRune",
 			"ValidRune",
 		},
 		"reflect": {

src/unicode/utf8/utf8.go

@@ -369,6 +369,32 @@ func EncodeRune(p []byte, r rune) int {
 	}
 }
 
+// AppendRune appends the UTF-8 encoding of r to the end of p and
+// returns the extended buffer. If the rune is out of range,
+// it appends the encoding of RuneError.
+func AppendRune(p []byte, r rune) []byte {
+	// This function is inlineable for fast handling of ASCII.
+	if uint32(r) <= rune1Max {
+		return append(p, byte(r))
+	}
+	return appendRuneNonASCII(p, r)
+}
+
+func appendRuneNonASCII(p []byte, r rune) []byte {
+	// Negative values are erroneous. Making it unsigned addresses the problem.
+	switch i := uint32(r); {
+	case i <= rune2Max:
+		return append(p, t2|byte(r>>6), tx|byte(r)&maskx)
+	case i > MaxRune, surrogateMin <= i && i <= surrogateMax:
+		r = RuneError
+		fallthrough
+	case i <= rune3Max:
+		return append(p, t3|byte(r>>12), tx|byte(r>>6)&maskx, tx|byte(r)&maskx)
+	default:
+		return append(p, t4|byte(r>>18), tx|byte(r>>12)&maskx, tx|byte(r>>6)&maskx, tx|byte(r)&maskx)
+	}
+}
+
 // RuneCount returns the number of runes in p. Erroneous and short
 // encodings are treated as single runes of width 1 byte.
 func RuneCount(p []byte) int {

src/unicode/utf8/utf8_test.go

@@ -127,6 +127,17 @@ func TestEncodeRune(t *testing.T) {
 	}
 }
 
+func TestAppendRune(t *testing.T) {
+	for _, m := range utf8map {
+		if buf := AppendRune(nil, m.r); string(buf) != m.str {
+			t.Errorf("AppendRune(nil, %#04x) = %s, want %s", m.r, buf, m.str)
+		}
+		if buf := AppendRune([]byte("init"), m.r); string(buf) != "init"+m.str {
+			t.Errorf("AppendRune(nil, %#04x) = %s, want %s", m.r, buf, "init"+m.str)
+		}
+	}
+}
+
 func TestDecodeRune(t *testing.T) {
 	for _, m := range utf8map {
 		b := []byte(m.str)
@@ -583,6 +594,20 @@ func BenchmarkEncodeJapaneseRune(b *testing.B) {
 	}
 }
 
+func BenchmarkAppendASCIIRune(b *testing.B) {
+	buf := make([]byte, UTFMax)
+	for i := 0; i < b.N; i++ {
+		AppendRune(buf[:0], 'a')
+	}
+}
+
+func BenchmarkAppendJapaneseRune(b *testing.B) {
+	buf := make([]byte, UTFMax)
+	for i := 0; i < b.N; i++ {
+		AppendRune(buf[:0], '本')
+	}
+}
+
 func BenchmarkDecodeASCIIRune(b *testing.B) {
 	a := []byte{'a'}
 	for i := 0; i < b.N; i++ {