unicode/utf16: add func RuneLen

This CL adds func RuneLen, while here, also uses RuneLen to simplify
code in Encode.

Fixes #44940

Change-Id: Ifd3b537f69880dfd32a69a6733d8d3c2b5d4ecba
Reviewed-on: https://go-review.googlesource.com/c/go/+/569755
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Commit-Queue: Ian Lance Taylor <iant@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>

api/next/44940.txt

@@ -0,0 +1 @@
+pkg unicode/utf16, func RuneLen(int32) int #44940

doc/next/6-stdlib/99-minor/unicode/utf16/44940.md

@@ -0,0 +1,3 @@
+The [`unicode/utf16.RuneLen`](/pkg/unicode/utf16#RuneLen) function returns
+the number of 16-bit words in the UTF-16 encoding of the rune. It returns -1
+if the rune is not a valid value to encode in UTF-16.

src/unicode/utf16/export_test.go

@@ -6,6 +6,9 @@ package utf16
 
 // Extra names for constants so we can validate them during testing.
 const (
+	Surr1           = surr1
+	Surr3           = surr3
+	SurrSelf        = surrSelf
 	MaxRune         = maxRune
 	ReplacementChar = replacementChar
 )

src/unicode/utf16/utf16.go

@@ -52,6 +52,19 @@ func EncodeRune(r rune) (r1, r2 rune) {
 	return surr1 + (r>>10)&0x3ff, surr2 + r&0x3ff
 }
 
+// RuneLen returns the number of 16-bit words in the UTF-16 encoding of the rune.
+// It returns -1 if the rune is not a valid value to encode in UTF-16.
+func RuneLen(r rune) int {
+	switch {
+	case 0 <= r && r < surr1, surr3 <= r && r < surrSelf:
+		return 1
+	case surrSelf <= r && r <= maxRune:
+		return 2
+	default:
+		return -1
+	}
+}
+
 // Encode returns the UTF-16 encoding of the Unicode code point sequence s.
 func Encode(s []rune) []uint16 {
 	n := len(s)
@@ -64,13 +77,11 @@ func Encode(s []rune) []uint16 {
 	a := make([]uint16, n)
 	n = 0
 	for _, v := range s {
-		switch {
-		case 0 <= v && v < surr1, surr3 <= v && v < surrSelf:
-			// normal rune
+		switch RuneLen(v) {
+		case 1: // normal rune
 			a[n] = uint16(v)
 			n++
-		case surrSelf <= v && v <= maxRune:
-			// needs surrogate sequence
+		case 2: // needs surrogate sequence
 			r1, r2 := EncodeRune(v)
 			a[n] = uint16(r1)
 			a[n+1] = uint16(r2)

src/unicode/utf16/utf16_test.go

@@ -22,6 +22,26 @@ func TestConstants(t *testing.T) {
 	}
 }
 
+func TestRuneLen(t *testing.T) {
+	for _, tt := range []struct {
+		r      rune
+		length int
+	}{
+		{0, 1},
+		{Surr1 - 1, 1},
+		{Surr3, 1},
+		{SurrSelf - 1, 1},
+		{SurrSelf, 2},
+		{MaxRune, 2},
+		{MaxRune + 1, -1},
+		{-1, -1},
+	} {
+		if length := RuneLen(tt.r); length != tt.length {
+			t.Errorf("RuneLen(%#U) = %d, want %d", tt.r, length, tt.length)
+		}
+	}
+}
+
 type encodeTest struct {
 	in  []rune
 	out []uint16