// the Unicode replacement code point U+FFFD.
func DecodeRune(r1, r2 rune) rune {
	if surr1 <= r1 && r1 < surr2 && surr2 <= r2 && r2 < surr3 {
		return (r1-surr1)<<10 | (r2 - surr2) + surrSelf
	}
	return replacementChar
}

// EncodeRune returns the UTF-16 surrogate pair r1, r2 for the given rune.
// If the rune is not a valid Unicode code point or does not need encoding,
// EncodeRune returns U+FFFD, U+FFFD.

func decodeWTF16(s []uint16, buf []byte) []byte {
	for i := 0; i < len(s); i++ {
		var ar rune
		switch r := s[i]; {
		case r < surr1, surr3 <= r:
			// normal rune
			ar = rune(r)
		case surr1 <= r && r < surr2 && i+1 < len(s) &&
			surr2 <= s[i+1] && s[i+1] < surr3:
			// valid surrogate sequence
			ar = utf16.DecodeRune(rune(r), rune(s[i+1]))
			i++
		default:
			// WTF-8 fallback.

	{'\U0010FFFD', false}, // PRIVATE USE CHARACTER-10FFFD (last Unicode code point)

	{rune(0xd7ff), false}, // surr1-1
	{rune(0xd800), true},  // surr1
	{rune(0xdc00), true},  // surr2
	{rune(0xe000), false}, // surr3
	{rune(0xdfff), true},  // surr3-1
}

func TestIsSurrogate(t *testing.T) {
	for i, tt := range surrogateTests {
		got := IsSurrogate(tt.r)
		if got != tt.want {

// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

package utf16

// Extra names for constants so we can validate them during testing.
const (
	Surr1           = surr1
	Surr3           = surr3
	SurrSelf        = surrSelf
	MaxRune         = maxRune
	ReplacementChar = replacementChar