Source: utf16.go in package unicode/utf16

Package utf16 implements encoding and decoding of UTF-16 sequences.

package utf16

The conditions replacementChar==unicode.ReplacementChar and maxRune==unicode.MaxRune are verified in the tests. Defining them locally avoids this package depending on package unicode.


const (
	replacementChar = '\uFFFD'     // Unicode replacement character
	maxRune         = '\U0010FFFF' // Maximum valid Unicode code point.
)

0xd800-0xdc00 encodes the high 10 bits of a pair. 0xdc00-0xe000 encodes the low 10 bits of a pair. the value is those 20 bits plus 0x10000.

	surr1 = 0xd800
	surr2 = 0xdc00
	surr3 = 0xe000

	surrSelf = 0x10000
)

IsSurrogate reports whether the specified Unicode code point can appear in a surrogate pair.

func IsSurrogate(r rune) bool {
	return surr1 <= r && r < surr3
}

DecodeRune returns the UTF-16 decoding of a surrogate pair. If the pair is not a valid UTF-16 surrogate pair, DecodeRune returns 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 EncodeRune(r rune) (r1, r2 rune) {
	if r < surrSelf || r > maxRune {
		return replacementChar, replacementChar
	}
	r -= surrSelf
	return surr1 + (r>>10)&0x3ff, surr2 + r&0x3ff
}

Encode returns the UTF-16 encoding of the Unicode code point sequence s.

func Encode(s []rune) []uint16 {
	n := len(s)
	for _, v := range s {
		if v >= surrSelf {
			n++
		}
	}

	a := make([]uint16, n)
	n = 0
	for _, v := range s {
		switch {

normal rune

			a[n] = uint16(v)
			n++

needs surrogate sequence

			r1, r2 := EncodeRune(v)
			a[n] = uint16(r1)
			a[n+1] = uint16(r2)
			n += 2
		default:
			a[n] = uint16(replacementChar)
			n++
		}
	}
	return a[:n]
}

Decode returns the Unicode code point sequence represented by the UTF-16 encoding s.

func Decode(s []uint16) []rune {
	a := make([]rune, len(s))
	n := 0
	for i := 0; i < len(s); i++ {
		switch r := s[i]; {

normal rune

			a[n] = rune(r)
		case surr1 <= r && r < surr2 && i+1 < len(s) &&

valid surrogate sequence

			a[n] = DecodeRune(rune(r), rune(s[i+1]))
			i++

invalid surrogate sequence

			a[n] = replacementChar
		}
		n++
	}
	return a[:n]