return 0, nil
	}
	fd.rmu.Lock()
	if fd.rtimedout {
		fd.rmu.Unlock()
		return 0, ErrDeadlineExceeded
	}
	fd.raio = newAsyncIO(fn, b)
	fd.rmu.Unlock()
	n, err := fd.raio.Wait()
	fd.raio = nil
	if isHangup(err) {
		err = io.EOF
	}
	if isInterrupted(err) {
		err = ErrDeadlineExceeded
	}
	return n, err
}

// Write implements io.Writer.
func (fd *FD) Write(fn func([]byte) (int, error), b []byte) (int, error) {

}

func (in *input) hangul(p int) (r rune) {
	var size int
	if in.bytes == nil {
		if !isHangulString(in.str[p:]) {
			return 0
		}
		r, size = utf8.DecodeRuneInString(in.str[p:])
	} else {
		if !isHangul(in.bytes[p:]) {
			return 0
		}
		r, size = utf8.DecodeRune(in.bytes[p:])
	}
	if size != hangulUTF8Size {
		return 0
	}
	return r

}

func (in *input) hangul(p int) (r rune) {
	var size int
	if in.bytes == nil {
		if !isHangulString(in.str[p:]) {
			return 0
		}
		r, size = utf8.DecodeRuneInString(in.str[p:])
	} else {
		if !isHangul(in.bytes[p:]) {
			return 0
		}
		r, size = utf8.DecodeRune(in.bytes[p:])
	}
	if size != hangulUTF8Size {
		return 0
	}
	return r

	jamoTBase  = 0x11A7
	jamoTEnd   = 0x11C3

	jamoTCount   = 28
	jamoVCount   = 21
	jamoVTCount  = 21 * 28
	jamoLVTCount = 19 * 21 * 28
)

const hangulUTF8Size = 3

func isHangul(b []byte) bool {
	if len(b) < hangulUTF8Size {
		return false
	}
	b0 := b[0]
	if b0 < hangulBase0 {
		return false
	}
	b1 := b[1]
	switch {
	case b0 == hangulBase0:

	jamoTBase  = 0x11A7
	jamoTEnd   = 0x11C3

	jamoTCount   = 28
	jamoVCount   = 21
	jamoVTCount  = 21 * 28
	jamoLVTCount = 19 * 21 * 28
)

const hangulUTF8Size = 3

func isHangul(b []byte) bool {
	if len(b) < hangulUTF8Size {
		return false
	}
	b0 := b[0]
	if b0 < hangulBase0 {
		return false
	}
	b1 := b[1]
	switch {
	case b0 == hangulBase0: