func (r *Reader) Len() int {
	if r.i >= int64(len(r.s)) {
		return 0
	}
	return int(int64(len(r.s)) - r.i)
}
Len returns the number of bytes of the unread portion of the
slice.
package skykiss
import (
	"errors"
	"io"
	"unicode/utf8"
)
type Reader struct {
	s        []byte
	i        int64  
	prevRune int    
}