Example:

PACKAGE

package utf8
    import "unicode/utf8"

    Package utf8 implements functions and constants to support text
    encoded in UTF-8.  This package calls a Unicode character a rune for
    brevity.

CONSTANTS

const (
    RuneError = unicode.ReplacementChar // the "error" Rune or "replacement character".
    RuneSelf  = 0x80                    // characters below Runeself are represented as themselves in a single byte.
    UTFMax    = 4                       // maximum number of bytes of a UTF-8 encoded Unicode character.
)
    Numbers fundamental to the encoding.

FUNCTIONS

func DecodeLastRune(p []byte) (r rune, size int)
    DecodeLastRune unpacks the last UTF-8 encoding in p and returns the
    rune and its width in bytes.

func DecodeLastRuneInString(s string) (r rune, size int)
    DecodeLastRuneInString is like DecodeLastRune but its input is a
    string.

func DecodeRune(p []byte) (r rune, size int)
    DecodeRune unpacks the first UTF-8 encoding in p and returns the rune
    and its width in bytes.

func DecodeRuneInString(s string) (r rune, size int)
    DecodeRuneInString is like DecodeRune but its input is a string.

func EncodeRune(p []byte, r rune) int
    EncodeRune writes into p (which must be large enough) the UTF-8
    encoding of the rune.  It returns the number of bytes written.

func FullRune(p []byte) bool
    FullRune reports whether the bytes in p begin with a full UTF-8
    encoding of a rune.  An invalid encoding is considered a full Rune
    since it will convert as a width-1 error rune.

func FullRuneInString(s string) bool
    FullRuneInString is like FullRune but its input is a string.

func RuneCount(p []byte) int
    RuneCount returns the number of runes in p.  Erroneous and short
    encodings are treated as single runes of width 1 byte.

func RuneCountInString(s string) (n int)
    RuneCountInString is like RuneCount but its input is a string.

func RuneLen(r rune) int
    RuneLen returns the number of bytes required to encode the rune.

func RuneStart(b byte) bool
    RuneStart reports whether the byte could be the first byte of an
    encoded rune.  Second and subsequent bytes always have the top two
    bits set to 10.

func Valid(p []byte) bool
    Valid reports whether p consists entirely of valid UTF-8-encoded
    runes.

func ValidString(s string) bool
    ValidString reports whether s consists entirely of valid UTF-8-encoded
    runes.

TYPES

type String struct {
    // contains filtered or unexported fields
}
    String wraps a regular string with a small structure that provides
    more efficient indexing by code point index, as opposed to byte index.
    Scanning incrementally forwards or backwards is O(1) per index
    operation (although not as fast a range clause going forwards).
    Random access is O(N) in the length of the string, but the overhead is
    less than always scanning from the beginning.  If the string is ASCII,
    random access is O(1).  Unlike the built-in string type, String has
    internal mutable state and is not thread-safe.

func NewString(contents string) *String
    NewString returns a new UTF-8 string with the provided contents.

func (s *String) At(i int) rune
    At returns the rune with index i in the String.  The sequence of runes
    is the same as iterating over the contents with a "for range" clause.

func (s *String) Init(contents string) *String
    Init initializes an existing String to hold the provided contents.
    It returns a pointer to the initialized String.

func (s *String) IsASCII() bool
    IsASCII returns a boolean indicating whether the String contains only
    ASCII bytes.

func (s *String) RuneCount() int
    RuneCount returns the number of runes (Unicode code points) in the
    String.

func (s *String) Slice(i, j int) string
    Slice returns the string sliced at rune positions [i:j].

func (s *String) String() string
    String returns the contents of the String.  This method also means the
    String is directly printable by fmt.Print.

Fixes #2479.

R=golang-dev, dsymonds, mattn.jp, r, gri, r
CC=golang-dev
https://golang.org/cl/5472051