go源码解读-strings.Reader

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Reader

strings.Reader结构体

  • Reader结构体包三个参数
    • s,带读取的字符串
    • i,当前读取到的字符索引值
    • prevRune上一个读取到的字符索引值
    • Reader结构体的零值和从空字符串读取创建Reader是等价的
    • Reader结构体实现了io.Reader, io.ReaderAt, io.Seeker, io.WriterTo,io.ByteScanner, and io.RuneScanner等接口
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type Reader struct {
	s        string
	i        int64 // current reading index
	prevRune int   // index of previous rune; or < 0
}
// 结构体初始化方法
func NewReader(s string) *Reader { return &Reader{s, 0, -1} }

结构体的方法

  • Len方法返回字符串s未读取部分的字节数
  • Size方法返回字符串s的字节数
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func (r *Reader) Len() int {
	if r.i >= int64(len(r.s)) {
		return 0
	}
	return int(int64(len(r.s)) - r.i)
}
func (r *Reader) Size() int64 { return int64(len(r.s)) }
  • Read方法，从字符串s中读取数据到b中，读取到字节数为len(b)和len(s[s.r:])之间的较小值
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func (r *Reader) Read(b []byte) (n int, err error) {
	if r.i >= int64(len(r.s)) {
		return 0, io.EOF
	}
	r.prevRune = -1
	n = copy(b, r.s[r.i:])
	r.i += int64(n)
	return
}
  • ReadAt方法读取s[off:]之后的字节切片到字符串到b中
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func (r *Reader) ReadAt(b []byte, off int64) (n int, err error) {
	// cannot modify state - see io.ReaderAt
	if off < 0 {
		return 0, errors.New("strings.Reader.ReadAt: negative offset")
	}
	if off >= int64(len(r.s)) {
		return 0, io.EOF
	}
	n = copy(b, r.s[off:])
	if n < len(b) {
		err = io.EOF
	}
	return
}
  • ReadByte方法从s中读取i指向的字节
  • UnreadByte方法将reader的索引值i–，还原读取一个字节之前的状态
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func (r *Reader) ReadByte() (byte, error) {
	r.prevRune = -1
	if r.i >= int64(len(r.s)) {
		return 0, io.EOF
	}
	b := r.s[r.i]
	r.i++
	return b, nil
}
func (r *Reader) UnreadByte() error {
	if r.i <= 0 {
		return errors.New("strings.Reader.UnreadByte: at beginning of string")
	}
	r.prevRune = -1
	r.i--
	return nil
}
  • ReadRune方法从s中读取i指向的字符
  • UnreadRune方法完成对ReadRune方法的重置
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func (r *Reader) ReadRune() (ch rune, size int, err error) {
	if r.i >= int64(len(r.s)) {
		r.prevRune = -1
		return 0, 0, io.EOF
	}
	r.prevRune = int(r.i)
	if c := r.s[r.i]; c < utf8.RuneSelf {
		r.i++
		return rune(c), 1, nil
	}
	ch, size = utf8.DecodeRuneInString(r.s[r.i:])
	r.i += int64(size)
	return
}
func (r *Reader) UnreadRune() error {
	if r.i <= 0 {
		return errors.New("strings.Reader.UnreadRune: at beginning of string")
	}
	if r.prevRune < 0 {
		return errors.New("strings.Reader.UnreadRune: previous operation was not ReadRune")
	}
	r.i = int64(r.prevRune)
	r.prevRune = -1
	return nil
}
  • Seek方法根据whence设定下一个读取的字符的索引值
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func (r *Reader) Seek(offset int64, whence int) (int64, error) {
	r.prevRune = -1
	var abs int64
	switch whence {
	case io.SeekStart:
		abs = offset
	case io.SeekCurrent:
		abs = r.i + offset
	case io.SeekEnd:
		abs = int64(len(r.s)) + offset
	default:
		return 0, errors.New("strings.Reader.Seek: invalid whence")
	}
	if abs < 0 {
		return 0, errors.New("strings.Reader.Seek: negative position")
	}
	r.i = abs
	return abs, nil
}
  • WriterTo方法将w中的数据写入到reader中
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func (r *Reader) WriteTo(w io.Writer) (n int64, err error) {
	r.prevRune = -1
	if r.i >= int64(len(r.s)) {
		return 0, nil
	}
	s := r.s[r.i:]
	m, err := io.WriteString(w, s)
	if m > len(s) {
		panic("strings.Reader.WriteTo: invalid WriteString count")
	}
	r.i += int64(m)
	n = int64(m)
	if m != len(s) && err == nil {
		err = io.ErrShortWrite
	}
	return
}
  • Reset方法完成Reader的重置操作
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func (r *Reader) Reset(s string) { *r = Reader{s, 0, -1} }
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