Source: reader.go in package strings


package strings

import (
	"errors"
	"io"
	"unicode/utf8"
)

A Reader implements the io.Reader, io.ReaderAt, io.ByteReader, io.ByteScanner, io.RuneReader, io.RuneScanner, io.Seeker, and io.WriterTo interfaces by reading from a string. The zero value for Reader operates like a Reader of an empty string.

type Reader struct {
	s        string
	i        int64 // current reading index
	prevRune int   // index of previous rune; or < 0
}

Len returns the number of bytes of the unread portion of the string.

func (r *Reader) Len() int {
	if r.i >= int64(len(r.s)) {
		return 0
	}
	return int(int64(len(r.s)) - r.i)
}

Size returns the original length of the underlying string. Size is the number of bytes available for reading via ReadAt. The returned value is always the same and is not affected by calls to any other method.

func (r *Reader) Size() int64 { return int64(len(r.s)) }

Read implements the io.Reader interface.

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 implements the io.ReaderAt interface.

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 implements the io.ByteReader interface.

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
}

UnreadByte implements the io.ByteScanner interface.

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 implements the io.RuneReader interface.

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
}

UnreadRune implements the io.RuneScanner interface.

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 implements the io.Seeker interface.

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
}

WriteTo implements the io.WriterTo interface.

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 resets the Reader to be reading from s.

func (r *Reader) Reset(s string) { *r = Reader{s, 0, -1} }

NewReader returns a new Reader reading from s. It is similar to bytes.NewBufferString but more efficient and read-only.