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// Copyright 2009 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
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 [Reader.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.
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 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 non-writable.
func NewReader(s string) *Reader { return &Reader{s, 0, -1} }