Source file src/strings/reader.go

Documentation: strings

     1  // Copyright 2009 The Go Authors. All rights reserved.
     2  // Use of this source code is governed by a BSD-style
     3  // license that can be found in the LICENSE file.
     4  
     5  package strings
     6  
     7  import (
     8  	"errors"
     9  	"io"
    10  	"unicode/utf8"
    11  )
    12  
    13  // A Reader implements the io.Reader, io.ReaderAt, io.Seeker, io.WriterTo,
    14  // io.ByteScanner, and io.RuneScanner interfaces by reading
    15  // from a string.
    16  // The zero value for Reader operates like a Reader of an empty string.
    17  type Reader struct {
    18  	s        string
    19  	i        int64 // current reading index
    20  	prevRune int   // index of previous rune; or < 0
    21  }
    22  
    23  // Len returns the number of bytes of the unread portion of the
    24  // string.
    25  func (r *Reader) Len() int {
    26  	if r.i >= int64(len(r.s)) {
    27  		return 0
    28  	}
    29  	return int(int64(len(r.s)) - r.i)
    30  }
    31  
    32  // Size returns the original length of the underlying string.
    33  // Size is the number of bytes available for reading via ReadAt.
    34  // The returned value is always the same and is not affected by calls
    35  // to any other method.
    36  func (r *Reader) Size() int64 { return int64(len(r.s)) }
    37  
    38  func (r *Reader) Read(b []byte) (n int, err error) {
    39  	if r.i >= int64(len(r.s)) {
    40  		return 0, io.EOF
    41  	}
    42  	r.prevRune = -1
    43  	n = copy(b, r.s[r.i:])
    44  	r.i += int64(n)
    45  	return
    46  }
    47  
    48  func (r *Reader) ReadAt(b []byte, off int64) (n int, err error) {
    49  	// cannot modify state - see io.ReaderAt
    50  	if off < 0 {
    51  		return 0, errors.New("strings.Reader.ReadAt: negative offset")
    52  	}
    53  	if off >= int64(len(r.s)) {
    54  		return 0, io.EOF
    55  	}
    56  	n = copy(b, r.s[off:])
    57  	if n < len(b) {
    58  		err = io.EOF
    59  	}
    60  	return
    61  }
    62  
    63  func (r *Reader) ReadByte() (byte, error) {
    64  	r.prevRune = -1
    65  	if r.i >= int64(len(r.s)) {
    66  		return 0, io.EOF
    67  	}
    68  	b := r.s[r.i]
    69  	r.i++
    70  	return b, nil
    71  }
    72  
    73  func (r *Reader) UnreadByte() error {
    74  	if r.i <= 0 {
    75  		return errors.New("strings.Reader.UnreadByte: at beginning of string")
    76  	}
    77  	r.prevRune = -1
    78  	r.i--
    79  	return nil
    80  }
    81  
    82  func (r *Reader) ReadRune() (ch rune, size int, err error) {
    83  	if r.i >= int64(len(r.s)) {
    84  		r.prevRune = -1
    85  		return 0, 0, io.EOF
    86  	}
    87  	r.prevRune = int(r.i)
    88  	if c := r.s[r.i]; c < utf8.RuneSelf {
    89  		r.i++
    90  		return rune(c), 1, nil
    91  	}
    92  	ch, size = utf8.DecodeRuneInString(r.s[r.i:])
    93  	r.i += int64(size)
    94  	return
    95  }
    96  
    97  func (r *Reader) UnreadRune() error {
    98  	if r.i <= 0 {
    99  		return errors.New("strings.Reader.UnreadRune: at beginning of string")
   100  	}
   101  	if r.prevRune < 0 {
   102  		return errors.New("strings.Reader.UnreadRune: previous operation was not ReadRune")
   103  	}
   104  	r.i = int64(r.prevRune)
   105  	r.prevRune = -1
   106  	return nil
   107  }
   108  
   109  // Seek implements the io.Seeker interface.
   110  func (r *Reader) Seek(offset int64, whence int) (int64, error) {
   111  	r.prevRune = -1
   112  	var abs int64
   113  	switch whence {
   114  	case io.SeekStart:
   115  		abs = offset
   116  	case io.SeekCurrent:
   117  		abs = r.i + offset
   118  	case io.SeekEnd:
   119  		abs = int64(len(r.s)) + offset
   120  	default:
   121  		return 0, errors.New("strings.Reader.Seek: invalid whence")
   122  	}
   123  	if abs < 0 {
   124  		return 0, errors.New("strings.Reader.Seek: negative position")
   125  	}
   126  	r.i = abs
   127  	return abs, nil
   128  }
   129  
   130  // WriteTo implements the io.WriterTo interface.
   131  func (r *Reader) WriteTo(w io.Writer) (n int64, err error) {
   132  	r.prevRune = -1
   133  	if r.i >= int64(len(r.s)) {
   134  		return 0, nil
   135  	}
   136  	s := r.s[r.i:]
   137  	m, err := io.WriteString(w, s)
   138  	if m > len(s) {
   139  		panic("strings.Reader.WriteTo: invalid WriteString count")
   140  	}
   141  	r.i += int64(m)
   142  	n = int64(m)
   143  	if m != len(s) && err == nil {
   144  		err = io.ErrShortWrite
   145  	}
   146  	return
   147  }
   148  
   149  // Reset resets the Reader to be reading from s.
   150  func (r *Reader) Reset(s string) { *r = Reader{s, 0, -1} }
   151  
   152  // NewReader returns a new Reader reading from s.
   153  // It is similar to bytes.NewBufferString but more efficient and read-only.
   154  func NewReader(s string) *Reader { return &Reader{s, 0, -1} }
   155  

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