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