Source file src/strings/reader.go

     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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