Source file src/bytes/buffer.go

Documentation: bytes

     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 bytes
     6  
     7  // Simple byte buffer for marshaling data.
     8  
     9  import (
    10  	"errors"
    11  	"io"
    12  	"unicode/utf8"
    13  )
    14  
    15  // smallBufferSize is an initial allocation minimal capacity.
    16  const smallBufferSize = 64
    17  
    18  // A Buffer is a variable-sized buffer of bytes with Read and Write methods.
    19  // The zero value for Buffer is an empty buffer ready to use.
    20  type Buffer struct {
    21  	buf      []byte // contents are the bytes buf[off : len(buf)]
    22  	off      int    // read at &buf[off], write at &buf[len(buf)]
    23  	lastRead readOp // last read operation, so that Unread* can work correctly.
    24  }
    25  
    26  // The readOp constants describe the last action performed on
    27  // the buffer, so that UnreadRune and UnreadByte can check for
    28  // invalid usage. opReadRuneX constants are chosen such that
    29  // converted to int they correspond to the rune size that was read.
    30  type readOp int8
    31  
    32  // Don't use iota for these, as the values need to correspond with the
    33  // names and comments, which is easier to see when being explicit.
    34  const (
    35  	opRead      readOp = -1 // Any other read operation.
    36  	opInvalid   readOp = 0  // Non-read operation.
    37  	opReadRune1 readOp = 1  // Read rune of size 1.
    38  	opReadRune2 readOp = 2  // Read rune of size 2.
    39  	opReadRune3 readOp = 3  // Read rune of size 3.
    40  	opReadRune4 readOp = 4  // Read rune of size 4.
    41  )
    42  
    43  // ErrTooLarge is passed to panic if memory cannot be allocated to store data in a buffer.
    44  var ErrTooLarge = errors.New("bytes.Buffer: too large")
    45  var errNegativeRead = errors.New("bytes.Buffer: reader returned negative count from Read")
    46  
    47  const maxInt = int(^uint(0) >> 1)
    48  
    49  // Bytes returns a slice of length b.Len() holding the unread portion of the buffer.
    50  // The slice is valid for use only until the next buffer modification (that is,
    51  // only until the next call to a method like Read, Write, Reset, or Truncate).
    52  // The slice aliases the buffer content at least until the next buffer modification,
    53  // so immediate changes to the slice will affect the result of future reads.
    54  func (b *Buffer) Bytes() []byte { return b.buf[b.off:] }
    55  
    56  // String returns the contents of the unread portion of the buffer
    57  // as a string. If the Buffer is a nil pointer, it returns "<nil>".
    58  //
    59  // To build strings more efficiently, see the strings.Builder type.
    60  func (b *Buffer) String() string {
    61  	if b == nil {
    62  		// Special case, useful in debugging.
    63  		return "<nil>"
    64  	}
    65  	return string(b.buf[b.off:])
    66  }
    67  
    68  // empty reports whether the unread portion of the buffer is empty.
    69  func (b *Buffer) empty() bool { return len(b.buf) <= b.off }
    70  
    71  // Len returns the number of bytes of the unread portion of the buffer;
    72  // b.Len() == len(b.Bytes()).
    73  func (b *Buffer) Len() int { return len(b.buf) - b.off }
    74  
    75  // Cap returns the capacity of the buffer's underlying byte slice, that is, the
    76  // total space allocated for the buffer's data.
    77  func (b *Buffer) Cap() int { return cap(b.buf) }
    78  
    79  // Truncate discards all but the first n unread bytes from the buffer
    80  // but continues to use the same allocated storage.
    81  // It panics if n is negative or greater than the length of the buffer.
    82  func (b *Buffer) Truncate(n int) {
    83  	if n == 0 {
    84  		b.Reset()
    85  		return
    86  	}
    87  	b.lastRead = opInvalid
    88  	if n < 0 || n > b.Len() {
    89  		panic("bytes.Buffer: truncation out of range")
    90  	}
    91  	b.buf = b.buf[:b.off+n]
    92  }
    93  
    94  // Reset resets the buffer to be empty,
    95  // but it retains the underlying storage for use by future writes.
    96  // Reset is the same as Truncate(0).
    97  func (b *Buffer) Reset() {
    98  	b.buf = b.buf[:0]
    99  	b.off = 0
   100  	b.lastRead = opInvalid
   101  }
   102  
   103  // tryGrowByReslice is a inlineable version of grow for the fast-case where the
   104  // internal buffer only needs to be resliced.
   105  // It returns the index where bytes should be written and whether it succeeded.
   106  func (b *Buffer) tryGrowByReslice(n int) (int, bool) {
   107  	if l := len(b.buf); n <= cap(b.buf)-l {
   108  		b.buf = b.buf[:l+n]
   109  		return l, true
   110  	}
   111  	return 0, false
   112  }
   113  
   114  // grow grows the buffer to guarantee space for n more bytes.
   115  // It returns the index where bytes should be written.
   116  // If the buffer can't grow it will panic with ErrTooLarge.
   117  func (b *Buffer) grow(n int) int {
   118  	m := b.Len()
   119  	// If buffer is empty, reset to recover space.
   120  	if m == 0 && b.off != 0 {
   121  		b.Reset()
   122  	}
   123  	// Try to grow by means of a reslice.
   124  	if i, ok := b.tryGrowByReslice(n); ok {
   125  		return i
   126  	}
   127  	if b.buf == nil && n <= smallBufferSize {
   128  		b.buf = make([]byte, n, smallBufferSize)
   129  		return 0
   130  	}
   131  	c := cap(b.buf)
   132  	if n <= c/2-m {
   133  		// We can slide things down instead of allocating a new
   134  		// slice. We only need m+n <= c to slide, but
   135  		// we instead let capacity get twice as large so we
   136  		// don't spend all our time copying.
   137  		copy(b.buf, b.buf[b.off:])
   138  	} else if c > maxInt-c-n {
   139  		panic(ErrTooLarge)
   140  	} else {
   141  		// Not enough space anywhere, we need to allocate.
   142  		buf := makeSlice(2*c + n)
   143  		copy(buf, b.buf[b.off:])
   144  		b.buf = buf
   145  	}
   146  	// Restore b.off and len(b.buf).
   147  	b.off = 0
   148  	b.buf = b.buf[:m+n]
   149  	return m
   150  }
   151  
   152  // Grow grows the buffer's capacity, if necessary, to guarantee space for
   153  // another n bytes. After Grow(n), at least n bytes can be written to the
   154  // buffer without another allocation.
   155  // If n is negative, Grow will panic.
   156  // If the buffer can't grow it will panic with ErrTooLarge.
   157  func (b *Buffer) Grow(n int) {
   158  	if n < 0 {
   159  		panic("bytes.Buffer.Grow: negative count")
   160  	}
   161  	m := b.grow(n)
   162  	b.buf = b.buf[:m]
   163  }
   164  
   165  // Write appends the contents of p to the buffer, growing the buffer as
   166  // needed. The return value n is the length of p; err is always nil. If the
   167  // buffer becomes too large, Write will panic with ErrTooLarge.
   168  func (b *Buffer) Write(p []byte) (n int, err error) {
   169  	b.lastRead = opInvalid
   170  	m, ok := b.tryGrowByReslice(len(p))
   171  	if !ok {
   172  		m = b.grow(len(p))
   173  	}
   174  	return copy(b.buf[m:], p), nil
   175  }
   176  
   177  // WriteString appends the contents of s to the buffer, growing the buffer as
   178  // needed. The return value n is the length of s; err is always nil. If the
   179  // buffer becomes too large, WriteString will panic with ErrTooLarge.
   180  func (b *Buffer) WriteString(s string) (n int, err error) {
   181  	b.lastRead = opInvalid
   182  	m, ok := b.tryGrowByReslice(len(s))
   183  	if !ok {
   184  		m = b.grow(len(s))
   185  	}
   186  	return copy(b.buf[m:], s), nil
   187  }
   188  
   189  // MinRead is the minimum slice size passed to a Read call by
   190  // Buffer.ReadFrom. As long as the Buffer has at least MinRead bytes beyond
   191  // what is required to hold the contents of r, ReadFrom will not grow the
   192  // underlying buffer.
   193  const MinRead = 512
   194  
   195  // ReadFrom reads data from r until EOF and appends it to the buffer, growing
   196  // the buffer as needed. The return value n is the number of bytes read. Any
   197  // error except io.EOF encountered during the read is also returned. If the
   198  // buffer becomes too large, ReadFrom will panic with ErrTooLarge.
   199  func (b *Buffer) ReadFrom(r io.Reader) (n int64, err error) {
   200  	b.lastRead = opInvalid
   201  	for {
   202  		i := b.grow(MinRead)
   203  		b.buf = b.buf[:i]
   204  		m, e := r.Read(b.buf[i:cap(b.buf)])
   205  		if m < 0 {
   206  			panic(errNegativeRead)
   207  		}
   208  
   209  		b.buf = b.buf[:i+m]
   210  		n += int64(m)
   211  		if e == io.EOF {
   212  			return n, nil // e is EOF, so return nil explicitly
   213  		}
   214  		if e != nil {
   215  			return n, e
   216  		}
   217  	}
   218  }
   219  
   220  // makeSlice allocates a slice of size n. If the allocation fails, it panics
   221  // with ErrTooLarge.
   222  func makeSlice(n int) []byte {
   223  	// If the make fails, give a known error.
   224  	defer func() {
   225  		if recover() != nil {
   226  			panic(ErrTooLarge)
   227  		}
   228  	}()
   229  	return make([]byte, n)
   230  }
   231  
   232  // WriteTo writes data to w until the buffer is drained or an error occurs.
   233  // The return value n is the number of bytes written; it always fits into an
   234  // int, but it is int64 to match the io.WriterTo interface. Any error
   235  // encountered during the write is also returned.
   236  func (b *Buffer) WriteTo(w io.Writer) (n int64, err error) {
   237  	b.lastRead = opInvalid
   238  	if nBytes := b.Len(); nBytes > 0 {
   239  		m, e := w.Write(b.buf[b.off:])
   240  		if m > nBytes {
   241  			panic("bytes.Buffer.WriteTo: invalid Write count")
   242  		}
   243  		b.off += m
   244  		n = int64(m)
   245  		if e != nil {
   246  			return n, e
   247  		}
   248  		// all bytes should have been written, by definition of
   249  		// Write method in io.Writer
   250  		if m != nBytes {
   251  			return n, io.ErrShortWrite
   252  		}
   253  	}
   254  	// Buffer is now empty; reset.
   255  	b.Reset()
   256  	return n, nil
   257  }
   258  
   259  // WriteByte appends the byte c to the buffer, growing the buffer as needed.
   260  // The returned error is always nil, but is included to match bufio.Writer's
   261  // WriteByte. If the buffer becomes too large, WriteByte will panic with
   262  // ErrTooLarge.
   263  func (b *Buffer) WriteByte(c byte) error {
   264  	b.lastRead = opInvalid
   265  	m, ok := b.tryGrowByReslice(1)
   266  	if !ok {
   267  		m = b.grow(1)
   268  	}
   269  	b.buf[m] = c
   270  	return nil
   271  }
   272  
   273  // WriteRune appends the UTF-8 encoding of Unicode code point r to the
   274  // buffer, returning its length and an error, which is always nil but is
   275  // included to match bufio.Writer's WriteRune. The buffer is grown as needed;
   276  // if it becomes too large, WriteRune will panic with ErrTooLarge.
   277  func (b *Buffer) WriteRune(r rune) (n int, err error) {
   278  	if r < utf8.RuneSelf {
   279  		b.WriteByte(byte(r))
   280  		return 1, nil
   281  	}
   282  	b.lastRead = opInvalid
   283  	m, ok := b.tryGrowByReslice(utf8.UTFMax)
   284  	if !ok {
   285  		m = b.grow(utf8.UTFMax)
   286  	}
   287  	n = utf8.EncodeRune(b.buf[m:m+utf8.UTFMax], r)
   288  	b.buf = b.buf[:m+n]
   289  	return n, nil
   290  }
   291  
   292  // Read reads the next len(p) bytes from the buffer or until the buffer
   293  // is drained. The return value n is the number of bytes read. If the
   294  // buffer has no data to return, err is io.EOF (unless len(p) is zero);
   295  // otherwise it is nil.
   296  func (b *Buffer) Read(p []byte) (n int, err error) {
   297  	b.lastRead = opInvalid
   298  	if b.empty() {
   299  		// Buffer is empty, reset to recover space.
   300  		b.Reset()
   301  		if len(p) == 0 {
   302  			return 0, nil
   303  		}
   304  		return 0, io.EOF
   305  	}
   306  	n = copy(p, b.buf[b.off:])
   307  	b.off += n
   308  	if n > 0 {
   309  		b.lastRead = opRead
   310  	}
   311  	return n, nil
   312  }
   313  
   314  // Next returns a slice containing the next n bytes from the buffer,
   315  // advancing the buffer as if the bytes had been returned by Read.
   316  // If there are fewer than n bytes in the buffer, Next returns the entire buffer.
   317  // The slice is only valid until the next call to a read or write method.
   318  func (b *Buffer) Next(n int) []byte {
   319  	b.lastRead = opInvalid
   320  	m := b.Len()
   321  	if n > m {
   322  		n = m
   323  	}
   324  	data := b.buf[b.off : b.off+n]
   325  	b.off += n
   326  	if n > 0 {
   327  		b.lastRead = opRead
   328  	}
   329  	return data
   330  }
   331  
   332  // ReadByte reads and returns the next byte from the buffer.
   333  // If no byte is available, it returns error io.EOF.
   334  func (b *Buffer) ReadByte() (byte, error) {
   335  	if b.empty() {
   336  		// Buffer is empty, reset to recover space.
   337  		b.Reset()
   338  		return 0, io.EOF
   339  	}
   340  	c := b.buf[b.off]
   341  	b.off++
   342  	b.lastRead = opRead
   343  	return c, nil
   344  }
   345  
   346  // ReadRune reads and returns the next UTF-8-encoded
   347  // Unicode code point from the buffer.
   348  // If no bytes are available, the error returned is io.EOF.
   349  // If the bytes are an erroneous UTF-8 encoding, it
   350  // consumes one byte and returns U+FFFD, 1.
   351  func (b *Buffer) ReadRune() (r rune, size int, err error) {
   352  	if b.empty() {
   353  		// Buffer is empty, reset to recover space.
   354  		b.Reset()
   355  		return 0, 0, io.EOF
   356  	}
   357  	c := b.buf[b.off]
   358  	if c < utf8.RuneSelf {
   359  		b.off++
   360  		b.lastRead = opReadRune1
   361  		return rune(c), 1, nil
   362  	}
   363  	r, n := utf8.DecodeRune(b.buf[b.off:])
   364  	b.off += n
   365  	b.lastRead = readOp(n)
   366  	return r, n, nil
   367  }
   368  
   369  // UnreadRune unreads the last rune returned by ReadRune.
   370  // If the most recent read or write operation on the buffer was
   371  // not a successful ReadRune, UnreadRune returns an error.  (In this regard
   372  // it is stricter than UnreadByte, which will unread the last byte
   373  // from any read operation.)
   374  func (b *Buffer) UnreadRune() error {
   375  	if b.lastRead <= opInvalid {
   376  		return errors.New("bytes.Buffer: UnreadRune: previous operation was not a successful ReadRune")
   377  	}
   378  	if b.off >= int(b.lastRead) {
   379  		b.off -= int(b.lastRead)
   380  	}
   381  	b.lastRead = opInvalid
   382  	return nil
   383  }
   384  
   385  var errUnreadByte = errors.New("bytes.Buffer: UnreadByte: previous operation was not a successful read")
   386  
   387  // UnreadByte unreads the last byte returned by the most recent successful
   388  // read operation that read at least one byte. If a write has happened since
   389  // the last read, if the last read returned an error, or if the read read zero
   390  // bytes, UnreadByte returns an error.
   391  func (b *Buffer) UnreadByte() error {
   392  	if b.lastRead == opInvalid {
   393  		return errUnreadByte
   394  	}
   395  	b.lastRead = opInvalid
   396  	if b.off > 0 {
   397  		b.off--
   398  	}
   399  	return nil
   400  }
   401  
   402  // ReadBytes reads until the first occurrence of delim in the input,
   403  // returning a slice containing the data up to and including the delimiter.
   404  // If ReadBytes encounters an error before finding a delimiter,
   405  // it returns the data read before the error and the error itself (often io.EOF).
   406  // ReadBytes returns err != nil if and only if the returned data does not end in
   407  // delim.
   408  func (b *Buffer) ReadBytes(delim byte) (line []byte, err error) {
   409  	slice, err := b.readSlice(delim)
   410  	// return a copy of slice. The buffer's backing array may
   411  	// be overwritten by later calls.
   412  	line = append(line, slice...)
   413  	return line, err
   414  }
   415  
   416  // readSlice is like ReadBytes but returns a reference to internal buffer data.
   417  func (b *Buffer) readSlice(delim byte) (line []byte, err error) {
   418  	i := IndexByte(b.buf[b.off:], delim)
   419  	end := b.off + i + 1
   420  	if i < 0 {
   421  		end = len(b.buf)
   422  		err = io.EOF
   423  	}
   424  	line = b.buf[b.off:end]
   425  	b.off = end
   426  	b.lastRead = opRead
   427  	return line, err
   428  }
   429  
   430  // ReadString reads until the first occurrence of delim in the input,
   431  // returning a string containing the data up to and including the delimiter.
   432  // If ReadString encounters an error before finding a delimiter,
   433  // it returns the data read before the error and the error itself (often io.EOF).
   434  // ReadString returns err != nil if and only if the returned data does not end
   435  // in delim.
   436  func (b *Buffer) ReadString(delim byte) (line string, err error) {
   437  	slice, err := b.readSlice(delim)
   438  	return string(slice), err
   439  }
   440  
   441  // NewBuffer creates and initializes a new Buffer using buf as its
   442  // initial contents. The new Buffer takes ownership of buf, and the
   443  // caller should not use buf after this call. NewBuffer is intended to
   444  // prepare a Buffer to read existing data. It can also be used to set
   445  // the initial size of the internal buffer for writing. To do that,
   446  // buf should have the desired capacity but a length of zero.
   447  //
   448  // In most cases, new(Buffer) (or just declaring a Buffer variable) is
   449  // sufficient to initialize a Buffer.
   450  func NewBuffer(buf []byte) *Buffer { return &Buffer{buf: buf} }
   451  
   452  // NewBufferString creates and initializes a new Buffer using string s as its
   453  // initial contents. It is intended to prepare a buffer to read an existing
   454  // string.
   455  //
   456  // In most cases, new(Buffer) (or just declaring a Buffer variable) is
   457  // sufficient to initialize a Buffer.
   458  func NewBufferString(s string) *Buffer {
   459  	return &Buffer{buf: []byte(s)}
   460  }
   461  

View as plain text