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Source file src/bytes/buffer.go

Documentation: bytes

  // 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 bytes
  
  // Simple byte buffer for marshaling data.
  
  import (
  	"errors"
  	"io"
  	"unicode/utf8"
  )
  
  // A Buffer is a variable-sized buffer of bytes with Read and Write methods.
  // The zero value for Buffer is an empty buffer ready to use.
  type Buffer struct {
  	buf       []byte   // contents are the bytes buf[off : len(buf)]
  	off       int      // read at &buf[off], write at &buf[len(buf)]
  	bootstrap [64]byte // memory to hold first slice; helps small buffers avoid allocation.
  	lastRead  readOp   // last read operation, so that Unread* can work correctly.
  }
  
  // The readOp constants describe the last action performed on
  // the buffer, so that UnreadRune and UnreadByte can check for
  // invalid usage. opReadRuneX constants are chosen such that
  // converted to int they correspond to the rune size that was read.
  type readOp int
  
  const (
  	opRead      readOp = -1 // Any other read operation.
  	opInvalid          = 0  // Non-read operation.
  	opReadRune1        = 1  // Read rune of size 1.
  	opReadRune2        = 2  // Read rune of size 2.
  	opReadRune3        = 3  // Read rune of size 3.
  	opReadRune4        = 4  // Read rune of size 4.
  )
  
  // ErrTooLarge is passed to panic if memory cannot be allocated to store data in a buffer.
  var ErrTooLarge = errors.New("bytes.Buffer: too large")
  
  // Bytes returns a slice of length b.Len() holding the unread portion of the buffer.
  // The slice is valid for use only until the next buffer modification (that is,
  // only until the next call to a method like Read, Write, Reset, or Truncate).
  // The slice aliases the buffer content at least until the next buffer modification,
  // so immediate changes to the slice will affect the result of future reads.
  func (b *Buffer) Bytes() []byte { return b.buf[b.off:] }
  
  // String returns the contents of the unread portion of the buffer
  // as a string. If the Buffer is a nil pointer, it returns "<nil>".
  func (b *Buffer) String() string {
  	if b == nil {
  		// Special case, useful in debugging.
  		return "<nil>"
  	}
  	return string(b.buf[b.off:])
  }
  
  // Len returns the number of bytes of the unread portion of the buffer;
  // b.Len() == len(b.Bytes()).
  func (b *Buffer) Len() int { return len(b.buf) - b.off }
  
  // Cap returns the capacity of the buffer's underlying byte slice, that is, the
  // total space allocated for the buffer's data.
  func (b *Buffer) Cap() int { return cap(b.buf) }
  
  // Truncate discards all but the first n unread bytes from the buffer
  // but continues to use the same allocated storage.
  // It panics if n is negative or greater than the length of the buffer.
  func (b *Buffer) Truncate(n int) {
  	b.lastRead = opInvalid
  	switch {
  	case n < 0 || n > b.Len():
  		panic("bytes.Buffer: truncation out of range")
  	case n == 0:
  		// Reuse buffer space.
  		b.off = 0
  	}
  	b.buf = b.buf[0 : b.off+n]
  }
  
  // Reset resets the buffer to be empty,
  // but it retains the underlying storage for use by future writes.
  // Reset is the same as Truncate(0).
  func (b *Buffer) Reset() { b.Truncate(0) }
  
  // grow grows the buffer to guarantee space for n more bytes.
  // It returns the index where bytes should be written.
  // If the buffer can't grow it will panic with ErrTooLarge.
  func (b *Buffer) grow(n int) int {
  	m := b.Len()
  	// If buffer is empty, reset to recover space.
  	if m == 0 && b.off != 0 {
  		b.Truncate(0)
  	}
  	if len(b.buf)+n > cap(b.buf) {
  		var buf []byte
  		if b.buf == nil && n <= len(b.bootstrap) {
  			buf = b.bootstrap[0:]
  		} else if m+n <= cap(b.buf)/2 {
  			// We can slide things down instead of allocating a new
  			// slice. We only need m+n <= cap(b.buf) to slide, but
  			// we instead let capacity get twice as large so we
  			// don't spend all our time copying.
  			copy(b.buf[:], b.buf[b.off:])
  			buf = b.buf[:m]
  		} else {
  			// not enough space anywhere
  			buf = makeSlice(2*cap(b.buf) + n)
  			copy(buf, b.buf[b.off:])
  		}
  		b.buf = buf
  		b.off = 0
  	}
  	b.buf = b.buf[0 : b.off+m+n]
  	return b.off + m
  }
  
  // Grow grows the buffer's capacity, if necessary, to guarantee space for
  // another n bytes. After Grow(n), at least n bytes can be written to the
  // buffer without another allocation.
  // If n is negative, Grow will panic.
  // If the buffer can't grow it will panic with ErrTooLarge.
  func (b *Buffer) Grow(n int) {
  	if n < 0 {
  		panic("bytes.Buffer.Grow: negative count")
  	}
  	m := b.grow(n)
  	b.buf = b.buf[0:m]
  }
  
  // Write appends the contents of p to the buffer, growing the buffer as
  // needed. The return value n is the length of p; err is always nil. If the
  // buffer becomes too large, Write will panic with ErrTooLarge.
  func (b *Buffer) Write(p []byte) (n int, err error) {
  	b.lastRead = opInvalid
  	m := b.grow(len(p))
  	return copy(b.buf[m:], p), nil
  }
  
  // WriteString appends the contents of s to the buffer, growing the buffer as
  // needed. The return value n is the length of s; err is always nil. If the
  // buffer becomes too large, WriteString will panic with ErrTooLarge.
  func (b *Buffer) WriteString(s string) (n int, err error) {
  	b.lastRead = opInvalid
  	m := b.grow(len(s))
  	return copy(b.buf[m:], s), nil
  }
  
  // MinRead is the minimum slice size passed to a Read call by
  // Buffer.ReadFrom. As long as the Buffer has at least MinRead bytes beyond
  // what is required to hold the contents of r, ReadFrom will not grow the
  // underlying buffer.
  const MinRead = 512
  
  // ReadFrom reads data from r until EOF and appends it to the buffer, growing
  // the buffer as needed. The return value n is the number of bytes read. Any
  // error except io.EOF encountered during the read is also returned. If the
  // buffer becomes too large, ReadFrom will panic with ErrTooLarge.
  func (b *Buffer) ReadFrom(r io.Reader) (n int64, err error) {
  	b.lastRead = opInvalid
  	// If buffer is empty, reset to recover space.
  	if b.off >= len(b.buf) {
  		b.Truncate(0)
  	}
  	for {
  		if free := cap(b.buf) - len(b.buf); free < MinRead {
  			// not enough space at end
  			newBuf := b.buf
  			if b.off+free < MinRead {
  				// not enough space using beginning of buffer;
  				// double buffer capacity
  				newBuf = makeSlice(2*cap(b.buf) + MinRead)
  			}
  			copy(newBuf, b.buf[b.off:])
  			b.buf = newBuf[:len(b.buf)-b.off]
  			b.off = 0
  		}
  		m, e := r.Read(b.buf[len(b.buf):cap(b.buf)])
  		b.buf = b.buf[0 : len(b.buf)+m]
  		n += int64(m)
  		if e == io.EOF {
  			break
  		}
  		if e != nil {
  			return n, e
  		}
  	}
  	return n, nil // err is EOF, so return nil explicitly
  }
  
  // makeSlice allocates a slice of size n. If the allocation fails, it panics
  // with ErrTooLarge.
  func makeSlice(n int) []byte {
  	// If the make fails, give a known error.
  	defer func() {
  		if recover() != nil {
  			panic(ErrTooLarge)
  		}
  	}()
  	return make([]byte, n)
  }
  
  // WriteTo writes data to w until the buffer is drained or an error occurs.
  // The return value n is the number of bytes written; it always fits into an
  // int, but it is int64 to match the io.WriterTo interface. Any error
  // encountered during the write is also returned.
  func (b *Buffer) WriteTo(w io.Writer) (n int64, err error) {
  	b.lastRead = opInvalid
  	if b.off < len(b.buf) {
  		nBytes := b.Len()
  		m, e := w.Write(b.buf[b.off:])
  		if m > nBytes {
  			panic("bytes.Buffer.WriteTo: invalid Write count")
  		}
  		b.off += m
  		n = int64(m)
  		if e != nil {
  			return n, e
  		}
  		// all bytes should have been written, by definition of
  		// Write method in io.Writer
  		if m != nBytes {
  			return n, io.ErrShortWrite
  		}
  	}
  	// Buffer is now empty; reset.
  	b.Truncate(0)
  	return
  }
  
  // WriteByte appends the byte c to the buffer, growing the buffer as needed.
  // The returned error is always nil, but is included to match bufio.Writer's
  // WriteByte. If the buffer becomes too large, WriteByte will panic with
  // ErrTooLarge.
  func (b *Buffer) WriteByte(c byte) error {
  	b.lastRead = opInvalid
  	m := b.grow(1)
  	b.buf[m] = c
  	return nil
  }
  
  // WriteRune appends the UTF-8 encoding of Unicode code point r to the
  // buffer, returning its length and an error, which is always nil but is
  // included to match bufio.Writer's WriteRune. The buffer is grown as needed;
  // if it becomes too large, WriteRune will panic with ErrTooLarge.
  func (b *Buffer) WriteRune(r rune) (n int, err error) {
  	if r < utf8.RuneSelf {
  		b.WriteByte(byte(r))
  		return 1, nil
  	}
  	b.lastRead = opInvalid
  	m := b.grow(utf8.UTFMax)
  	n = utf8.EncodeRune(b.buf[m:m+utf8.UTFMax], r)
  	b.buf = b.buf[:m+n]
  	return n, nil
  }
  
  // Read reads the next len(p) bytes from the buffer or until the buffer
  // is drained. The return value n is the number of bytes read. If the
  // buffer has no data to return, err is io.EOF (unless len(p) is zero);
  // otherwise it is nil.
  func (b *Buffer) Read(p []byte) (n int, err error) {
  	b.lastRead = opInvalid
  	if b.off >= len(b.buf) {
  		// Buffer is empty, reset to recover space.
  		b.Truncate(0)
  		if len(p) == 0 {
  			return
  		}
  		return 0, io.EOF
  	}
  	n = copy(p, b.buf[b.off:])
  	b.off += n
  	if n > 0 {
  		b.lastRead = opRead
  	}
  	return
  }
  
  // Next returns a slice containing the next n bytes from the buffer,
  // advancing the buffer as if the bytes had been returned by Read.
  // If there are fewer than n bytes in the buffer, Next returns the entire buffer.
  // The slice is only valid until the next call to a read or write method.
  func (b *Buffer) Next(n int) []byte {
  	b.lastRead = opInvalid
  	m := b.Len()
  	if n > m {
  		n = m
  	}
  	data := b.buf[b.off : b.off+n]
  	b.off += n
  	if n > 0 {
  		b.lastRead = opRead
  	}
  	return data
  }
  
  // ReadByte reads and returns the next byte from the buffer.
  // If no byte is available, it returns error io.EOF.
  func (b *Buffer) ReadByte() (byte, error) {
  	b.lastRead = opInvalid
  	if b.off >= len(b.buf) {
  		// Buffer is empty, reset to recover space.
  		b.Truncate(0)
  		return 0, io.EOF
  	}
  	c := b.buf[b.off]
  	b.off++
  	b.lastRead = opRead
  	return c, nil
  }
  
  // ReadRune reads and returns the next UTF-8-encoded
  // Unicode code point from the buffer.
  // If no bytes are available, the error returned is io.EOF.
  // If the bytes are an erroneous UTF-8 encoding, it
  // consumes one byte and returns U+FFFD, 1.
  func (b *Buffer) ReadRune() (r rune, size int, err error) {
  	b.lastRead = opInvalid
  	if b.off >= len(b.buf) {
  		// Buffer is empty, reset to recover space.
  		b.Truncate(0)
  		return 0, 0, io.EOF
  	}
  	c := b.buf[b.off]
  	if c < utf8.RuneSelf {
  		b.off++
  		b.lastRead = opReadRune1
  		return rune(c), 1, nil
  	}
  	r, n := utf8.DecodeRune(b.buf[b.off:])
  	b.off += n
  	b.lastRead = readOp(n)
  	return r, n, nil
  }
  
  // UnreadRune unreads the last rune returned by ReadRune.
  // If the most recent read or write operation on the buffer was
  // not a ReadRune, UnreadRune returns an error.  (In this regard
  // it is stricter than UnreadByte, which will unread the last byte
  // from any read operation.)
  func (b *Buffer) UnreadRune() error {
  	if b.lastRead <= opInvalid {
  		return errors.New("bytes.Buffer: UnreadRune: previous operation was not ReadRune")
  	}
  	if b.off >= int(b.lastRead) {
  		b.off -= int(b.lastRead)
  	}
  	b.lastRead = opInvalid
  	return nil
  }
  
  // UnreadByte unreads the last byte returned by the most recent
  // read operation. If write has happened since the last read, UnreadByte
  // returns an error.
  func (b *Buffer) UnreadByte() error {
  	if b.lastRead == opInvalid {
  		return errors.New("bytes.Buffer: UnreadByte: previous operation was not a read")
  	}
  	b.lastRead = opInvalid
  	if b.off > 0 {
  		b.off--
  	}
  	return nil
  }
  
  // ReadBytes reads until the first occurrence of delim in the input,
  // returning a slice containing the data up to and including the delimiter.
  // If ReadBytes encounters an error before finding a delimiter,
  // it returns the data read before the error and the error itself (often io.EOF).
  // ReadBytes returns err != nil if and only if the returned data does not end in
  // delim.
  func (b *Buffer) ReadBytes(delim byte) (line []byte, err error) {
  	slice, err := b.readSlice(delim)
  	// return a copy of slice. The buffer's backing array may
  	// be overwritten by later calls.
  	line = append(line, slice...)
  	return
  }
  
  // readSlice is like ReadBytes but returns a reference to internal buffer data.
  func (b *Buffer) readSlice(delim byte) (line []byte, err error) {
  	i := IndexByte(b.buf[b.off:], delim)
  	end := b.off + i + 1
  	if i < 0 {
  		end = len(b.buf)
  		err = io.EOF
  	}
  	line = b.buf[b.off:end]
  	b.off = end
  	b.lastRead = opRead
  	return line, err
  }
  
  // ReadString reads until the first occurrence of delim in the input,
  // returning a string containing the data up to and including the delimiter.
  // If ReadString encounters an error before finding a delimiter,
  // it returns the data read before the error and the error itself (often io.EOF).
  // ReadString returns err != nil if and only if the returned data does not end
  // in delim.
  func (b *Buffer) ReadString(delim byte) (line string, err error) {
  	slice, err := b.readSlice(delim)
  	return string(slice), err
  }
  
  // NewBuffer creates and initializes a new Buffer using buf as its initial
  // contents. It is intended to prepare a Buffer to read existing data. It
  // can also be used to size the internal buffer for writing. To do that,
  // buf should have the desired capacity but a length of zero.
  //
  // In most cases, new(Buffer) (or just declaring a Buffer variable) is
  // sufficient to initialize a Buffer.
  func NewBuffer(buf []byte) *Buffer { return &Buffer{buf: buf} }
  
  // NewBufferString creates and initializes a new Buffer using string s as its
  // initial contents. It is intended to prepare a buffer to read an existing
  // string.
  //
  // In most cases, new(Buffer) (or just declaring a Buffer variable) is
  // sufficient to initialize a Buffer.
  func NewBufferString(s string) *Buffer {
  	return &Buffer{buf: []byte(s)}
  }
  

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