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

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