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 [Buffer.Read] and [Buffer.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 // Copying and modifying a non-zero Buffer is prone to error,
26 // but we cannot employ the noCopy trick used by WaitGroup and Mutex,
27 // which causes vet's copylocks checker to report misuse, as vet
28 // cannot reliably distinguish the zero and non-zero cases.
29 // See #26462, #25907, #47276, #48398 for history.
30 }
31 32 // The readOp constants describe the last action performed on
33 // the buffer, so that UnreadRune and UnreadByte can check for
34 // invalid usage. opReadRuneX constants are chosen such that
35 // converted to int they correspond to the rune size that was read.
36 type readOp int8
37 38 // Don't use iota for these, as the values need to correspond with the
39 // names and comments, which is easier to see when being explicit.
40 const (
41 opRead readOp = -1 // Any other read operation.
42 opInvalid readOp = 0 // Non-read operation.
43 opReadRune1 readOp = 1 // Read rune of size 1.
44 opReadRune2 readOp = 2 // Read rune of size 2.
45 opReadRune3 readOp = 3 // Read rune of size 3.
46 opReadRune4 readOp = 4 // Read rune of size 4.
47 )
48 49 // ErrTooLarge is passed to panic if memory cannot be allocated to store data in a buffer.
50 var ErrTooLarge = errors.New("bytes.Buffer: too large")
51 var errNegativeRead = errors.New("bytes.Buffer: reader returned negative count from Read")
52 53 const maxInt = int(^uint(0) >> 1)
54 55 // Bytes returns a slice of length b.Len() holding the unread portion of the buffer.
56 // The slice is valid for use only until the next buffer modification (that is,
57 // only until the next call to a method like [Buffer.Read], [Buffer.Write], [Buffer.Reset], or [Buffer.Truncate]).
58 // The slice aliases the buffer content at least until the next buffer modification,
59 // so immediate changes to the slice will affect the result of future reads.
60 func (b *Buffer) Bytes() []byte { return b.buf[b.off:] }
61 62 // AvailableBuffer returns an empty buffer with b.Available() capacity.
63 // This buffer is intended to be appended to and
64 // passed to an immediately succeeding [Buffer.Write] call.
65 // The buffer is only valid until the next write operation on b.
66 func (b *Buffer) AvailableBuffer() []byte { return b.buf[len(b.buf):] }
67 68 // String returns the contents of the unread portion of the buffer
69 // as a string. If the [Buffer] is a nil pointer, it returns "<nil>".
70 //
71 // To build strings more efficiently, see the [strings.Builder] type.
72 func (b *Buffer) String() string {
73 if b == nil {
74 // Special case, useful in debugging.
75 return "<nil>"
76 }
77 return string(b.buf[b.off:])
78 }
79 80 // empty reports whether the unread portion of the buffer is empty.
81 func (b *Buffer) empty() bool { return len(b.buf) <= b.off }
82 83 // Len returns the number of bytes of the unread portion of the buffer;
84 // b.Len() == len(b.Bytes()).
85 func (b *Buffer) Len() int { return len(b.buf) - b.off }
86 87 // Cap returns the capacity of the buffer's underlying byte slice, that is, the
88 // total space allocated for the buffer's data.
89 func (b *Buffer) Cap() int { return cap(b.buf) }
90 91 // Available returns how many bytes are unused in the buffer.
92 func (b *Buffer) Available() int { return cap(b.buf) - len(b.buf) }
93 94 // Truncate discards all but the first n unread bytes from the buffer
95 // but continues to use the same allocated storage.
96 // It panics if n is negative or greater than the length of the buffer.
97 func (b *Buffer) Truncate(n int) {
98 if n == 0 {
99 b.Reset()
100 return
101 }
102 b.lastRead = opInvalid
103 if n < 0 || n > b.Len() {
104 panic("bytes.Buffer: truncation out of range")
105 }
106 b.buf = b.buf[:b.off+n]
107 }
108 109 // Reset resets the buffer to be empty,
110 // but it retains the underlying storage for use by future writes.
111 // Reset is the same as [Buffer.Truncate](0).
112 func (b *Buffer) Reset() {
113 b.buf = b.buf[:0]
114 b.off = 0
115 b.lastRead = opInvalid
116 }
117 118 // tryGrowByReslice is an inlineable version of grow for the fast-case where the
119 // internal buffer only needs to be resliced.
120 // It returns the index where bytes should be written and whether it succeeded.
121 func (b *Buffer) tryGrowByReslice(n int) (int, bool) {
122 if l := len(b.buf); n <= cap(b.buf)-l {
123 b.buf = b.buf[:l+n]
124 return l, true
125 }
126 return 0, false
127 }
128 129 // grow grows the buffer to guarantee space for n more bytes.
130 // It returns the index where bytes should be written.
131 // If the buffer can't grow it will panic with ErrTooLarge.
132 func (b *Buffer) grow(n int) int {
133 m := b.Len()
134 // If buffer is empty, reset to recover space.
135 if m == 0 && b.off != 0 {
136 b.Reset()
137 }
138 // Try to grow by means of a reslice.
139 if i, ok := b.tryGrowByReslice(n); ok {
140 return i
141 }
142 if b.buf == nil && n <= smallBufferSize {
143 b.buf = []byte{:n:smallBufferSize}
144 return 0
145 }
146 c := cap(b.buf)
147 if n <= c/2-m {
148 // We can slide things down instead of allocating a new
149 // slice. We only need m+n <= c to slide, but
150 // we instead let capacity get twice as large so we
151 // don't spend all our time copying.
152 copy(b.buf, b.buf[b.off:])
153 } else if c > maxInt-c-n {
154 panic(ErrTooLarge)
155 } else {
156 // Add b.off to account for b.buf[:b.off] being sliced off the front.
157 b.buf = growSlice(b.buf[b.off:], b.off+n)
158 }
159 // Restore b.off and len(b.buf).
160 b.off = 0
161 b.buf = b.buf[:m+n]
162 return m
163 }
164 165 // Grow grows the buffer's capacity, if necessary, to guarantee space for
166 // another n bytes. After Grow(n), at least n bytes can be written to the
167 // buffer without another allocation.
168 // If n is negative, Grow will panic.
169 // If the buffer can't grow it will panic with [ErrTooLarge].
170 func (b *Buffer) Grow(n int) {
171 if n < 0 {
172 panic("bytes.Buffer.Grow: negative count")
173 }
174 m := b.grow(n)
175 b.buf = b.buf[:m]
176 }
177 178 // Write appends the contents of p to the buffer, growing the buffer as
179 // needed. The return value n is the length of p; err is always nil. If the
180 // buffer becomes too large, Write will panic with [ErrTooLarge].
181 func (b *Buffer) Write(p []byte) (n int, err error) {
182 b.lastRead = opInvalid
183 m, ok := b.tryGrowByReslice(len(p))
184 if !ok {
185 m = b.grow(len(p))
186 }
187 return copy(b.buf[m:], p), nil
188 }
189 190 // WriteString appends the contents of s to the buffer, growing the buffer as
191 // needed. The return value n is the length of s; err is always nil. If the
192 // buffer becomes too large, WriteString will panic with [ErrTooLarge].
193 func (b *Buffer) WriteString(s []byte) (n int, err error) {
194 b.lastRead = opInvalid
195 m, ok := b.tryGrowByReslice(len(s))
196 if !ok {
197 m = b.grow(len(s))
198 }
199 return copy(b.buf[m:], s), nil
200 }
201 202 // MinRead is the minimum slice size passed to a [Buffer.Read] call by
203 // [Buffer.ReadFrom]. As long as the [Buffer] has at least MinRead bytes beyond
204 // what is required to hold the contents of r, [Buffer.ReadFrom] will not grow the
205 // underlying buffer.
206 const MinRead = 512
207 208 // ReadFrom reads data from r until EOF and appends it to the buffer, growing
209 // the buffer as needed. The return value n is the number of bytes read. Any
210 // error except io.EOF encountered during the read is also returned. If the
211 // buffer becomes too large, ReadFrom will panic with [ErrTooLarge].
212 func (b *Buffer) ReadFrom(r io.Reader) (n int64, err error) {
213 b.lastRead = opInvalid
214 for {
215 i := b.grow(MinRead)
216 b.buf = b.buf[:i]
217 m, e := r.Read(b.buf[i:cap(b.buf)])
218 if m < 0 {
219 panic(errNegativeRead)
220 }
221 222 b.buf = b.buf[:i+m]
223 n += int64(m)
224 if e == io.EOF {
225 return n, nil // e is EOF, so return nil explicitly
226 }
227 if e != nil {
228 return n, e
229 }
230 }
231 }
232 233 // growSlice grows b by n, preserving the original content of b.
234 // If the allocation fails, it panics with ErrTooLarge.
235 func growSlice(b []byte, n int) []byte {
236 defer func() {
237 if recover() != nil {
238 panic(ErrTooLarge)
239 }
240 }()
241 // TODO(http://golang.org/issue/51462): We should rely on the append-make
242 // pattern so that the compiler can call runtime.growslice. For example:
243 // return append(b, make([]byte, n)...)
244 // This avoids unnecessary zero-ing of the first len(b) bytes of the
245 // allocated slice, but this pattern causes b to escape onto the heap.
246 //
247 // Instead use the append-make pattern with a nil slice to ensure that
248 // we allocate buffers rounded up to the closest size class.
249 c := len(b) + n // ensure enough space for n elements
250 if c < 2*cap(b) {
251 // The growth rate has historically always been 2x. In the future,
252 // we could rely purely on append to determine the growth rate.
253 c = 2 * cap(b)
254 }
255 b2 := append([]byte(nil), []byte{:c}...)
256 i := copy(b2, b)
257 return b2[:i]
258 }
259 260 // WriteTo writes data to w until the buffer is drained or an error occurs.
261 // The return value n is the number of bytes written; it always fits into an
262 // int, but it is int64 to match the [io.WriterTo] interface. Any error
263 // encountered during the write is also returned.
264 func (b *Buffer) WriteTo(w io.Writer) (n int64, err error) {
265 b.lastRead = opInvalid
266 if nBytes := b.Len(); nBytes > 0 {
267 m, e := w.Write(b.buf[b.off:])
268 if m > nBytes {
269 panic("bytes.Buffer.WriteTo: invalid Write count")
270 }
271 b.off += m
272 n = int64(m)
273 if e != nil {
274 return n, e
275 }
276 // all bytes should have been written, by definition of
277 // Write method in io.Writer
278 if m != nBytes {
279 return n, io.ErrShortWrite
280 }
281 }
282 // Buffer is now empty; reset.
283 b.Reset()
284 return n, nil
285 }
286 287 // WriteByte appends the byte c to the buffer, growing the buffer as needed.
288 // The returned error is always nil, but is included to match [bufio.Writer]'s
289 // WriteByte. If the buffer becomes too large, WriteByte will panic with
290 // [ErrTooLarge].
291 func (b *Buffer) WriteByte(c byte) error {
292 b.lastRead = opInvalid
293 m, ok := b.tryGrowByReslice(1)
294 if !ok {
295 m = b.grow(1)
296 }
297 b.buf[m] = c
298 return nil
299 }
300 301 // WriteRune appends the UTF-8 encoding of Unicode code point r to the
302 // buffer, returning its length and an error, which is always nil but is
303 // included to match [bufio.Writer]'s WriteRune. The buffer is grown as needed;
304 // if it becomes too large, WriteRune will panic with [ErrTooLarge].
305 func (b *Buffer) WriteRune(r rune) (n int, err error) {
306 // Compare as uint32 to correctly handle negative runes.
307 if uint32(r) < utf8.RuneSelf {
308 b.WriteByte(byte(r))
309 return 1, nil
310 }
311 b.lastRead = opInvalid
312 m, ok := b.tryGrowByReslice(utf8.UTFMax)
313 if !ok {
314 m = b.grow(utf8.UTFMax)
315 }
316 b.buf = utf8.AppendRune(b.buf[:m], r)
317 return len(b.buf) - m, nil
318 }
319 320 // Read reads the next len(p) bytes from the buffer or until the buffer
321 // is drained. The return value n is the number of bytes read. If the
322 // buffer has no data to return, err is [io.EOF] (unless len(p) is zero);
323 // otherwise it is nil.
324 func (b *Buffer) Read(p []byte) (n int, err error) {
325 b.lastRead = opInvalid
326 if b.empty() {
327 // Buffer is empty, reset to recover space.
328 b.Reset()
329 if len(p) == 0 {
330 return 0, nil
331 }
332 return 0, io.EOF
333 }
334 n = copy(p, b.buf[b.off:])
335 b.off += n
336 if n > 0 {
337 b.lastRead = opRead
338 }
339 return n, nil
340 }
341 342 // Next returns a slice containing the next n bytes from the buffer,
343 // advancing the buffer as if the bytes had been returned by [Buffer.Read].
344 // If there are fewer than n bytes in the buffer, Next returns the entire buffer.
345 // The slice is only valid until the next call to a read or write method.
346 func (b *Buffer) Next(n int) []byte {
347 b.lastRead = opInvalid
348 m := b.Len()
349 if n > m {
350 n = m
351 }
352 data := b.buf[b.off : b.off+n]
353 b.off += n
354 if n > 0 {
355 b.lastRead = opRead
356 }
357 return data
358 }
359 360 // ReadByte reads and returns the next byte from the buffer.
361 // If no byte is available, it returns error [io.EOF].
362 func (b *Buffer) ReadByte() (byte, error) {
363 if b.empty() {
364 // Buffer is empty, reset to recover space.
365 b.Reset()
366 return 0, io.EOF
367 }
368 c := b.buf[b.off]
369 b.off++
370 b.lastRead = opRead
371 return c, nil
372 }
373 374 // ReadRune reads and returns the next UTF-8-encoded
375 // Unicode code point from the buffer.
376 // If no bytes are available, the error returned is io.EOF.
377 // If the bytes are an erroneous UTF-8 encoding, it
378 // consumes one byte and returns U+FFFD, 1.
379 func (b *Buffer) ReadRune() (r rune, size int, err error) {
380 if b.empty() {
381 // Buffer is empty, reset to recover space.
382 b.Reset()
383 return 0, 0, io.EOF
384 }
385 c := b.buf[b.off]
386 if c < utf8.RuneSelf {
387 b.off++
388 b.lastRead = opReadRune1
389 return rune(c), 1, nil
390 }
391 r, n := utf8.DecodeRune(b.buf[b.off:])
392 b.off += n
393 b.lastRead = readOp(n)
394 return r, n, nil
395 }
396 397 // UnreadRune unreads the last rune returned by [Buffer.ReadRune].
398 // If the most recent read or write operation on the buffer was
399 // not a successful [Buffer.ReadRune], UnreadRune returns an error. (In this regard
400 // it is stricter than [Buffer.UnreadByte], which will unread the last byte
401 // from any read operation.)
402 func (b *Buffer) UnreadRune() error {
403 if b.lastRead <= opInvalid {
404 return errors.New("bytes.Buffer: UnreadRune: previous operation was not a successful ReadRune")
405 }
406 if b.off >= int(b.lastRead) {
407 b.off -= int(b.lastRead)
408 }
409 b.lastRead = opInvalid
410 return nil
411 }
412 413 var errUnreadByte = errors.New("bytes.Buffer: UnreadByte: previous operation was not a successful read")
414 415 // UnreadByte unreads the last byte returned by the most recent successful
416 // read operation that read at least one byte. If a write has happened since
417 // the last read, if the last read returned an error, or if the read read zero
418 // bytes, UnreadByte returns an error.
419 func (b *Buffer) UnreadByte() error {
420 if b.lastRead == opInvalid {
421 return errUnreadByte
422 }
423 b.lastRead = opInvalid
424 if b.off > 0 {
425 b.off--
426 }
427 return nil
428 }
429 430 // ReadBytes reads until the first occurrence of delim in the input,
431 // returning a slice containing the data up to and including the delimiter.
432 // If ReadBytes encounters an error before finding a delimiter,
433 // it returns the data read before the error and the error itself (often [io.EOF]).
434 // ReadBytes returns err != nil if and only if the returned data does not end in
435 // delim.
436 func (b *Buffer) ReadBytes(delim byte) (line []byte, err error) {
437 slice, err := b.readSlice(delim)
438 // return a copy of slice. The buffer's backing array may
439 // be overwritten by later calls.
440 line = append(line, slice...)
441 return line, err
442 }
443 444 // readSlice is like ReadBytes but returns a reference to internal buffer data.
445 func (b *Buffer) readSlice(delim byte) (line []byte, err error) {
446 i := IndexByte(b.buf[b.off:], delim)
447 end := b.off + i + 1
448 if i < 0 {
449 end = len(b.buf)
450 err = io.EOF
451 }
452 line = b.buf[b.off:end]
453 b.off = end
454 b.lastRead = opRead
455 return line, err
456 }
457 458 // ReadString reads until the first occurrence of delim in the input,
459 // returning a string containing the data up to and including the delimiter.
460 // If ReadString encounters an error before finding a delimiter,
461 // it returns the data read before the error and the error itself (often [io.EOF]).
462 // ReadString returns err != nil if and only if the returned data does not end
463 // in delim.
464 func (b *Buffer) ReadString(delim byte) (line []byte, err error) {
465 slice, err := b.readSlice(delim)
466 return []byte(slice), err
467 }
468 469 // NewBuffer creates and initializes a new [Buffer] using buf as its
470 // initial contents. The new [Buffer] takes ownership of buf, and the
471 // caller should not use buf after this call. NewBuffer is intended to
472 // prepare a [Buffer] to read existing data. It can also be used to set
473 // the initial size of the internal buffer for writing. To do that,
474 // buf should have the desired capacity but a length of zero.
475 //
476 // In most cases, new([Buffer]) (or just declaring a [Buffer] variable) is
477 // sufficient to initialize a [Buffer].
478 func NewBuffer(buf []byte) *Buffer { return &Buffer{buf: buf} }
479 480 // NewBufferString creates and initializes a new [Buffer] using string s as its
481 // initial contents. It is intended to prepare a buffer to read an existing
482 // string.
483 //
484 // In most cases, new([Buffer]) (or just declaring a [Buffer] variable) is
485 // sufficient to initialize a [Buffer].
486 func NewBufferString(s []byte) *Buffer {
487 return &Buffer{buf: []byte(s)}
488 }
489