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