github.com/megatontech/mynoteforgo@v0.0.0-20200507084910-5d0c6ea6e890/源码/bytes/buffer.go (about) 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 // FIXME: it would be advisable to align Buffer to cachelines to avoid false 26 // sharing. 27 } 28 29 // The readOp constants describe the last action performed on 30 // the buffer, so that UnreadRune and UnreadByte can check for 31 // invalid usage. opReadRuneX constants are chosen such that 32 // converted to int they correspond to the rune size that was read. 33 type readOp int8 34 35 // Don't use iota for these, as the values need to correspond with the 36 // names and comments, which is easier to see when being explicit. 37 const ( 38 opRead readOp = -1 // Any other read operation. 39 opInvalid readOp = 0 // Non-read operation. 40 opReadRune1 readOp = 1 // Read rune of size 1. 41 opReadRune2 readOp = 2 // Read rune of size 2. 42 opReadRune3 readOp = 3 // Read rune of size 3. 43 opReadRune4 readOp = 4 // Read rune of size 4. 44 ) 45 46 // ErrTooLarge is passed to panic if memory cannot be allocated to store data in a buffer. 47 var ErrTooLarge = errors.New("bytes.Buffer: too large") 48 var errNegativeRead = errors.New("bytes.Buffer: reader returned negative count from Read") 49 50 const maxInt = int(^uint(0) >> 1) 51 52 // Bytes returns a slice of length b.Len() holding the unread portion of the buffer. 53 // The slice is valid for use only until the next buffer modification (that is, 54 // only until the next call to a method like Read, Write, Reset, or Truncate). 55 // The slice aliases the buffer content at least until the next buffer modification, 56 // so immediate changes to the slice will affect the result of future reads. 57 func (b *Buffer) Bytes() []byte { return b.buf[b.off:] } 58 59 // String returns the contents of the unread portion of the buffer 60 // as a string. If the Buffer is a nil pointer, it returns "<nil>". 61 // 62 // To build strings more efficiently, see the strings.Builder type. 63 func (b *Buffer) String() string { 64 if b == nil { 65 // Special case, useful in debugging. 66 return "<nil>" 67 } 68 return string(b.buf[b.off:]) 69 } 70 71 // empty reports whether the unread portion of the buffer is empty. 72 func (b *Buffer) empty() bool { return len(b.buf) <= b.off } 73 74 // Len returns the number of bytes of the unread portion of the buffer; 75 // b.Len() == len(b.Bytes()). 76 func (b *Buffer) Len() int { return len(b.buf) - b.off } 77 78 // Cap returns the capacity of the buffer's underlying byte slice, that is, the 79 // total space allocated for the buffer's data. 80 func (b *Buffer) Cap() int { return cap(b.buf) } 81 82 // Truncate discards all but the first n unread bytes from the buffer 83 // but continues to use the same allocated storage. 84 // It panics if n is negative or greater than the length of the buffer. 85 func (b *Buffer) Truncate(n int) { 86 if n == 0 { 87 b.Reset() 88 return 89 } 90 b.lastRead = opInvalid 91 if n < 0 || n > b.Len() { 92 panic("bytes.Buffer: truncation out of range") 93 } 94 b.buf = b.buf[:b.off+n] 95 } 96 97 // Reset resets the buffer to be empty, 98 // but it retains the underlying storage for use by future writes. 99 // Reset is the same as Truncate(0). 100 func (b *Buffer) Reset() { 101 b.buf = b.buf[:0] 102 b.off = 0 103 b.lastRead = opInvalid 104 } 105 106 // tryGrowByReslice is a inlineable version of grow for the fast-case where the 107 // internal buffer only needs to be resliced. 108 // It returns the index where bytes should be written and whether it succeeded. 109 func (b *Buffer) tryGrowByReslice(n int) (int, bool) { 110 if l := len(b.buf); n <= cap(b.buf)-l { 111 b.buf = b.buf[:l+n] 112 return l, true 113 } 114 return 0, false 115 } 116 117 // grow grows the buffer to guarantee space for n more bytes. 118 // It returns the index where bytes should be written. 119 // If the buffer can't grow it will panic with ErrTooLarge. 120 func (b *Buffer) grow(n int) int { 121 m := b.Len() 122 // If buffer is empty, reset to recover space. 123 if m == 0 && b.off != 0 { 124 b.Reset() 125 } 126 // Try to grow by means of a reslice. 127 if i, ok := b.tryGrowByReslice(n); ok { 128 return i 129 } 130 if b.buf == nil && n <= smallBufferSize { 131 b.buf = make([]byte, n, smallBufferSize) 132 return 0 133 } 134 c := cap(b.buf) 135 if n <= c/2-m { 136 // We can slide things down instead of allocating a new 137 // slice. We only need m+n <= c to slide, but 138 // we instead let capacity get twice as large so we 139 // don't spend all our time copying. 140 copy(b.buf, b.buf[b.off:]) 141 } else if c > maxInt-c-n { 142 panic(ErrTooLarge) 143 } else { 144 // Not enough space anywhere, we need to allocate. 145 buf := makeSlice(2*c + n) 146 copy(buf, b.buf[b.off:]) 147 b.buf = buf 148 } 149 // Restore b.off and len(b.buf). 150 b.off = 0 151 b.buf = b.buf[:m+n] 152 return m 153 } 154 155 // Grow grows the buffer's capacity, if necessary, to guarantee space for 156 // another n bytes. After Grow(n), at least n bytes can be written to the 157 // buffer without another allocation. 158 // If n is negative, Grow will panic. 159 // If the buffer can't grow it will panic with ErrTooLarge. 160 func (b *Buffer) Grow(n int) { 161 if n < 0 { 162 panic("bytes.Buffer.Grow: negative count") 163 } 164 m := b.grow(n) 165 b.buf = b.buf[:m] 166 } 167 168 // Write appends the contents of p to the buffer, growing the buffer as 169 // needed. The return value n is the length of p; err is always nil. If the 170 // buffer becomes too large, Write will panic with ErrTooLarge. 171 func (b *Buffer) Write(p []byte) (n int, err error) { 172 b.lastRead = opInvalid 173 m, ok := b.tryGrowByReslice(len(p)) 174 if !ok { 175 m = b.grow(len(p)) 176 } 177 return copy(b.buf[m:], p), nil 178 } 179 180 // WriteString appends the contents of s to the buffer, growing the buffer as 181 // needed. The return value n is the length of s; err is always nil. If the 182 // buffer becomes too large, WriteString will panic with ErrTooLarge. 183 func (b *Buffer) WriteString(s string) (n int, err error) { 184 b.lastRead = opInvalid 185 m, ok := b.tryGrowByReslice(len(s)) 186 if !ok { 187 m = b.grow(len(s)) 188 } 189 return copy(b.buf[m:], s), nil 190 } 191 192 // MinRead is the minimum slice size passed to a Read call by 193 // Buffer.ReadFrom. As long as the Buffer has at least MinRead bytes beyond 194 // what is required to hold the contents of r, ReadFrom will not grow the 195 // underlying buffer. 196 const MinRead = 512 197 198 // ReadFrom reads data from r until EOF and appends it to the buffer, growing 199 // the buffer as needed. The return value n is the number of bytes read. Any 200 // error except io.EOF encountered during the read is also returned. If the 201 // buffer becomes too large, ReadFrom will panic with ErrTooLarge. 202 func (b *Buffer) ReadFrom(r io.Reader) (n int64, err error) { 203 b.lastRead = opInvalid 204 for { 205 i := b.grow(MinRead) 206 b.buf = b.buf[:i] 207 m, e := r.Read(b.buf[i:cap(b.buf)]) 208 if m < 0 { 209 panic(errNegativeRead) 210 } 211 212 b.buf = b.buf[:i+m] 213 n += int64(m) 214 if e == io.EOF { 215 return n, nil // e is EOF, so return nil explicitly 216 } 217 if e != nil { 218 return n, e 219 } 220 } 221 } 222 223 // makeSlice allocates a slice of size n. If the allocation fails, it panics 224 // with ErrTooLarge. 225 func makeSlice(n int) []byte { 226 // If the make fails, give a known error. 227 defer func() { 228 if recover() != nil { 229 panic(ErrTooLarge) 230 } 231 }() 232 return make([]byte, n) 233 } 234 235 // WriteTo writes data to w until the buffer is drained or an error occurs. 236 // The return value n is the number of bytes written; it always fits into an 237 // int, but it is int64 to match the io.WriterTo interface. Any error 238 // encountered during the write is also returned. 239 func (b *Buffer) WriteTo(w io.Writer) (n int64, err error) { 240 b.lastRead = opInvalid 241 if nBytes := b.Len(); nBytes > 0 { 242 m, e := w.Write(b.buf[b.off:]) 243 if m > nBytes { 244 panic("bytes.Buffer.WriteTo: invalid Write count") 245 } 246 b.off += m 247 n = int64(m) 248 if e != nil { 249 return n, e 250 } 251 // all bytes should have been written, by definition of 252 // Write method in io.Writer 253 if m != nBytes { 254 return n, io.ErrShortWrite 255 } 256 } 257 // Buffer is now empty; reset. 258 b.Reset() 259 return n, nil 260 } 261 262 // WriteByte appends the byte c to the buffer, growing the buffer as needed. 263 // The returned error is always nil, but is included to match bufio.Writer's 264 // WriteByte. If the buffer becomes too large, WriteByte will panic with 265 // ErrTooLarge. 266 func (b *Buffer) WriteByte(c byte) error { 267 b.lastRead = opInvalid 268 m, ok := b.tryGrowByReslice(1) 269 if !ok { 270 m = b.grow(1) 271 } 272 b.buf[m] = c 273 return nil 274 } 275 276 // WriteRune appends the UTF-8 encoding of Unicode code point r to the 277 // buffer, returning its length and an error, which is always nil but is 278 // included to match bufio.Writer's WriteRune. The buffer is grown as needed; 279 // if it becomes too large, WriteRune will panic with ErrTooLarge. 280 func (b *Buffer) WriteRune(r rune) (n int, err error) { 281 if r < utf8.RuneSelf { 282 b.WriteByte(byte(r)) 283 return 1, nil 284 } 285 b.lastRead = opInvalid 286 m, ok := b.tryGrowByReslice(utf8.UTFMax) 287 if !ok { 288 m = b.grow(utf8.UTFMax) 289 } 290 n = utf8.EncodeRune(b.buf[m:m+utf8.UTFMax], r) 291 b.buf = b.buf[:m+n] 292 return n, nil 293 } 294 295 // Read reads the next len(p) bytes from the buffer or until the buffer 296 // is drained. The return value n is the number of bytes read. If the 297 // buffer has no data to return, err is io.EOF (unless len(p) is zero); 298 // otherwise it is nil. 299 func (b *Buffer) Read(p []byte) (n int, err error) { 300 b.lastRead = opInvalid 301 if b.empty() { 302 // Buffer is empty, reset to recover space. 303 b.Reset() 304 if len(p) == 0 { 305 return 0, nil 306 } 307 return 0, io.EOF 308 } 309 n = copy(p, b.buf[b.off:]) 310 b.off += n 311 if n > 0 { 312 b.lastRead = opRead 313 } 314 return n, nil 315 } 316 317 // Next returns a slice containing the next n bytes from the buffer, 318 // advancing the buffer as if the bytes had been returned by Read. 319 // If there are fewer than n bytes in the buffer, Next returns the entire buffer. 320 // The slice is only valid until the next call to a read or write method. 321 func (b *Buffer) Next(n int) []byte { 322 b.lastRead = opInvalid 323 m := b.Len() 324 if n > m { 325 n = m 326 } 327 data := b.buf[b.off : b.off+n] 328 b.off += n 329 if n > 0 { 330 b.lastRead = opRead 331 } 332 return data 333 } 334 335 // ReadByte reads and returns the next byte from the buffer. 336 // If no byte is available, it returns error io.EOF. 337 func (b *Buffer) ReadByte() (byte, error) { 338 if b.empty() { 339 // Buffer is empty, reset to recover space. 340 b.Reset() 341 return 0, io.EOF 342 } 343 c := b.buf[b.off] 344 b.off++ 345 b.lastRead = opRead 346 return c, nil 347 } 348 349 // ReadRune reads and returns the next UTF-8-encoded 350 // Unicode code point from the buffer. 351 // If no bytes are available, the error returned is io.EOF. 352 // If the bytes are an erroneous UTF-8 encoding, it 353 // consumes one byte and returns U+FFFD, 1. 354 func (b *Buffer) ReadRune() (r rune, size int, err error) { 355 if b.empty() { 356 // Buffer is empty, reset to recover space. 357 b.Reset() 358 return 0, 0, io.EOF 359 } 360 c := b.buf[b.off] 361 if c < utf8.RuneSelf { 362 b.off++ 363 b.lastRead = opReadRune1 364 return rune(c), 1, nil 365 } 366 r, n := utf8.DecodeRune(b.buf[b.off:]) 367 b.off += n 368 b.lastRead = readOp(n) 369 return r, n, nil 370 } 371 372 // UnreadRune unreads the last rune returned by ReadRune. 373 // If the most recent read or write operation on the buffer was 374 // not a successful ReadRune, UnreadRune returns an error. (In this regard 375 // it is stricter than UnreadByte, which will unread the last byte 376 // from any read operation.) 377 func (b *Buffer) UnreadRune() error { 378 if b.lastRead <= opInvalid { 379 return errors.New("bytes.Buffer: UnreadRune: previous operation was not a successful ReadRune") 380 } 381 if b.off >= int(b.lastRead) { 382 b.off -= int(b.lastRead) 383 } 384 b.lastRead = opInvalid 385 return nil 386 } 387 388 // UnreadByte unreads the last byte returned by the most recent successful 389 // read operation that read at least one byte. If a write has happened since 390 // the last read, if the last read returned an error, or if the read read zero 391 // bytes, UnreadByte returns an error. 392 func (b *Buffer) UnreadByte() error { 393 if b.lastRead == opInvalid { 394 return errors.New("bytes.Buffer: UnreadByte: previous operation was not a successful read") 395 } 396 b.lastRead = opInvalid 397 if b.off > 0 { 398 b.off-- 399 } 400 return nil 401 } 402 403 // ReadBytes reads until the first occurrence of delim in the input, 404 // returning a slice containing the data up to and including the delimiter. 405 // If ReadBytes encounters an error before finding a delimiter, 406 // it returns the data read before the error and the error itself (often io.EOF). 407 // ReadBytes returns err != nil if and only if the returned data does not end in 408 // delim. 409 func (b *Buffer) ReadBytes(delim byte) (line []byte, err error) { 410 slice, err := b.readSlice(delim) 411 // return a copy of slice. The buffer's backing array may 412 // be overwritten by later calls. 413 line = append(line, slice...) 414 return line, err 415 } 416 417 // readSlice is like ReadBytes but returns a reference to internal buffer data. 418 func (b *Buffer) readSlice(delim byte) (line []byte, err error) { 419 i := IndexByte(b.buf[b.off:], delim) 420 end := b.off + i + 1 421 if i < 0 { 422 end = len(b.buf) 423 err = io.EOF 424 } 425 line = b.buf[b.off:end] 426 b.off = end 427 b.lastRead = opRead 428 return line, err 429 } 430 431 // ReadString reads until the first occurrence of delim in the input, 432 // returning a string containing the data up to and including the delimiter. 433 // If ReadString encounters an error before finding a delimiter, 434 // it returns the data read before the error and the error itself (often io.EOF). 435 // ReadString returns err != nil if and only if the returned data does not end 436 // in delim. 437 func (b *Buffer) ReadString(delim byte) (line string, err error) { 438 slice, err := b.readSlice(delim) 439 return string(slice), err 440 } 441 442 // NewBuffer creates and initializes a new Buffer using buf as its 443 // initial contents. The new Buffer takes ownership of buf, and the 444 // caller should not use buf after this call. NewBuffer is intended to 445 // prepare a Buffer to read existing data. It can also be used to set 446 // the initial size of the internal buffer for writing. To do that, 447 // buf should have the desired capacity but a length of zero. 448 // 449 // In most cases, new(Buffer) (or just declaring a Buffer variable) is 450 // sufficient to initialize a Buffer. 451 func NewBuffer(buf []byte) *Buffer { return &Buffer{buf: buf} } 452 453 // NewBufferString creates and initializes a new Buffer using string s as its 454 // initial contents. It is intended to prepare a buffer to read an existing 455 // string. 456 // 457 // In most cases, new(Buffer) (or just declaring a Buffer variable) is 458 // sufficient to initialize a Buffer. 459 func NewBufferString(s string) *Buffer { 460 return &Buffer{buf: []byte(s)} 461 }