1 // Copyright 2013 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 bufio 6 7 import ( 8 "bytes" 9 "errors" 10 "io" 11 "unicode/utf8" 12 ) 13 14 // Scanner provides a convenient interface for reading data such as 15 // a file of newline-delimited lines of text. Successive calls to 16 // the Scan method will step through the 'tokens' of a file, skipping 17 // the bytes between the tokens. The specification of a token is 18 // defined by a split function of type SplitFunc; the default split 19 // function breaks the input into lines with line termination stripped. Split 20 // functions are defined in this package for scanning a file into 21 // lines, bytes, UTF-8-encoded runes, and space-delimited words. The 22 // client may instead provide a custom split function. 23 // 24 // Scanning stops unrecoverably at EOF, the first I/O error, or a token too 25 // large to fit in the buffer. When a scan stops, the reader may have 26 // advanced arbitrarily far past the last token. Programs that need more 27 // control over error handling or large tokens, or must run sequential scans 28 // on a reader, should use bufio.Reader instead. 29 // 30 type Scanner struct { 31 r io.Reader // The reader provided by the client. 32 split SplitFunc // The function to split the tokens. 33 maxTokenSize int // Maximum size of a token; modified by tests. 34 token []byte // Last token returned by split. 35 buf []byte // Buffer used as argument to split. 36 start int // First non-processed byte in buf. 37 end int // End of data in buf. 38 err error // Sticky error. 39 empties int // Count of successive empty tokens. 40 scanCalled bool // Scan has been called; buffer is in use. 41 done bool // Scan has finished. 42 } 43 44 // SplitFunc is the signature of the split function used to tokenize the 45 // input. The arguments are an initial substring of the remaining unprocessed 46 // data and a flag, atEOF, that reports whether the Reader has no more data 47 // to give. The return values are the number of bytes to advance the input 48 // and the next token to return to the user, if any, plus an error, if any. 49 // 50 // Scanning stops if the function returns an error, in which case some of 51 // the input may be discarded. 52 // 53 // Otherwise, the Scanner advances the input. If the token is not nil, 54 // the Scanner returns it to the user. If the token is nil, the 55 // Scanner reads more data and continues scanning; if there is no more 56 // data--if atEOF was true--the Scanner returns. If the data does not 57 // yet hold a complete token, for instance if it has no newline while 58 // scanning lines, a SplitFunc can return (0, nil, nil) to signal the 59 // Scanner to read more data into the slice and try again with a 60 // longer slice starting at the same point in the input. 61 // 62 // The function is never called with an empty data slice unless atEOF 63 // is true. If atEOF is true, however, data may be non-empty and, 64 // as always, holds unprocessed text. 65 type SplitFunc func(data []byte, atEOF bool) (advance int, token []byte, err error) 66 67 // Errors returned by Scanner. 68 var ( 69 ErrTooLong = errors.New("bufio.Scanner: token too long") 70 ErrNegativeAdvance = errors.New("bufio.Scanner: SplitFunc returns negative advance count") 71 ErrAdvanceTooFar = errors.New("bufio.Scanner: SplitFunc returns advance count beyond input") 72 ) 73 74 const ( 75 // MaxScanTokenSize is the maximum size used to buffer a token 76 // unless the user provides an explicit buffer with Scanner.Buffer. 77 // The actual maximum token size may be smaller as the buffer 78 // may need to include, for instance, a newline. 79 MaxScanTokenSize = 64 * 1024 80 81 startBufSize = 4096 // Size of initial allocation for buffer. 82 ) 83 84 // NewScanner returns a new Scanner to read from r. 85 // The split function defaults to ScanLines. 86 func NewScanner(r io.Reader) *Scanner { 87 return &Scanner{ 88 r: r, 89 split: ScanLines, 90 maxTokenSize: MaxScanTokenSize, 91 } 92 } 93 94 // Err returns the first non-EOF error that was encountered by the Scanner. 95 func (s *Scanner) Err() error { 96 if s.err == io.EOF { 97 return nil 98 } 99 return s.err 100 } 101 102 // Bytes returns the most recent token generated by a call to Scan. 103 // The underlying array may point to data that will be overwritten 104 // by a subsequent call to Scan. It does no allocation. 105 func (s *Scanner) Bytes() []byte { 106 return s.token 107 } 108 109 // Text returns the most recent token generated by a call to Scan 110 // as a newly allocated string holding its bytes. 111 func (s *Scanner) Text() string { 112 return string(s.token) 113 } 114 115 // ErrFinalToken is a special sentinel error value. It is intended to be 116 // returned by a Split function to indicate that the token being delivered 117 // with the error is the last token and scanning should stop after this one. 118 // After ErrFinalToken is received by Scan, scanning stops with no error. 119 // The value is useful to stop processing early or when it is necessary to 120 // deliver a final empty token. One could achieve the same behavior 121 // with a custom error value but providing one here is tidier. 122 // See the emptyFinalToken example for a use of this value. 123 var ErrFinalToken = errors.New("final token") 124 125 // Scan advances the Scanner to the next token, which will then be 126 // available through the Bytes or Text method. It returns false when the 127 // scan stops, either by reaching the end of the input or an error. 128 // After Scan returns false, the Err method will return any error that 129 // occurred during scanning, except that if it was io.EOF, Err 130 // will return nil. 131 // Scan panics if the split function returns too many empty 132 // tokens without advancing the input. This is a common error mode for 133 // scanners. 134 func (s *Scanner) Scan() bool { 135 if s.done { 136 return false 137 } 138 s.scanCalled = true 139 // Loop until we have a token. 140 for { 141 // See if we can get a token with what we already have. 142 // If we've run out of data but have an error, give the split function 143 // a chance to recover any remaining, possibly empty token. 144 if s.end > s.start || s.err != nil { 145 advance, token, err := s.split(s.buf[s.start:s.end], s.err != nil) 146 if err != nil { 147 if err == ErrFinalToken { 148 s.token = token 149 s.done = true 150 return true 151 } 152 s.setErr(err) 153 return false 154 } 155 if !s.advance(advance) { 156 return false 157 } 158 s.token = token 159 if token != nil { 160 if s.err == nil || advance > 0 { 161 s.empties = 0 162 } else { 163 // Returning tokens not advancing input at EOF. 164 s.empties++ 165 if s.empties > maxConsecutiveEmptyReads { 166 panic("bufio.Scan: too many empty tokens without progressing") 167 } 168 } 169 return true 170 } 171 } 172 // We cannot generate a token with what we are holding. 173 // If we've already hit EOF or an I/O error, we are done. 174 if s.err != nil { 175 // Shut it down. 176 s.start = 0 177 s.end = 0 178 return false 179 } 180 // Must read more data. 181 // First, shift data to beginning of buffer if there's lots of empty space 182 // or space is needed. 183 if s.start > 0 && (s.end == len(s.buf) || s.start > len(s.buf)/2) { 184 copy(s.buf, s.buf[s.start:s.end]) 185 s.end -= s.start 186 s.start = 0 187 } 188 // Is the buffer full? If so, resize. 189 if s.end == len(s.buf) { 190 // Guarantee no overflow in the multiplication below. 191 const maxInt = int(^uint(0) >> 1) 192 if len(s.buf) >= s.maxTokenSize || len(s.buf) > maxInt/2 { 193 s.setErr(ErrTooLong) 194 return false 195 } 196 newSize := len(s.buf) * 2 197 if newSize == 0 { 198 newSize = startBufSize 199 } 200 if newSize > s.maxTokenSize { 201 newSize = s.maxTokenSize 202 } 203 newBuf := make([]byte, newSize) 204 copy(newBuf, s.buf[s.start:s.end]) 205 s.buf = newBuf 206 s.end -= s.start 207 s.start = 0 208 } 209 // Finally we can read some input. Make sure we don't get stuck with 210 // a misbehaving Reader. Officially we don't need to do this, but let's 211 // be extra careful: Scanner is for safe, simple jobs. 212 for loop := 0; ; { 213 n, err := s.r.Read(s.buf[s.end:len(s.buf)]) 214 s.end += n 215 if err != nil { 216 s.setErr(err) 217 break 218 } 219 if n > 0 { 220 s.empties = 0 221 break 222 } 223 loop++ 224 if loop > maxConsecutiveEmptyReads { 225 s.setErr(io.ErrNoProgress) 226 break 227 } 228 } 229 } 230 } 231 232 // advance consumes n bytes of the buffer. It reports whether the advance was legal. 233 func (s *Scanner) advance(n int) bool { 234 if n < 0 { 235 s.setErr(ErrNegativeAdvance) 236 return false 237 } 238 if n > s.end-s.start { 239 s.setErr(ErrAdvanceTooFar) 240 return false 241 } 242 s.start += n 243 return true 244 } 245 246 // setErr records the first error encountered. 247 func (s *Scanner) setErr(err error) { 248 if s.err == nil || s.err == io.EOF { 249 s.err = err 250 } 251 } 252 253 // Buffer sets the initial buffer to use when scanning and the maximum 254 // size of buffer that may be allocated during scanning. The maximum 255 // token size is the larger of max and cap(buf). If max <= cap(buf), 256 // Scan will use this buffer only and do no allocation. 257 // 258 // By default, Scan uses an internal buffer and sets the 259 // maximum token size to MaxScanTokenSize. 260 // 261 // Buffer panics if it is called after scanning has started. 262 func (s *Scanner) Buffer(buf []byte, max int) { 263 if s.scanCalled { 264 panic("Buffer called after Scan") 265 } 266 s.buf = buf[0:cap(buf)] 267 s.maxTokenSize = max 268 } 269 270 // Split sets the split function for the Scanner. 271 // The default split function is ScanLines. 272 // 273 // Split panics if it is called after scanning has started. 274 func (s *Scanner) Split(split SplitFunc) { 275 if s.scanCalled { 276 panic("Split called after Scan") 277 } 278 s.split = split 279 } 280 281 // Split functions 282 283 // ScanBytes is a split function for a Scanner that returns each byte as a token. 284 func ScanBytes(data []byte, atEOF bool) (advance int, token []byte, err error) { 285 if atEOF && len(data) == 0 { 286 return 0, nil, nil 287 } 288 return 1, data[0:1], nil 289 } 290 291 var errorRune = []byte(string(utf8.RuneError)) 292 293 // ScanRunes is a split function for a Scanner that returns each 294 // UTF-8-encoded rune as a token. The sequence of runes returned is 295 // equivalent to that from a range loop over the input as a string, which 296 // means that erroneous UTF-8 encodings translate to U+FFFD = "\xef\xbf\xbd". 297 // Because of the Scan interface, this makes it impossible for the client to 298 // distinguish correctly encoded replacement runes from encoding errors. 299 func ScanRunes(data []byte, atEOF bool) (advance int, token []byte, err error) { 300 if atEOF && len(data) == 0 { 301 return 0, nil, nil 302 } 303 304 // Fast path 1: ASCII. 305 if data[0] < utf8.RuneSelf { 306 return 1, data[0:1], nil 307 } 308 309 // Fast path 2: Correct UTF-8 decode without error. 310 _, width := utf8.DecodeRune(data) 311 if width > 1 { 312 // It's a valid encoding. Width cannot be one for a correctly encoded 313 // non-ASCII rune. 314 return width, data[0:width], nil 315 } 316 317 // We know it's an error: we have width==1 and implicitly r==utf8.RuneError. 318 // Is the error because there wasn't a full rune to be decoded? 319 // FullRune distinguishes correctly between erroneous and incomplete encodings. 320 if !atEOF && !utf8.FullRune(data) { 321 // Incomplete; get more bytes. 322 return 0, nil, nil 323 } 324 325 // We have a real UTF-8 encoding error. Return a properly encoded error rune 326 // but advance only one byte. This matches the behavior of a range loop over 327 // an incorrectly encoded string. 328 return 1, errorRune, nil 329 } 330 331 // dropCR drops a terminal \r from the data. 332 func dropCR(data []byte) []byte { 333 if len(data) > 0 && data[len(data)-1] == '\r' { 334 return data[0 : len(data)-1] 335 } 336 return data 337 } 338 339 // ScanLines is a split function for a Scanner that returns each line of 340 // text, stripped of any trailing end-of-line marker. The returned line may 341 // be empty. The end-of-line marker is one optional carriage return followed 342 // by one mandatory newline. In regular expression notation, it is `\r?\n`. 343 // The last non-empty line of input will be returned even if it has no 344 // newline. 345 func ScanLines(data []byte, atEOF bool) (advance int, token []byte, err error) { 346 if atEOF && len(data) == 0 { 347 return 0, nil, nil 348 } 349 if i := bytes.IndexByte(data, '\n'); i >= 0 { 350 // We have a full newline-terminated line. 351 return i + 1, dropCR(data[0:i]), nil 352 } 353 // If we're at EOF, we have a final, non-terminated line. Return it. 354 if atEOF { 355 return len(data), dropCR(data), nil 356 } 357 // Request more data. 358 return 0, nil, nil 359 } 360 361 // isSpace reports whether the character is a Unicode white space character. 362 // We avoid dependency on the unicode package, but check validity of the implementation 363 // in the tests. 364 func isSpace(r rune) bool { 365 if r <= '\u00FF' { 366 // Obvious ASCII ones: \t through \r plus space. Plus two Latin-1 oddballs. 367 switch r { 368 case ' ', '\t', '\n', '\v', '\f', '\r': 369 return true 370 case '\u0085', '\u00A0': 371 return true 372 } 373 return false 374 } 375 // High-valued ones. 376 if '\u2000' <= r && r <= '\u200a' { 377 return true 378 } 379 switch r { 380 case '\u1680', '\u2028', '\u2029', '\u202f', '\u205f', '\u3000': 381 return true 382 } 383 return false 384 } 385 386 // ScanWords is a split function for a Scanner that returns each 387 // space-separated word of text, with surrounding spaces deleted. It will 388 // never return an empty string. The definition of space is set by 389 // unicode.IsSpace. 390 func ScanWords(data []byte, atEOF bool) (advance int, token []byte, err error) { 391 // Skip leading spaces. 392 start := 0 393 for width := 0; start < len(data); start += width { 394 var r rune 395 r, width = utf8.DecodeRune(data[start:]) 396 if !isSpace(r) { 397 break 398 } 399 } 400 // Scan until space, marking end of word. 401 for width, i := 0, start; i < len(data); i += width { 402 var r rune 403 r, width = utf8.DecodeRune(data[i:]) 404 if isSpace(r) { 405 return i + width, data[start:i], nil 406 } 407 } 408 // If we're at EOF, we have a final, non-empty, non-terminated word. Return it. 409 if atEOF && len(data) > start { 410 return len(data), data[start:], nil 411 } 412 // Request more data. 413 return start, nil, nil 414 } 415