Source file src/text/tabwriter/tabwriter.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 tabwriter implements a write filter (tabwriter.Writer) that 6 // translates tabbed columns in input into properly aligned text. 7 // 8 // The package is using the Elastic Tabstops algorithm described at 9 // http://nickgravgaard.com/elastictabstops/index.html. 10 // 11 // The text/tabwriter package is frozen and is not accepting new features. 12 package tabwriter 13 14 import ( 15 "io" 16 "unicode/utf8" 17 ) 18 19 // ---------------------------------------------------------------------------- 20 // Filter implementation 21 22 // A cell represents a segment of text terminated by tabs or line breaks. 23 // The text itself is stored in a separate buffer; cell only describes the 24 // segment's size in bytes, its width in runes, and whether it's an htab 25 // ('\t') terminated cell. 26 // 27 type cell struct { 28 size int // cell size in bytes 29 width int // cell width in runes 30 htab bool // true if the cell is terminated by an htab ('\t') 31 } 32 33 // A Writer is a filter that inserts padding around tab-delimited 34 // columns in its input to align them in the output. 35 // 36 // The Writer treats incoming bytes as UTF-8-encoded text consisting 37 // of cells terminated by horizontal ('\t') or vertical ('\v') tabs, 38 // and newline ('\n') or formfeed ('\f') characters; both newline and 39 // formfeed act as line breaks. 40 // 41 // Tab-terminated cells in contiguous lines constitute a column. The 42 // Writer inserts padding as needed to make all cells in a column have 43 // the same width, effectively aligning the columns. It assumes that 44 // all characters have the same width, except for tabs for which a 45 // tabwidth must be specified. Column cells must be tab-terminated, not 46 // tab-separated: non-tab terminated trailing text at the end of a line 47 // forms a cell but that cell is not part of an aligned column. 48 // For instance, in this example (where | stands for a horizontal tab): 49 // 50 // aaaa|bbb|d 51 // aa |b |dd 52 // a | 53 // aa |cccc|eee 54 // 55 // the b and c are in distinct columns (the b column is not contiguous 56 // all the way). The d and e are not in a column at all (there's no 57 // terminating tab, nor would the column be contiguous). 58 // 59 // The Writer assumes that all Unicode code points have the same width; 60 // this may not be true in some fonts or if the string contains combining 61 // characters. 62 // 63 // If DiscardEmptyColumns is set, empty columns that are terminated 64 // entirely by vertical (or "soft") tabs are discarded. Columns 65 // terminated by horizontal (or "hard") tabs are not affected by 66 // this flag. 67 // 68 // If a Writer is configured to filter HTML, HTML tags and entities 69 // are passed through. The widths of tags and entities are 70 // assumed to be zero (tags) and one (entities) for formatting purposes. 71 // 72 // A segment of text may be escaped by bracketing it with Escape 73 // characters. The tabwriter passes escaped text segments through 74 // unchanged. In particular, it does not interpret any tabs or line 75 // breaks within the segment. If the StripEscape flag is set, the 76 // Escape characters are stripped from the output; otherwise they 77 // are passed through as well. For the purpose of formatting, the 78 // width of the escaped text is always computed excluding the Escape 79 // characters. 80 // 81 // The formfeed character acts like a newline but it also terminates 82 // all columns in the current line (effectively calling Flush). Tab- 83 // terminated cells in the next line start new columns. Unless found 84 // inside an HTML tag or inside an escaped text segment, formfeed 85 // characters appear as newlines in the output. 86 // 87 // The Writer must buffer input internally, because proper spacing 88 // of one line may depend on the cells in future lines. Clients must 89 // call Flush when done calling Write. 90 // 91 type Writer struct { 92 // configuration 93 output io.Writer 94 minwidth int 95 tabwidth int 96 padding int 97 padbytes [8]byte 98 flags uint 99 100 // current state 101 buf []byte // collected text excluding tabs or line breaks 102 pos int // buffer position up to which cell.width of incomplete cell has been computed 103 cell cell // current incomplete cell; cell.width is up to buf[pos] excluding ignored sections 104 endChar byte // terminating char of escaped sequence (Escape for escapes, '>', ';' for HTML tags/entities, or 0) 105 lines [][]cell // list of lines; each line is a list of cells 106 widths []int // list of column widths in runes - re-used during formatting 107 } 108 109 // addLine adds a new line. 110 // flushed is a hint indicating whether the underlying writer was just flushed. 111 // If so, the previous line is not likely to be a good indicator of the new line's cells. 112 func (b *Writer) addLine(flushed bool) { 113 // Grow slice instead of appending, 114 // as that gives us an opportunity 115 // to re-use an existing []cell. 116 if n := len(b.lines) + 1; n <= cap(b.lines) { 117 b.lines = b.lines[:n] 118 b.lines[n-1] = b.lines[n-1][:0] 119 } else { 120 b.lines = append(b.lines, nil) 121 } 122 123 if !flushed { 124 // The previous line is probably a good indicator 125 // of how many cells the current line will have. 126 // If the current line's capacity is smaller than that, 127 // abandon it and make a new one. 128 if n := len(b.lines); n >= 2 { 129 if prev := len(b.lines[n-2]); prev > cap(b.lines[n-1]) { 130 b.lines[n-1] = make([]cell, 0, prev) 131 } 132 } 133 } 134 } 135 136 // Reset the current state. 137 func (b *Writer) reset() { 138 b.buf = b.buf[:0] 139 b.pos = 0 140 b.cell = cell{} 141 b.endChar = 0 142 b.lines = b.lines[0:0] 143 b.widths = b.widths[0:0] 144 b.addLine(true) 145 } 146 147 // Internal representation (current state): 148 // 149 // - all text written is appended to buf; tabs and line breaks are stripped away 150 // - at any given time there is a (possibly empty) incomplete cell at the end 151 // (the cell starts after a tab or line break) 152 // - cell.size is the number of bytes belonging to the cell so far 153 // - cell.width is text width in runes of that cell from the start of the cell to 154 // position pos; html tags and entities are excluded from this width if html 155 // filtering is enabled 156 // - the sizes and widths of processed text are kept in the lines list 157 // which contains a list of cells for each line 158 // - the widths list is a temporary list with current widths used during 159 // formatting; it is kept in Writer because it's re-used 160 // 161 // |<---------- size ---------->| 162 // | | 163 // |<- width ->|<- ignored ->| | 164 // | | | | 165 // [---processed---tab------------<tag>...</tag>...] 166 // ^ ^ ^ 167 // | | | 168 // buf start of incomplete cell pos 169 170 // Formatting can be controlled with these flags. 171 const ( 172 // Ignore html tags and treat entities (starting with '&' 173 // and ending in ';') as single characters (width = 1). 174 FilterHTML uint = 1 << iota 175 176 // Strip Escape characters bracketing escaped text segments 177 // instead of passing them through unchanged with the text. 178 StripEscape 179 180 // Force right-alignment of cell content. 181 // Default is left-alignment. 182 AlignRight 183 184 // Handle empty columns as if they were not present in 185 // the input in the first place. 186 DiscardEmptyColumns 187 188 // Always use tabs for indentation columns (i.e., padding of 189 // leading empty cells on the left) independent of padchar. 190 TabIndent 191 192 // Print a vertical bar ('|') between columns (after formatting). 193 // Discarded columns appear as zero-width columns ("||"). 194 Debug 195 ) 196 197 // A Writer must be initialized with a call to Init. The first parameter (output) 198 // specifies the filter output. The remaining parameters control the formatting: 199 // 200 // minwidth minimal cell width including any padding 201 // tabwidth width of tab characters (equivalent number of spaces) 202 // padding padding added to a cell before computing its width 203 // padchar ASCII char used for padding 204 // if padchar == '\t', the Writer will assume that the 205 // width of a '\t' in the formatted output is tabwidth, 206 // and cells are left-aligned independent of align_left 207 // (for correct-looking results, tabwidth must correspond 208 // to the tab width in the viewer displaying the result) 209 // flags formatting control 210 // 211 func (b *Writer) Init(output io.Writer, minwidth, tabwidth, padding int, padchar byte, flags uint) *Writer { 212 if minwidth < 0 || tabwidth < 0 || padding < 0 { 213 panic("negative minwidth, tabwidth, or padding") 214 } 215 b.output = output 216 b.minwidth = minwidth 217 b.tabwidth = tabwidth 218 b.padding = padding 219 for i := range b.padbytes { 220 b.padbytes[i] = padchar 221 } 222 if padchar == '\t' { 223 // tab padding enforces left-alignment 224 flags &^= AlignRight 225 } 226 b.flags = flags 227 228 b.reset() 229 230 return b 231 } 232 233 // debugging support (keep code around) 234 func (b *Writer) dump() { 235 pos := 0 236 for i, line := range b.lines { 237 print("(", i, ") ") 238 for _, c := range line { 239 print("[", string(b.buf[pos:pos+c.size]), "]") 240 pos += c.size 241 } 242 print("\n") 243 } 244 print("\n") 245 } 246 247 // local error wrapper so we can distinguish errors we want to return 248 // as errors from genuine panics (which we don't want to return as errors) 249 type osError struct { 250 err error 251 } 252 253 func (b *Writer) write0(buf []byte) { 254 n, err := b.output.Write(buf) 255 if n != len(buf) && err == nil { 256 err = io.ErrShortWrite 257 } 258 if err != nil { 259 panic(osError{err}) 260 } 261 } 262 263 func (b *Writer) writeN(src []byte, n int) { 264 for n > len(src) { 265 b.write0(src) 266 n -= len(src) 267 } 268 b.write0(src[0:n]) 269 } 270 271 var ( 272 newline = []byte{'\n'} 273 tabs = []byte("\t\t\t\t\t\t\t\t") 274 ) 275 276 func (b *Writer) writePadding(textw, cellw int, useTabs bool) { 277 if b.padbytes[0] == '\t' || useTabs { 278 // padding is done with tabs 279 if b.tabwidth == 0 { 280 return // tabs have no width - can't do any padding 281 } 282 // make cellw the smallest multiple of b.tabwidth 283 cellw = (cellw + b.tabwidth - 1) / b.tabwidth * b.tabwidth 284 n := cellw - textw // amount of padding 285 if n < 0 { 286 panic("internal error") 287 } 288 b.writeN(tabs, (n+b.tabwidth-1)/b.tabwidth) 289 return 290 } 291 292 // padding is done with non-tab characters 293 b.writeN(b.padbytes[0:], cellw-textw) 294 } 295 296 var vbar = []byte{'|'} 297 298 func (b *Writer) writeLines(pos0 int, line0, line1 int) (pos int) { 299 pos = pos0 300 for i := line0; i < line1; i++ { 301 line := b.lines[i] 302 303 // if TabIndent is set, use tabs to pad leading empty cells 304 useTabs := b.flags&TabIndent != 0 305 306 for j, c := range line { 307 if j > 0 && b.flags&Debug != 0 { 308 // indicate column break 309 b.write0(vbar) 310 } 311 312 if c.size == 0 { 313 // empty cell 314 if j < len(b.widths) { 315 b.writePadding(c.width, b.widths[j], useTabs) 316 } 317 } else { 318 // non-empty cell 319 useTabs = false 320 if b.flags&AlignRight == 0 { // align left 321 b.write0(b.buf[pos : pos+c.size]) 322 pos += c.size 323 if j < len(b.widths) { 324 b.writePadding(c.width, b.widths[j], false) 325 } 326 } else { // align right 327 if j < len(b.widths) { 328 b.writePadding(c.width, b.widths[j], false) 329 } 330 b.write0(b.buf[pos : pos+c.size]) 331 pos += c.size 332 } 333 } 334 } 335 336 if i+1 == len(b.lines) { 337 // last buffered line - we don't have a newline, so just write 338 // any outstanding buffered data 339 b.write0(b.buf[pos : pos+b.cell.size]) 340 pos += b.cell.size 341 } else { 342 // not the last line - write newline 343 b.write0(newline) 344 } 345 } 346 return 347 } 348 349 // Format the text between line0 and line1 (excluding line1); pos 350 // is the buffer position corresponding to the beginning of line0. 351 // Returns the buffer position corresponding to the beginning of 352 // line1 and an error, if any. 353 // 354 func (b *Writer) format(pos0 int, line0, line1 int) (pos int) { 355 pos = pos0 356 column := len(b.widths) 357 for this := line0; this < line1; this++ { 358 line := b.lines[this] 359 360 if column >= len(line)-1 { 361 continue 362 } 363 // cell exists in this column => this line 364 // has more cells than the previous line 365 // (the last cell per line is ignored because cells are 366 // tab-terminated; the last cell per line describes the 367 // text before the newline/formfeed and does not belong 368 // to a column) 369 370 // print unprinted lines until beginning of block 371 pos = b.writeLines(pos, line0, this) 372 line0 = this 373 374 // column block begin 375 width := b.minwidth // minimal column width 376 discardable := true // true if all cells in this column are empty and "soft" 377 for ; this < line1; this++ { 378 line = b.lines[this] 379 if column >= len(line)-1 { 380 break 381 } 382 // cell exists in this column 383 c := line[column] 384 // update width 385 if w := c.width + b.padding; w > width { 386 width = w 387 } 388 // update discardable 389 if c.width > 0 || c.htab { 390 discardable = false 391 } 392 } 393 // column block end 394 395 // discard empty columns if necessary 396 if discardable && b.flags&DiscardEmptyColumns != 0 { 397 width = 0 398 } 399 400 // format and print all columns to the right of this column 401 // (we know the widths of this column and all columns to the left) 402 b.widths = append(b.widths, width) // push width 403 pos = b.format(pos, line0, this) 404 b.widths = b.widths[0 : len(b.widths)-1] // pop width 405 line0 = this 406 } 407 408 // print unprinted lines until end 409 return b.writeLines(pos, line0, line1) 410 } 411 412 // Append text to current cell. 413 func (b *Writer) append(text []byte) { 414 b.buf = append(b.buf, text...) 415 b.cell.size += len(text) 416 } 417 418 // Update the cell width. 419 func (b *Writer) updateWidth() { 420 b.cell.width += utf8.RuneCount(b.buf[b.pos:]) 421 b.pos = len(b.buf) 422 } 423 424 // To escape a text segment, bracket it with Escape characters. 425 // For instance, the tab in this string "Ignore this tab: \xff\t\xff" 426 // does not terminate a cell and constitutes a single character of 427 // width one for formatting purposes. 428 // 429 // The value 0xff was chosen because it cannot appear in a valid UTF-8 sequence. 430 // 431 const Escape = '\xff' 432 433 // Start escaped mode. 434 func (b *Writer) startEscape(ch byte) { 435 switch ch { 436 case Escape: 437 b.endChar = Escape 438 case '<': 439 b.endChar = '>' 440 case '&': 441 b.endChar = ';' 442 } 443 } 444 445 // Terminate escaped mode. If the escaped text was an HTML tag, its width 446 // is assumed to be zero for formatting purposes; if it was an HTML entity, 447 // its width is assumed to be one. In all other cases, the width is the 448 // unicode width of the text. 449 // 450 func (b *Writer) endEscape() { 451 switch b.endChar { 452 case Escape: 453 b.updateWidth() 454 if b.flags&StripEscape == 0 { 455 b.cell.width -= 2 // don't count the Escape chars 456 } 457 case '>': // tag of zero width 458 case ';': 459 b.cell.width++ // entity, count as one rune 460 } 461 b.pos = len(b.buf) 462 b.endChar = 0 463 } 464 465 // Terminate the current cell by adding it to the list of cells of the 466 // current line. Returns the number of cells in that line. 467 // 468 func (b *Writer) terminateCell(htab bool) int { 469 b.cell.htab = htab 470 line := &b.lines[len(b.lines)-1] 471 *line = append(*line, b.cell) 472 b.cell = cell{} 473 return len(*line) 474 } 475 476 func (b *Writer) handlePanic(err *error, op string) { 477 if e := recover(); e != nil { 478 if op == "Flush" { 479 // If Flush ran into a panic, we still need to reset. 480 b.reset() 481 } 482 if nerr, ok := e.(osError); ok { 483 *err = nerr.err 484 return 485 } 486 panic("tabwriter: panic during " + op) 487 } 488 } 489 490 // Flush should be called after the last call to Write to ensure 491 // that any data buffered in the Writer is written to output. Any 492 // incomplete escape sequence at the end is considered 493 // complete for formatting purposes. 494 func (b *Writer) Flush() error { 495 return b.flush() 496 } 497 498 // flush is the internal version of Flush, with a named return value which we 499 // don't want to expose. 500 func (b *Writer) flush() (err error) { 501 defer b.handlePanic(&err, "Flush") 502 b.flushNoDefers() 503 return nil 504 } 505 506 // flushNoDefers is like flush, but without a deferred handlePanic call. This 507 // can be called from other methods which already have their own deferred 508 // handlePanic calls, such as Write, and avoid the extra defer work. 509 func (b *Writer) flushNoDefers() { 510 // add current cell if not empty 511 if b.cell.size > 0 { 512 if b.endChar != 0 { 513 // inside escape - terminate it even if incomplete 514 b.endEscape() 515 } 516 b.terminateCell(false) 517 } 518 519 // format contents of buffer 520 b.format(0, 0, len(b.lines)) 521 b.reset() 522 } 523 524 var hbar = []byte("---\n") 525 526 // Write writes buf to the writer b. 527 // The only errors returned are ones encountered 528 // while writing to the underlying output stream. 529 // 530 func (b *Writer) Write(buf []byte) (n int, err error) { 531 defer b.handlePanic(&err, "Write") 532 533 // split text into cells 534 n = 0 535 for i, ch := range buf { 536 if b.endChar == 0 { 537 // outside escape 538 switch ch { 539 case '\t', '\v', '\n', '\f': 540 // end of cell 541 b.append(buf[n:i]) 542 b.updateWidth() 543 n = i + 1 // ch consumed 544 ncells := b.terminateCell(ch == '\t') 545 if ch == '\n' || ch == '\f' { 546 // terminate line 547 b.addLine(ch == '\f') 548 if ch == '\f' || ncells == 1 { 549 // A '\f' always forces a flush. Otherwise, if the previous 550 // line has only one cell which does not have an impact on 551 // the formatting of the following lines (the last cell per 552 // line is ignored by format()), thus we can flush the 553 // Writer contents. 554 b.flushNoDefers() 555 if ch == '\f' && b.flags&Debug != 0 { 556 // indicate section break 557 b.write0(hbar) 558 } 559 } 560 } 561 562 case Escape: 563 // start of escaped sequence 564 b.append(buf[n:i]) 565 b.updateWidth() 566 n = i 567 if b.flags&StripEscape != 0 { 568 n++ // strip Escape 569 } 570 b.startEscape(Escape) 571 572 case '<', '&': 573 // possibly an html tag/entity 574 if b.flags&FilterHTML != 0 { 575 // begin of tag/entity 576 b.append(buf[n:i]) 577 b.updateWidth() 578 n = i 579 b.startEscape(ch) 580 } 581 } 582 583 } else { 584 // inside escape 585 if ch == b.endChar { 586 // end of tag/entity 587 j := i + 1 588 if ch == Escape && b.flags&StripEscape != 0 { 589 j = i // strip Escape 590 } 591 b.append(buf[n:j]) 592 n = i + 1 // ch consumed 593 b.endEscape() 594 } 595 } 596 } 597 598 // append leftover text 599 b.append(buf[n:]) 600 n = len(buf) 601 return 602 } 603 604 // NewWriter allocates and initializes a new tabwriter.Writer. 605 // The parameters are the same as for the Init function. 606 // 607 func NewWriter(output io.Writer, minwidth, tabwidth, padding int, padchar byte, flags uint) *Writer { 608 return new(Writer).Init(output, minwidth, tabwidth, padding, padchar, flags) 609 } 610