// Copyright 2009 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
// Package tabwriter implements a write filter (tabwriter.Writer) that
// translates tabbed columns in input into properly aligned text.
//
// The package is using the Elastic Tabstops algorithm described at
// http://nickgravgaard.com/elastictabstops/index.html.
//
// The text/tabwriter package is frozen and is not accepting new features.
package tabwriter
import (
"bytes"
"io"
"unicode/utf8"
)
// ----------------------------------------------------------------------------
// Filter implementation
// A cell represents a segment of text terminated by tabs or line breaks.
// The text itself is stored in a separate buffer; cell only describes the
// segment's size in bytes, its width in runes, and whether it's an htab
// ('\t') terminated cell.
//
type cell struct {
size int // cell size in bytes
width int // cell width in runes
htab bool // true if the cell is terminated by an htab ('\t')
}
// A Writer is a filter that inserts padding around tab-delimited
// columns in its input to align them in the output.
//
// The Writer treats incoming bytes as UTF-8-encoded text consisting
// of cells terminated by horizontal ('\t') or vertical ('\v') tabs,
// and newline ('\n') or formfeed ('\f') characters; both newline and
// formfeed act as line breaks.
//
// Tab-terminated cells in contiguous lines constitute a column. The
// Writer inserts padding as needed to make all cells in a column have
// the same width, effectively aligning the columns. It assumes that
// all characters have the same width, except for tabs for which a
// tabwidth must be specified. Column cells must be tab-terminated, not
// tab-separated: non-tab terminated trailing text at the end of a line
// forms a cell but that cell is not part of an aligned column.
// For instance, in this example (where | stands for a horizontal tab):
//
// aaaa|bbb|d
// aa |b |dd
// a |
// aa |cccc|eee
//
// the b and c are in distinct columns (the b column is not contiguous
// all the way). The d and e are not in a column at all (there's no
// terminating tab, nor would the column be contiguous).
//
// The Writer assumes that all Unicode code points have the same width;
// this may not be true in some fonts or if the string contains combining
// characters.
//
// If DiscardEmptyColumns is set, empty columns that are terminated
// entirely by vertical (or "soft") tabs are discarded. Columns
// terminated by horizontal (or "hard") tabs are not affected by
// this flag.
//
// If a Writer is configured to filter HTML, HTML tags and entities
// are passed through. The widths of tags and entities are
// assumed to be zero (tags) and one (entities) for formatting purposes.
//
// A segment of text may be escaped by bracketing it with Escape
// characters. The tabwriter passes escaped text segments through
// unchanged. In particular, it does not interpret any tabs or line
// breaks within the segment. If the StripEscape flag is set, the
// Escape characters are stripped from the output; otherwise they
// are passed through as well. For the purpose of formatting, the
// width of the escaped text is always computed excluding the Escape
// characters.
//
// The formfeed character acts like a newline but it also terminates
// all columns in the current line (effectively calling Flush). Tab-
// terminated cells in the next line start new columns. Unless found
// inside an HTML tag or inside an escaped text segment, formfeed
// characters appear as newlines in the output.
//
// The Writer must buffer input internally, because proper spacing
// of one line may depend on the cells in future lines. Clients must
// call Flush when done calling Write.
//
type Writer struct {
// configuration
output io.Writer
minwidth int
tabwidth int
padding int
padbytes [8]byte
flags uint
// current state
buf bytes.Buffer // collected text excluding tabs or line breaks
pos int // buffer position up to which cell.width of incomplete cell has been computed
cell cell // current incomplete cell; cell.width is up to buf[pos] excluding ignored sections
endChar byte // terminating char of escaped sequence (Escape for escapes, '>', ';' for HTML tags/entities, or 0)
lines [][]cell // list of lines; each line is a list of cells
widths []int // list of column widths in runes - re-used during formatting
}
func (b *Writer) addLine() { b.lines = append(b.lines, []cell{}) }
// Reset the current state.
func (b *Writer) reset() {
b.buf.Reset()
b.pos = 0
b.cell = cell{}
b.endChar = 0
b.lines = b.lines[0:0]
b.widths = b.widths[0:0]
b.addLine()
}
// Internal representation (current state):
//
// - all text written is appended to buf; tabs and line breaks are stripped away
// - at any given time there is a (possibly empty) incomplete cell at the end
// (the cell starts after a tab or line break)
// - cell.size is the number of bytes belonging to the cell so far
// - cell.width is text width in runes of that cell from the start of the cell to
// position pos; html tags and entities are excluded from this width if html
// filtering is enabled
// - the sizes and widths of processed text are kept in the lines list
// which contains a list of cells for each line
// - the widths list is a temporary list with current widths used during
// formatting; it is kept in Writer because it's re-used
//
// |<---------- size ---------->|
// | |
// |<- width ->|<- ignored ->| |
// | | | |
// [---processed---tab------------<tag>...</tag>...]
// ^ ^ ^
// | | |
// buf start of incomplete cell pos
// Formatting can be controlled with these flags.
const (
// Ignore html tags and treat entities (starting with '&'
// and ending in ';') as single characters (width = 1).
FilterHTML uint = 1 << iota
// Strip Escape characters bracketing escaped text segments
// instead of passing them through unchanged with the text.
StripEscape
// Force right-alignment of cell content.
// Default is left-alignment.
AlignRight
// Handle empty columns as if they were not present in
// the input in the first place.
DiscardEmptyColumns
// Always use tabs for indentation columns (i.e., padding of
// leading empty cells on the left) independent of padchar.
TabIndent
// Print a vertical bar ('|') between columns (after formatting).
// Discarded columns appear as zero-width columns ("||").
Debug
)
// A Writer must be initialized with a call to Init. The first parameter (output)
// specifies the filter output. The remaining parameters control the formatting:
//
// minwidth minimal cell width including any padding
// tabwidth width of tab characters (equivalent number of spaces)
// padding padding added to a cell before computing its width
// padchar ASCII char used for padding
// if padchar == '\t', the Writer will assume that the
// width of a '\t' in the formatted output is tabwidth,
// and cells are left-aligned independent of align_left
// (for correct-looking results, tabwidth must correspond
// to the tab width in the viewer displaying the result)
// flags formatting control
//
func (b *Writer) Init(output io.Writer, minwidth, tabwidth, padding int, padchar byte, flags uint) *Writer {
if minwidth < 0 || tabwidth < 0 || padding < 0 {
panic("negative minwidth, tabwidth, or padding")
}
b.output = output
b.minwidth = minwidth
b.tabwidth = tabwidth
b.padding = padding
for i := range b.padbytes {
b.padbytes[i] = padchar
}
if padchar == '\t' {
// tab padding enforces left-alignment
flags &^= AlignRight
}
b.flags = flags
b.reset()
return b
}
// debugging support (keep code around)
func (b *Writer) dump() {
pos := 0
for i, line := range b.lines {
print("(", i, ") ")
for _, c := range line {
print("[", string(b.buf.Bytes()[pos:pos+c.size]), "]")
pos += c.size
}
print("\n")
}
print("\n")
}
// local error wrapper so we can distinguish errors we want to return
// as errors from genuine panics (which we don't want to return as errors)
type osError struct {
err error
}
func (b *Writer) write0(buf []byte) {
n, err := b.output.Write(buf)
if n != len(buf) && err == nil {
err = io.ErrShortWrite
}
if err != nil {
panic(osError{err})
}
}
func (b *Writer) writeN(src []byte, n int) {
for n > len(src) {
b.write0(src)
n -= len(src)
}
b.write0(src[0:n])
}
var (
newline = []byte{'\n'}
tabs = []byte("\t\t\t\t\t\t\t\t")
)
func (b *Writer) writePadding(textw, cellw int, useTabs bool) {
if b.padbytes[0] == '\t' || useTabs {
// padding is done with tabs
if b.tabwidth == 0 {
return // tabs have no width - can't do any padding
}
// make cellw the smallest multiple of b.tabwidth
cellw = (cellw + b.tabwidth - 1) / b.tabwidth * b.tabwidth
n := cellw - textw // amount of padding
if n < 0 {
panic("internal error")
}
b.writeN(tabs, (n+b.tabwidth-1)/b.tabwidth)
return
}
// padding is done with non-tab characters
b.writeN(b.padbytes[0:], cellw-textw)
}
var vbar = []byte{'|'}
func (b *Writer) writeLines(pos0 int, line0, line1 int) (pos int) {
pos = pos0
for i := line0; i < line1; i++ {
line := b.lines[i]
// if TabIndent is set, use tabs to pad leading empty cells
useTabs := b.flags&TabIndent != 0
for j, c := range line {
if j > 0 && b.flags&Debug != 0 {
// indicate column break
b.write0(vbar)
}
if c.size == 0 {
// empty cell
if j < len(b.widths) {
b.writePadding(c.width, b.widths[j], useTabs)
}
} else {
// non-empty cell
useTabs = false
if b.flags&AlignRight == 0 { // align left
b.write0(b.buf.Bytes()[pos : pos+c.size])
pos += c.size
if j < len(b.widths) {
b.writePadding(c.width, b.widths[j], false)
}
} else { // align right
if j < len(b.widths) {
b.writePadding(c.width, b.widths[j], false)
}
b.write0(b.buf.Bytes()[pos : pos+c.size])
pos += c.size
}
}
}
if i+1 == len(b.lines) {
// last buffered line - we don't have a newline, so just write
// any outstanding buffered data
b.write0(b.buf.Bytes()[pos : pos+b.cell.size])
pos += b.cell.size
} else {
// not the last line - write newline
b.write0(newline)
}
}
return
}
// Format the text between line0 and line1 (excluding line1); pos
// is the buffer position corresponding to the beginning of line0.
// Returns the buffer position corresponding to the beginning of
// line1 and an error, if any.
//
func (b *Writer) format(pos0 int, line0, line1 int) (pos int) {
pos = pos0
column := len(b.widths)
for this := line0; this < line1; this++ {
line := b.lines[this]
if column >= len(line)-1 {
continue
}
// cell exists in this column => this line
// has more cells than the previous line
// (the last cell per line is ignored because cells are
// tab-terminated; the last cell per line describes the
// text before the newline/formfeed and does not belong
// to a column)
// print unprinted lines until beginning of block
pos = b.writeLines(pos, line0, this)
line0 = this
// column block begin
width := b.minwidth // minimal column width
discardable := true // true if all cells in this column are empty and "soft"
for ; this < line1; this++ {
line = b.lines[this]
if column >= len(line)-1 {
break
}
// cell exists in this column
c := line[column]
// update width
if w := c.width + b.padding; w > width {
width = w
}
// update discardable
if c.width > 0 || c.htab {
discardable = false
}
}
// column block end
// discard empty columns if necessary
if discardable && b.flags&DiscardEmptyColumns != 0 {
width = 0
}
// format and print all columns to the right of this column
// (we know the widths of this column and all columns to the left)
b.widths = append(b.widths, width) // push width
pos = b.format(pos, line0, this)
b.widths = b.widths[0 : len(b.widths)-1] // pop width
line0 = this
}
// print unprinted lines until end
return b.writeLines(pos, line0, line1)
}
// Append text to current cell.
func (b *Writer) append(text []byte) {
b.buf.Write(text)
b.cell.size += len(text)
}
// Update the cell width.
func (b *Writer) updateWidth() {
b.cell.width += utf8.RuneCount(b.buf.Bytes()[b.pos:b.buf.Len()])
b.pos = b.buf.Len()
}
// To escape a text segment, bracket it with Escape characters.
// For instance, the tab in this string "Ignore this tab: \xff\t\xff"
// does not terminate a cell and constitutes a single character of
// width one for formatting purposes.
//
// The value 0xff was chosen because it cannot appear in a valid UTF-8 sequence.
//
const Escape = '\xff'
// Start escaped mode.
func (b *Writer) startEscape(ch byte) {
switch ch {
case Escape:
b.endChar = Escape
case '<':
b.endChar = '>'
case '&':
b.endChar = ';'
}
}
// Terminate escaped mode. If the escaped text was an HTML tag, its width
// is assumed to be zero for formatting purposes; if it was an HTML entity,
// its width is assumed to be one. In all other cases, the width is the
// unicode width of the text.
//
func (b *Writer) endEscape() {
switch b.endChar {
case Escape:
b.updateWidth()
if b.flags&StripEscape == 0 {
b.cell.width -= 2 // don't count the Escape chars
}
case '>': // tag of zero width
case ';':
b.cell.width++ // entity, count as one rune
}
b.pos = b.buf.Len()
b.endChar = 0
}
// Terminate the current cell by adding it to the list of cells of the
// current line. Returns the number of cells in that line.
//
func (b *Writer) terminateCell(htab bool) int {
b.cell.htab = htab
line := &b.lines[len(b.lines)-1]
*line = append(*line, b.cell)
b.cell = cell{}
return len(*line)
}
func handlePanic(err *error, op string) {
if e := recover(); e != nil {
if nerr, ok := e.(osError); ok {
*err = nerr.err
return
}
panic("tabwriter: panic during " + op)
}
}
// Flush should be called after the last call to Write to ensure
// that any data buffered in the Writer is written to output. Any
// incomplete escape sequence at the end is considered
// complete for formatting purposes.
func (b *Writer) Flush() error {
return b.flush()
}
func (b *Writer) flush() (err error) {
defer b.reset() // even in the presence of errors
defer handlePanic(&err, "Flush")
// add current cell if not empty
if b.cell.size > 0 {
if b.endChar != 0 {
// inside escape - terminate it even if incomplete
b.endEscape()
}
b.terminateCell(false)
}
// format contents of buffer
b.format(0, 0, len(b.lines))
return nil
}
var hbar = []byte("---\n")
// Write writes buf to the writer b.
// The only errors returned are ones encountered
// while writing to the underlying output stream.
//
func (b *Writer) Write(buf []byte) (n int, err error) {
defer handlePanic(&err, "Write")
// split text into cells
n = 0
for i, ch := range buf {
if b.endChar == 0 {
// outside escape
switch ch {
case '\t', '\v', '\n', '\f':
// end of cell
b.append(buf[n:i])
b.updateWidth()
n = i + 1 // ch consumed
ncells := b.terminateCell(ch == '\t')
if ch == '\n' || ch == '\f' {
// terminate line
b.addLine()
if ch == '\f' || ncells == 1 {
// A '\f' always forces a flush. Otherwise, if the previous
// line has only one cell which does not have an impact on
// the formatting of the following lines (the last cell per
// line is ignored by format()), thus we can flush the
// Writer contents.
if err = b.Flush(); err != nil {
return
}
if ch == '\f' && b.flags&Debug != 0 {
// indicate section break
b.write0(hbar)
}
}
}
case Escape:
// start of escaped sequence
b.append(buf[n:i])
b.updateWidth()
n = i
if b.flags&StripEscape != 0 {
n++ // strip Escape
}
b.startEscape(Escape)
case '<', '&':
// possibly an html tag/entity
if b.flags&FilterHTML != 0 {
// begin of tag/entity
b.append(buf[n:i])
b.updateWidth()
n = i
b.startEscape(ch)
}
}
} else {
// inside escape
if ch == b.endChar {
// end of tag/entity
j := i + 1
if ch == Escape && b.flags&StripEscape != 0 {
j = i // strip Escape
}
b.append(buf[n:j])
n = i + 1 // ch consumed
b.endEscape()
}
}
}
// append leftover text
b.append(buf[n:])
n = len(buf)
return
}
// NewWriter allocates and initializes a new tabwriter.Writer.
// The parameters are the same as for the Init function.
//
func NewWriter(output io.Writer, minwidth, tabwidth, padding int, padchar byte, flags uint) *Writer {
return new(Writer).Init(output, minwidth, tabwidth, padding, padchar, flags)
}