textutil/line_writer.go - release.go.x.lib - Git at Google

 // Copyright 2015 The Vanadium 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 textutil

 import (
 	"fmt"
 	"io"
 	"unicode"
 )

 // LineWriter implements an io.Writer filter that formats input text into output
 // lines with a given target width in runes.
 //
 // Each input rune is classified into one of three kinds:
 //   EOL:    end-of-line, consisting of \f, \n, \r, \v, U+2028 or U+2029
 //   Space:  defined by unicode.IsSpace
 //   Letter: everything else
 //
 // The input text is expected to consist of words, defined as sequences of
 // letters.  Sequences of words form paragraphs, where paragraphs are separated
 // by either blank lines (that contain no letters), or an explicit U+2029
 // ParagraphSeparator.  Input lines with leading spaces are treated verbatim.
 //
 // Paragraphs are output as word-wrapped lines; line breaks only occur at word
 // boundaries.  Output lines are usually no longer than the target width.  The
 // exceptions are single words longer than the target width, which are output on
 // their own line, and verbatim lines, which may be arbitrarily longer or
 // shorter than the width.
 //
 // Output lines never contain trailing spaces.  Only verbatim output lines may
 // contain leading spaces.  Spaces separating input words are output verbatim,
 // unless it would result in a line with leading or trailing spaces.
 //
 // EOL runes within the input text are never written to the output; the output
 // line terminator and paragraph separator may be configured, and some EOL may
 // be output as a single space ' ' to maintain word separation.
 //
 // The algorithm greedily fills each output line with as many words as it can,
 // assuming that all Unicode code points have the same width.  Invalid UTF-8 is
 // silently transformed to the replacement character U+FFFD and treated as a
 // single rune.
 //
 // Flush must be called after the last call to Write; the input is buffered.
 //
 //   Implementation note: line breaking is a complicated topic.  This approach
 //   attempts to be simple and useful; a full implementation conforming to
 //   Unicode Standard Annex #14 would be complicated, and is not implemented.
 //   Languages that don't use spaces to separate words (e.g. CJK) won't work
 //   well under the current approach.
 //
 //   http://www.unicode.org/reports/tr14 [Unicode Line Breaking Algorithm]
 //   http://www.unicode.org/versions/Unicode4.0.0/ch05.pdf [5.8 Newline Guidelines]
 type LineWriter struct {
 	// State configured by the user.
 	w             io.Writer
 	runeDecoder   RuneChunkDecoder
 	width         runePos
 	lineTerm      []byte
 	paragraphSep  string
 	indents       []string
 	forceVerbatim bool

 	// The buffer contains a single output line.
 	lineBuf byteRuneBuffer

 	// Keep track of the previous state and rune.
 	prevState state
 	prevRune  rune

 	// Keep track of blank input lines.
 	inputLineHasLetter bool

 	// lineBuf positions where the line starts (after separators and indents), a
 	// new word has started and the last word has ended.
 	lineStart    bytePos
 	newWordStart bytePos
 	lastWordEnd  bytePos

 	// Keep track of paragraph terminations and line indices, so we can output the
 	// paragraph separator and indents correctly.
 	terminateParagraph bool
 	paragraphLineIndex int
 	wroteFirstLine     bool
 }

 type state int

 const (
 	stateWordWrap  state = iota // Perform word-wrapping [start state]
 	stateVerbatim               // Verbatim output-line, no word-wrapping
 	stateSkipSpace              // Skip spaces in input line.
 )

 // NewLineWriter returns a new LineWriter with the given target width in runes,
 // producing output on the underlying writer w.  The dec and enc are used to
 // respectively decode runes from Write calls, and encode runes to w.
 func NewLineWriter(w io.Writer, width int, dec RuneChunkDecoder, enc RuneEncoder) *LineWriter {
 	ret := &LineWriter{
 		w:            w,
 		runeDecoder:  dec,
 		width:        runePos(width),
 		lineTerm:     []byte("\n"),
 		paragraphSep: "\n",
 		prevState:    stateWordWrap,
 		prevRune:     LineSeparator,
 		lineBuf:      byteRuneBuffer{enc: enc},
 	}
 	ret.resetLine()
 	return ret
 }

 // NewUTF8LineWriter returns a new LineWriter filter that implements io.Writer,
 // and decodes and encodes runes in UTF-8.
 func NewUTF8LineWriter(w io.Writer, width int) *LineWriter {
 	return NewLineWriter(w, width, &UTF8ChunkDecoder{}, UTF8Encoder{})
 }

 // Width returns the target width in runes.  If width < 0 the width is
 // unlimited; each paragraph is output as a single line.
 func (w *LineWriter) Width() int { return int(w.width) }

 // SetLineTerminator sets the line terminator for subsequent Write calls.  Every
 // output line is terminated with term; EOL runes from the input are never
 // written to the output.  A new LineWriter instance uses "\n" as the default
 // line terminator.
 //
 // Calls Flush internally, and returns any Flush error.
 func (w *LineWriter) SetLineTerminator(term string) error {
 	if err := w.Flush(); err != nil {
 		return err
 	}
 	w.lineTerm = []byte(term)
 	w.resetLine()
 	return nil
 }

 // SetParagraphSeparator sets the paragraph separator for subsequent Write
 // calls.  Every consecutive pair of non-empty paragraphs is separated with sep;
 // EOL runes from the input are never written to the output.  A new LineWriter
 // instance uses "\n" as the default paragraph separator.
 //
 // Calls Flush internally, and returns any Flush error.
 func (w *LineWriter) SetParagraphSeparator(sep string) error {
 	if err := w.Flush(); err != nil {
 		return err
 	}
 	w.paragraphSep = sep
 	w.resetLine()
 	return nil
 }

 // SetIndents sets the indentation for subsequent Write calls.  Multiple indents
 // may be set, corresponding to the indent to use for the corresponding
 // paragraph line.  E.g. SetIndents("AA", "BBB", C") means the first line in
 // each paragraph is indented with "AA", the second line in each paragraph is
 // indented with "BBB", and all subsequent lines in each paragraph are indented
 // with "C".
 //
 // SetIndents() is equivalent to SetIndents(""), SetIndents("", ""), etc.
 //
 // A new LineWriter instance has no indents by default.
 //
 // Calls Flush internally, and returns any Flush error.
 func (w *LineWriter) SetIndents(indents ...string) error {
 	if err := w.Flush(); err != nil {
 		return err
 	}
 	// Copy indents in case the user passed the slice via SetIndents(p...), and
 	// canonicalize the all empty case to nil.
 	allEmpty := true
 	w.indents = make([]string, len(indents))
 	for ix, indent := range indents {
 		w.indents[ix] = indent
 		if indent != "" {
 			allEmpty = false
 		}
 	}
 	if allEmpty {
 		w.indents = nil
 	}
 	w.resetLine()
 	return nil
 }

 // ForceVerbatim forces w to stay in verbatim mode if v is true, or lets w
 // perform its regular line writing algorithm if v is false.  This is useful if
 // there is a sequence of lines that should be written verbatim, even if the
 // lines don't start with spaces.
 //
 // Calls Flush internally, and returns any Flush error.
 func (w *LineWriter) ForceVerbatim(v bool) error {
 	w.forceVerbatim = v
 	return w.Flush()
 }

 // Write implements io.Writer by buffering data into the LineWriter w.  Actual
 // writes to the underlying writer may occur, and may include data buffered in
 // either this Write call or previous Write calls.
 //
 // Flush must be called after the last call to Write.
 func (w *LineWriter) Write(data []byte) (int, error) {
 	return WriteRuneChunk(w.runeDecoder, w.addRune, data)
 }

 // Flush flushes any remaining buffered text, and resets the paragraph line
 // count back to 0, so that indents will be applied starting from the first
 // line.  It does not imply a paragraph separator; repeated calls to Flush with
 // no intervening calls to other methods is equivalent to a single Flush.
 //
 // Flush must be called after the last call to Write, and may be called an
 // arbitrary number of times before the last Write.
 func (w *LineWriter) Flush() error {
 	if err := FlushRuneChunk(w.runeDecoder, w.addRune); err != nil {
 		return err
 	}
 	// Add U+2028 to force the last line (if any) to be written.
 	if err := w.addRune(LineSeparator); err != nil {
 		return err
 	}
 	// Reset the paragraph line count.
 	w.paragraphLineIndex = 0
 	w.resetLine()
 	return nil
 }

 // addRune is called every time w.runeDecoder decodes a full rune.
 func (w *LineWriter) addRune(r rune) error {
 	state, lineBreak := w.nextState(r, w.updateRune(r))
 	if lineBreak {
 		if err := w.writeLine(); err != nil {
 			return err
 		}
 	}
 	w.bufferRune(r, state, lineBreak)
 	w.prevState = state
 	w.prevRune = r
 	return nil
 }

 // We classify each incoming rune into three kinds for easier handling.
 type kind int

 const (
 	kindEOL kind = iota
 	kindSpace
 	kindLetter
 )

 func runeKind(r rune) kind {
 	switch r {
 	case '\f', '\n', '\r', '\v', LineSeparator, ParagraphSeparator:
 		return kindEOL
 	}
 	if unicode.IsSpace(r) {
 		return kindSpace
 	}
 	return kindLetter
 }

 func (w *LineWriter) updateRune(r rune) bool {
 	forceLineBreak := false
 	switch kind := runeKind(r); kind {
 	case kindEOL:
 		// Update lastWordEnd if the last word just ended.
 		if w.newWordStart != -1 {
 			w.newWordStart = -1
 			w.lastWordEnd = w.lineBuf.ByteLen()
 		}
 		switch {
 		case w.prevRune == '\r' && r == '\n':
 			// Treat "\r\n" as a single EOL; we've already handled the logic for '\r',
 			// so there's nothing to do when we see '\n'.
 		case r == LineSeparator:
 			// Treat U+2028 as a pure line break; it's never a paragraph break.
 			forceLineBreak = true
 		case r == ParagraphSeparator || !w.inputLineHasLetter:
 			// The paragraph has just been terminated if we see an explicit U+2029, or
 			// if we see a blank line, which may contain spaces.
 			forceLineBreak = true
 			w.terminateParagraph = true
 		}
 		w.inputLineHasLetter = false
 	case kindSpace:
 		// Update lastWordEnd if the last word just ended.
 		if w.newWordStart != -1 {
 			w.newWordStart = -1
 			w.lastWordEnd = w.lineBuf.ByteLen()
 		}
 	case kindLetter:
 		// Update newWordStart if a new word just started.
 		if w.newWordStart == -1 {
 			w.newWordStart = w.lineBuf.ByteLen()
 		}
 		w.inputLineHasLetter = true
 		w.terminateParagraph = false
 	default:
 		panic(fmt.Errorf("textutil: updateRune unhandled kind %d", kind))
 	}
 	return forceLineBreak
 }

 // nextState returns the next state and whether we should break the line.
 //
 // Here's a handy table that describes all the scenarios in which we will line
 // break input text, grouped by the reason for the break.  The current position
 // is the last non-* rune in each pattern, which is where we decide to break.
 //
 //              w.prevState   Next state   Buffer reset
 //              -----------   ----------   ------------
 //   ===== Force line break (U+2028 / U+2029, blank line) =====
 //   a..*|***   *             wordWrap     empty
 //   a._.|***   *             wordWrap     empty
 //   a+**|***   *             wordWrap     empty
 //
 //   ===== verbatim: wait for any EOL =====
 //   _*.*|***   verbatim      wordWrap     empty
 //
 //   ===== wordWrap: switch to verbatim =====
 //   a._*|***   wordWrap      verbatim     empty
 //
 //   ===== wordWrap: line is too wide =====
 //   abc.|***   wordWrap      wordWrap     empty
 //   abcd|.**   wordWrap      wordWrap     empty
 //   abcd|e.*   wordWrap      wordWrap     empty
 //   a_cd|.**   wordWrap      wordWrap     empty
 //
 //   abc_|***   wordWrap      skipSpace    empty
 //   abcd|_**   wordWrap      skipSpace    empty
 //   abcd|e_*   wordWrap      skipSpace    empty
 //   a_cd|_**   wordWrap      skipSpace    empty
 //
 //   a_cd|e**   wordWrap      start        newWordStart
 //
 //   LEGEND
 //     abcde  Letter
 //     .      End-of-line
 //     +      End-of-line (only U+2028 / U+2029)
 //     _      Space
 //     *      Any rune (letter, line-end or space)
 //     |      Visual indication of width=4, has no width itself.
 //
 // Note that Flush calls behave exactly as if an explicit U+2028 line separator
 // were added to the end of all buffered data.
 func (w *LineWriter) nextState(r rune, forceLineBreak bool) (state, bool) {
 	kind := runeKind(r)
 	if w.forceVerbatim {
 		return stateVerbatim, forceLineBreak || kind == kindEOL
 	}
 	if forceLineBreak {
 		return stateWordWrap, true
 	}
 	// Handle non word-wrap states, which are easy.
 	switch w.prevState {
 	case stateVerbatim:
 		if kind == kindEOL {
 			return stateWordWrap, true
 		}
 		return stateVerbatim, false
 	case stateSkipSpace:
 		if kind == kindSpace {
 			return stateSkipSpace, false
 		}
 		return stateWordWrap, false
 	}
 	// Handle stateWordWrap, which is more complicated.

 	// Switch to the verbatim state when we see a space right after an EOL.
 	if runeKind(w.prevRune) == kindEOL && kind == kindSpace {
 		return stateVerbatim, true
 	}
 	// Break on EOL or space when the line is too wide.  See above table.
 	if w.width >= 0 && w.width <= w.lineBuf.RuneLen()+1 {
 		switch kind {
 		case kindEOL:
 			return stateWordWrap, true
 		case kindSpace:
 			return stateSkipSpace, true
 		}
 		// case kindLetter falls through
 	}
 	// Handle the newWordStart case in the above table.
 	if w.width >= 0 && w.width < w.lineBuf.RuneLen()+1 && w.newWordStart != w.lineStart {
 		return stateWordWrap, true
 	}
 	// Stay in the wordWrap state and don't break the line.
 	return stateWordWrap, false
 }

 func (w *LineWriter) writeLine() error {
 	if w.lastWordEnd == -1 {
 		// Don't write blank lines, but we must reset the line in case the paragraph
 		// has just been terminated.
 		w.resetLine()
 		return nil
 	}
 	// Write the line (without trailing spaces) followed by the line terminator.
 	line := w.lineBuf.Bytes()[:w.lastWordEnd]
 	if _, err := w.w.Write(line); err != nil {
 		return err
 	}
 	if _, err := w.w.Write(w.lineTerm); err != nil {
 		return err
 	}
 	// Reset the line buffer.
 	w.wroteFirstLine = true
 	w.paragraphLineIndex++
 	if w.newWordStart != -1 {
 		// If we have an unterminated new word, we must be in the newWordStart case
 		// in the table above.  Handle the special buffer reset here.
 		newWord := string(w.lineBuf.Bytes()[w.newWordStart:])
 		w.resetLine()
 		w.newWordStart = w.lineBuf.ByteLen()
 		w.lineBuf.WriteString(newWord)
 	} else {
 		w.resetLine()
 	}
 	return nil
 }

 func (w *LineWriter) resetLine() {
 	w.lineBuf.Reset()
 	w.newWordStart = -1
 	w.lastWordEnd = -1
 	// Write the paragraph separator if the previous paragraph has terminated.
 	// This consumes no runes from the line width.
 	if w.wroteFirstLine && w.terminateParagraph {
 		w.lineBuf.WriteString0Runes(w.paragraphSep)
 		w.paragraphLineIndex = 0
 	}
 	// Add indent; a non-empty indent consumes runes from the line width.
 	var indent string
 	switch {
 	case w.paragraphLineIndex < len(w.indents):
 		indent = w.indents[w.paragraphLineIndex]
 	case len(w.indents) > 0:
 		indent = w.indents[len(w.indents)-1]
 	}
 	w.lineBuf.WriteString(indent)
 	w.lineStart = w.lineBuf.ByteLen()
 }

 func (w *LineWriter) bufferRune(r rune, state state, lineBreak bool) {
 	// Never add leading spaces to the buffer in the wordWrap state.
 	wordWrapNoLeadingSpaces := state == stateWordWrap && !lineBreak
 	switch kind := runeKind(r); kind {
 	case kindEOL:
 		// When we're word-wrapping and we see a letter followed by EOL, we convert
 		// the EOL into a single space in the buffer, to break the previous word
 		// from the next word.
 		if wordWrapNoLeadingSpaces && runeKind(w.prevRune) == kindLetter {
 			w.lineBuf.WriteRune(' ')
 		}
 	case kindSpace:
 		if wordWrapNoLeadingSpaces || state == stateVerbatim {
 			w.lineBuf.WriteRune(r)
 		}
 	case kindLetter:
 		w.lineBuf.WriteRune(r)
 	default:
 		panic(fmt.Errorf("textutil: bufferRune unhandled kind %d", kind))
 	}
 }
	// Copyright 2015 The Vanadium 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 textutil

	import (
	"fmt"
	"io"
	"unicode"
	)

	// LineWriter implements an io.Writer filter that formats input text into output
	// lines with a given target width in runes.
	//
	// Each input rune is classified into one of three kinds:
	// EOL: end-of-line, consisting of \f, \n, \r, \v, U+2028 or U+2029
	// Space: defined by unicode.IsSpace
	// Letter: everything else
	//
	// The input text is expected to consist of words, defined as sequences of
	// letters. Sequences of words form paragraphs, where paragraphs are separated
	// by either blank lines (that contain no letters), or an explicit U+2029
	// ParagraphSeparator. Input lines with leading spaces are treated verbatim.
	//
	// Paragraphs are output as word-wrapped lines; line breaks only occur at word
	// boundaries. Output lines are usually no longer than the target width. The
	// exceptions are single words longer than the target width, which are output on
	// their own line, and verbatim lines, which may be arbitrarily longer or
	// shorter than the width.
	//
	// Output lines never contain trailing spaces. Only verbatim output lines may
	// contain leading spaces. Spaces separating input words are output verbatim,
	// unless it would result in a line with leading or trailing spaces.
	//
	// EOL runes within the input text are never written to the output; the output
	// line terminator and paragraph separator may be configured, and some EOL may
	// be output as a single space ' ' to maintain word separation.
	//
	// The algorithm greedily fills each output line with as many words as it can,
	// assuming that all Unicode code points have the same width. Invalid UTF-8 is
	// silently transformed to the replacement character U+FFFD and treated as a
	// single rune.
	//
	// Flush must be called after the last call to Write; the input is buffered.
	//
	// Implementation note: line breaking is a complicated topic. This approach
	// attempts to be simple and useful; a full implementation conforming to
	// Unicode Standard Annex #14 would be complicated, and is not implemented.
	// Languages that don't use spaces to separate words (e.g. CJK) won't work
	// well under the current approach.
	//
	// http://www.unicode.org/reports/tr14 [Unicode Line Breaking Algorithm]
	// http://www.unicode.org/versions/Unicode4.0.0/ch05.pdf [5.8 Newline Guidelines]
	type LineWriter struct {
	// State configured by the user.
	w io.Writer
	runeDecoder RuneChunkDecoder
	width runePos
	lineTerm []byte
	paragraphSep string
	indents []string
	forceVerbatim bool

	// The buffer contains a single output line.
	lineBuf byteRuneBuffer

	// Keep track of the previous state and rune.
	prevState state
	prevRune rune

	// Keep track of blank input lines.
	inputLineHasLetter bool

	// lineBuf positions where the line starts (after separators and indents), a
	// new word has started and the last word has ended.
	lineStart bytePos
	newWordStart bytePos
	lastWordEnd bytePos

	// Keep track of paragraph terminations and line indices, so we can output the
	// paragraph separator and indents correctly.
	terminateParagraph bool
	paragraphLineIndex int
	wroteFirstLine bool
	}

	type state int

	const (
	stateWordWrap state = iota // Perform word-wrapping [start state]
	stateVerbatim // Verbatim output-line, no word-wrapping
	stateSkipSpace // Skip spaces in input line.
	)

	// NewLineWriter returns a new LineWriter with the given target width in runes,
	// producing output on the underlying writer w. The dec and enc are used to
	// respectively decode runes from Write calls, and encode runes to w.
	func NewLineWriter(w io.Writer, width int, dec RuneChunkDecoder, enc RuneEncoder) *LineWriter {
	ret := &LineWriter{
	w: w,
	runeDecoder: dec,
	width: runePos(width),
	lineTerm: []byte("\n"),
	paragraphSep: "\n",
	prevState: stateWordWrap,
	prevRune: LineSeparator,
	lineBuf: byteRuneBuffer{enc: enc},
	}
	ret.resetLine()
	return ret
	}

	// NewUTF8LineWriter returns a new LineWriter filter that implements io.Writer,
	// and decodes and encodes runes in UTF-8.
	func NewUTF8LineWriter(w io.Writer, width int) *LineWriter {
	return NewLineWriter(w, width, &UTF8ChunkDecoder{}, UTF8Encoder{})
	}

	// Width returns the target width in runes. If width < 0 the width is
	// unlimited; each paragraph is output as a single line.
	func (w *LineWriter) Width() int { return int(w.width) }

	// SetLineTerminator sets the line terminator for subsequent Write calls. Every
	// output line is terminated with term; EOL runes from the input are never
	// written to the output. A new LineWriter instance uses "\n" as the default
	// line terminator.
	//
	// Calls Flush internally, and returns any Flush error.
	func (w *LineWriter) SetLineTerminator(term string) error {
	if err := w.Flush(); err != nil {
	return err
	}
	w.lineTerm = []byte(term)
	w.resetLine()
	return nil
	}

	// SetParagraphSeparator sets the paragraph separator for subsequent Write
	// calls. Every consecutive pair of non-empty paragraphs is separated with sep;
	// EOL runes from the input are never written to the output. A new LineWriter
	// instance uses "\n" as the default paragraph separator.
	//
	// Calls Flush internally, and returns any Flush error.
	func (w *LineWriter) SetParagraphSeparator(sep string) error {
	if err := w.Flush(); err != nil {
	return err
	}
	w.paragraphSep = sep
	w.resetLine()
	return nil
	}

	// SetIndents sets the indentation for subsequent Write calls. Multiple indents
	// may be set, corresponding to the indent to use for the corresponding
	// paragraph line. E.g. SetIndents("AA", "BBB", C") means the first line in
	// each paragraph is indented with "AA", the second line in each paragraph is
	// indented with "BBB", and all subsequent lines in each paragraph are indented
	// with "C".
	//
	// SetIndents() is equivalent to SetIndents(""), SetIndents("", ""), etc.
	//
	// A new LineWriter instance has no indents by default.
	//
	// Calls Flush internally, and returns any Flush error.
	func (w *LineWriter) SetIndents(indents ...string) error {
	if err := w.Flush(); err != nil {
	return err
	}
	// Copy indents in case the user passed the slice via SetIndents(p...), and
	// canonicalize the all empty case to nil.
	allEmpty := true
	w.indents = make([]string, len(indents))
	for ix, indent := range indents {
	w.indents[ix] = indent
	if indent != "" {
	allEmpty = false
	}
	}
	if allEmpty {
	w.indents = nil
	}
	w.resetLine()
	return nil
	}

	// ForceVerbatim forces w to stay in verbatim mode if v is true, or lets w
	// perform its regular line writing algorithm if v is false. This is useful if
	// there is a sequence of lines that should be written verbatim, even if the
	// lines don't start with spaces.
	//
	// Calls Flush internally, and returns any Flush error.
	func (w *LineWriter) ForceVerbatim(v bool) error {
	w.forceVerbatim = v
	return w.Flush()
	}

	// Write implements io.Writer by buffering data into the LineWriter w. Actual
	// writes to the underlying writer may occur, and may include data buffered in
	// either this Write call or previous Write calls.
	//
	// Flush must be called after the last call to Write.
	func (w *LineWriter) Write(data []byte) (int, error) {
	return WriteRuneChunk(w.runeDecoder, w.addRune, data)
	}

	// Flush flushes any remaining buffered text, and resets the paragraph line
	// count back to 0, so that indents will be applied starting from the first
	// line. It does not imply a paragraph separator; repeated calls to Flush with
	// no intervening calls to other methods is equivalent to a single Flush.
	//
	// Flush must be called after the last call to Write, and may be called an
	// arbitrary number of times before the last Write.
	func (w *LineWriter) Flush() error {
	if err := FlushRuneChunk(w.runeDecoder, w.addRune); err != nil {
	return err
	}
	// Add U+2028 to force the last line (if any) to be written.
	if err := w.addRune(LineSeparator); err != nil {
	return err
	}
	// Reset the paragraph line count.
	w.paragraphLineIndex = 0
	w.resetLine()
	return nil
	}

	// addRune is called every time w.runeDecoder decodes a full rune.
	func (w *LineWriter) addRune(r rune) error {
	state, lineBreak := w.nextState(r, w.updateRune(r))
	if lineBreak {
	if err := w.writeLine(); err != nil {
	return err
	}
	}
	w.bufferRune(r, state, lineBreak)
	w.prevState = state
	w.prevRune = r
	return nil
	}

	// We classify each incoming rune into three kinds for easier handling.
	type kind int

	const (
	kindEOL kind = iota
	kindSpace
	kindLetter
	)

	func runeKind(r rune) kind {
	switch r {
	case '\f', '\n', '\r', '\v', LineSeparator, ParagraphSeparator:
	return kindEOL
	}
	if unicode.IsSpace(r) {
	return kindSpace
	}
	return kindLetter
	}

	func (w *LineWriter) updateRune(r rune) bool {
	forceLineBreak := false
	switch kind := runeKind(r); kind {
	case kindEOL:
	// Update lastWordEnd if the last word just ended.
	if w.newWordStart != -1 {
	w.newWordStart = -1
	w.lastWordEnd = w.lineBuf.ByteLen()
	}
	switch {
	case w.prevRune == '\r' && r == '\n':
	// Treat "\r\n" as a single EOL; we've already handled the logic for '\r',
	// so there's nothing to do when we see '\n'.
	case r == LineSeparator:
	// Treat U+2028 as a pure line break; it's never a paragraph break.
	forceLineBreak = true
	case r == ParagraphSeparator \|\| !w.inputLineHasLetter:
	// The paragraph has just been terminated if we see an explicit U+2029, or
	// if we see a blank line, which may contain spaces.
	forceLineBreak = true
	w.terminateParagraph = true
	}
	w.inputLineHasLetter = false
	case kindSpace:
	// Update lastWordEnd if the last word just ended.
	if w.newWordStart != -1 {
	w.newWordStart = -1
	w.lastWordEnd = w.lineBuf.ByteLen()
	}
	case kindLetter:
	// Update newWordStart if a new word just started.
	if w.newWordStart == -1 {
	w.newWordStart = w.lineBuf.ByteLen()
	}
	w.inputLineHasLetter = true
	w.terminateParagraph = false
	default:
	panic(fmt.Errorf("textutil: updateRune unhandled kind %d", kind))
	}
	return forceLineBreak
	}

	// nextState returns the next state and whether we should break the line.
	//
	// Here's a handy table that describes all the scenarios in which we will line
	// break input text, grouped by the reason for the break. The current position
	// is the last non-* rune in each pattern, which is where we decide to break.
	//
	// w.prevState Next state Buffer reset
	// ----------- ---------- ------------
	// ===== Force line break (U+2028 / U+2029, blank line) =====
	// a..\|** * wordWrap empty
	// a._.\|*** * wordWrap empty
	// a+\|* * wordWrap empty
	//
	// ===== verbatim: wait for any EOL =====
	// _.\|*** verbatim wordWrap empty
	//
	// ===== wordWrap: switch to verbatim =====
	// a._\|** wordWrap verbatim empty
	//
	// ===== wordWrap: line is too wide =====
	// abc.\|*** wordWrap wordWrap empty
	// abcd\|.** wordWrap wordWrap empty
	// abcd\|e.* wordWrap wordWrap empty
	// a_cd\|.** wordWrap wordWrap empty
	//
	// abc_\|*** wordWrap skipSpace empty
	// abcd\|_** wordWrap skipSpace empty
	// abcd\|e_* wordWrap skipSpace empty
	// a_cd\|_** wordWrap skipSpace empty
	//
	// a_cd\|e** wordWrap start newWordStart
	//
	// LEGEND
	// abcde Letter
	// . End-of-line
	// + End-of-line (only U+2028 / U+2029)
	// _ Space
	// * Any rune (letter, line-end or space)
	// \| Visual indication of width=4, has no width itself.
	//
	// Note that Flush calls behave exactly as if an explicit U+2028 line separator
	// were added to the end of all buffered data.
	func (w *LineWriter) nextState(r rune, forceLineBreak bool) (state, bool) {
	kind := runeKind(r)
	if w.forceVerbatim {
	return stateVerbatim, forceLineBreak \|\| kind == kindEOL
	}
	if forceLineBreak {
	return stateWordWrap, true
	}
	// Handle non word-wrap states, which are easy.
	switch w.prevState {
	case stateVerbatim:
	if kind == kindEOL {
	return stateWordWrap, true
	}
	return stateVerbatim, false
	case stateSkipSpace:
	if kind == kindSpace {
	return stateSkipSpace, false
	}
	return stateWordWrap, false
	}
	// Handle stateWordWrap, which is more complicated.

	// Switch to the verbatim state when we see a space right after an EOL.
	if runeKind(w.prevRune) == kindEOL && kind == kindSpace {
	return stateVerbatim, true
	}
	// Break on EOL or space when the line is too wide. See above table.
	if w.width >= 0 && w.width <= w.lineBuf.RuneLen()+1 {
	switch kind {
	case kindEOL:
	return stateWordWrap, true
	case kindSpace:
	return stateSkipSpace, true
	}
	// case kindLetter falls through
	}
	// Handle the newWordStart case in the above table.
	if w.width >= 0 && w.width < w.lineBuf.RuneLen()+1 && w.newWordStart != w.lineStart {
	return stateWordWrap, true
	}
	// Stay in the wordWrap state and don't break the line.
	return stateWordWrap, false
	}

	func (w *LineWriter) writeLine() error {
	if w.lastWordEnd == -1 {
	// Don't write blank lines, but we must reset the line in case the paragraph
	// has just been terminated.
	w.resetLine()
	return nil
	}
	// Write the line (without trailing spaces) followed by the line terminator.
	line := w.lineBuf.Bytes()[:w.lastWordEnd]
	if _, err := w.w.Write(line); err != nil {
	return err
	}
	if _, err := w.w.Write(w.lineTerm); err != nil {
	return err
	}
	// Reset the line buffer.
	w.wroteFirstLine = true
	w.paragraphLineIndex++
	if w.newWordStart != -1 {
	// If we have an unterminated new word, we must be in the newWordStart case
	// in the table above. Handle the special buffer reset here.
	newWord := string(w.lineBuf.Bytes()[w.newWordStart:])
	w.resetLine()
	w.newWordStart = w.lineBuf.ByteLen()
	w.lineBuf.WriteString(newWord)
	} else {
	w.resetLine()
	}
	return nil
	}

	func (w *LineWriter) resetLine() {
	w.lineBuf.Reset()
	w.newWordStart = -1
	w.lastWordEnd = -1
	// Write the paragraph separator if the previous paragraph has terminated.
	// This consumes no runes from the line width.
	if w.wroteFirstLine && w.terminateParagraph {
	w.lineBuf.WriteString0Runes(w.paragraphSep)
	w.paragraphLineIndex = 0
	}
	// Add indent; a non-empty indent consumes runes from the line width.
	var indent string
	switch {
	case w.paragraphLineIndex < len(w.indents):
	indent = w.indents[w.paragraphLineIndex]
	case len(w.indents) > 0:
	indent = w.indents[len(w.indents)-1]
	}
	w.lineBuf.WriteString(indent)
	w.lineStart = w.lineBuf.ByteLen()
	}

	func (w *LineWriter) bufferRune(r rune, state state, lineBreak bool) {
	// Never add leading spaces to the buffer in the wordWrap state.
	wordWrapNoLeadingSpaces := state == stateWordWrap && !lineBreak
	switch kind := runeKind(r); kind {
	case kindEOL:
	// When we're word-wrapping and we see a letter followed by EOL, we convert
	// the EOL into a single space in the buffer, to break the previous word
	// from the next word.
	if wordWrapNoLeadingSpaces && runeKind(w.prevRune) == kindLetter {
	w.lineBuf.WriteRune(' ')
	}
	case kindSpace:
	if wordWrapNoLeadingSpaces \|\| state == stateVerbatim {
	w.lineBuf.WriteRune(r)
	}
	case kindLetter:
	w.lineBuf.WriteRune(r)
	default:
	panic(fmt.Errorf("textutil: bufferRune unhandled kind %d", kind))
	}
	}