src/pkg/go/printer/printer.go

// 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 printer implements printing of AST nodes.
package printer

import (
	"fmt"
	"go/ast"
	"go/token"
	"io"
	"os"
	"strconv"
	"strings"
	"text/tabwriter"
)

const (
	maxNewlines = 2     // max. number of newlines between source text
	debug       = false // enable for debugging
	infinity    = 1 << 30
)

type whiteSpace byte

const (
	ignore   = whiteSpace(0)
	blank    = whiteSpace(' ')
	vtab     = whiteSpace('\v')
	newline  = whiteSpace('\n')
	formfeed = whiteSpace('\f')
	indent   = whiteSpace('>')
	unindent = whiteSpace('<')
)

// A pmode value represents the current printer mode.
type pmode int

const (
	noExtraLinebreak pmode = 1 << iota
)

type printer struct {
	// Configuration (does not change after initialization)
	Config
	fset *token.FileSet

	// Current state
	output      []byte       // raw printer result
	indent      int          // current indentation
	mode        pmode        // current printer mode
	impliedSemi bool         // if set, a linebreak implies a semicolon
	lastTok     token.Token  // the last token printed (token.ILLEGAL if it's whitespace)
	wsbuf       []whiteSpace // delayed white space

	// Positions
	// The out position differs from the pos position when the result
	// formatting differs from the source formatting (in the amount of
	// white space). If there's a difference and SourcePos is set in
	// ConfigMode, //line comments are used in the output to restore
	// original source positions for a reader.
	pos  token.Position // current position in AST (source) space
	out  token.Position // current position in output space
	last token.Position // value of pos after calling writeString

	// The list of all source comments, in order of appearance.
	comments        []*ast.CommentGroup // may be nil
	cindex          int                 // current comment index
	useNodeComments bool                // if not set, ignore lead and line comments of nodes

	// Information about p.comments[p.cindex]; set up by nextComment.
	comment        *ast.CommentGroup // = p.comments[p.cindex]; or nil
	commentOffset  int               // = p.posFor(p.comments[p.cindex].List[0].Pos()).Offset; or infinity
	commentNewline bool              // true if the comment group contains newlines

	// Cache of already computed node sizes.
	nodeSizes map[ast.Node]int

	// Cache of most recently computed line position.
	cachedPos  token.Pos
	cachedLine int // line corresponding to cachedPos
}

func (p *printer) init(cfg *Config, fset *token.FileSet, nodeSizes map[ast.Node]int) {
	p.Config = *cfg
	p.fset = fset
	p.pos = token.Position{Line: 1, Column: 1}
	p.out = token.Position{Line: 1, Column: 1}
	p.wsbuf = make([]whiteSpace, 0, 16) // whitespace sequences are short
	p.nodeSizes = nodeSizes
	p.cachedPos = -1
}

// commentsHaveNewline reports whether a list of comments belonging to
// an *ast.CommentGroup contains newlines. Because the position information
// may only be partially correct, we also have to read the comment text.
func (p *printer) commentsHaveNewline(list []*ast.Comment) bool {
	// len(list) > 0
	line := p.lineFor(list[0].Pos())
	for i, c := range list {
		if i > 0 && p.lineFor(list[i].Pos()) != line {
			// not all comments on the same line
			return true
		}
		if t := c.Text; len(t) >= 2 && (t[1] == '/' || strings.Contains(t, "\n")) {
			return true
		}
	}
	_ = line
	return false
}

func (p *printer) nextComment() {
	for p.cindex < len(p.comments) {
		c := p.comments[p.cindex]
		p.cindex++
		if list := c.List; len(list) > 0 {
			p.comment = c
			p.commentOffset = p.posFor(list[0].Pos()).Offset
			p.commentNewline = p.commentsHaveNewline(list)
			return
		}
		// we should not reach here (correct ASTs don't have empty
		// ast.CommentGroup nodes), but be conservative and try again
	}
	// no more comments
	p.commentOffset = infinity
}

func (p *printer) internalError(msg ...interface{}) {
	if debug {
		fmt.Print(p.pos.String() + ": ")
		fmt.Println(msg...)
		panic("go/printer")
	}
}

func (p *printer) posFor(pos token.Pos) token.Position {
	// not used frequently enough to cache entire token.Position
	return p.fset.Position(pos)
}

func (p *printer) lineFor(pos token.Pos) int {
	if pos != p.cachedPos {
		p.cachedPos = pos
		p.cachedLine = p.fset.Position(pos).Line
	}
	return p.cachedLine
}

// atLineBegin emits a //line comment if necessary and prints indentation.
func (p *printer) atLineBegin(pos token.Position) {
	// write a //line comment if necessary
	if p.Config.Mode&SourcePos != 0 && pos.IsValid() && (p.out.Line != pos.Line || p.out.Filename != pos.Filename) {
		p.output = append(p.output, tabwriter.Escape) // protect '\n' in //line from tabwriter interpretation
		p.output = append(p.output, fmt.Sprintf("//line %s:%d\n", pos.Filename, pos.Line)...)
		p.output = append(p.output, tabwriter.Escape)
		// p.out must match the //line comment
		p.out.Filename = pos.Filename
		p.out.Line = pos.Line
	}

	// write indentation
	// use "hard" htabs - indentation columns
	// must not be discarded by the tabwriter
	for i := 0; i < p.indent; i++ {
		p.output = append(p.output, '\t')
	}

	// update positions
	i := p.indent
	p.pos.Offset += i
	p.pos.Column += i
	p.out.Column += i
}

// writeByte writes ch n times to p.output and updates p.pos.
func (p *printer) writeByte(ch byte, n int) {
	if p.out.Column == 1 {
		p.atLineBegin(p.pos)
	}

	for i := 0; i < n; i++ {
		p.output = append(p.output, ch)
	}

	// update positions
	p.pos.Offset += n
	if ch == '\n' || ch == '\f' {
		p.pos.Line += n
		p.out.Line += n
		p.pos.Column = 1
		p.out.Column = 1
		return
	}
	p.pos.Column += n
	p.out.Column += n
}

// writeString writes the string s to p.output and updates p.pos, p.out,
// and p.last. If isLit is set, s is escaped w/ tabwriter.Escape characters
// to protect s from being interpreted by the tabwriter.
//
// Note: writeString is only used to write Go tokens, literals, and
// comments, all of which must be written literally. Thus, it is correct
// to always set isLit = true. However, setting it explicitly only when
// needed (i.e., when we don't know that s contains no tabs or line breaks)
// avoids processing extra escape characters and reduces run time of the
// printer benchmark by up to 10%.
//
func (p *printer) writeString(pos token.Position, s string, isLit bool) {
	if p.out.Column == 1 {
		p.atLineBegin(pos)
	}

	if pos.IsValid() {
		// update p.pos (if pos is invalid, continue with existing p.pos)
		// Note: Must do this after handling line beginnings because
		// atLineBegin updates p.pos if there's indentation, but p.pos
		// is the position of s.
		p.pos = pos
		// reset state if the file changed
		// (used when printing merged ASTs of different files
		// e.g., the result of ast.MergePackageFiles)
		if p.last.IsValid() && p.last.Filename != pos.Filename {
			p.indent = 0
			p.mode = 0
			p.wsbuf = p.wsbuf[0:0]
		}
	}

	if isLit {
		// Protect s such that is passes through the tabwriter
		// unchanged. Note that valid Go programs cannot contain
		// tabwriter.Escape bytes since they do not appear in legal
		// UTF-8 sequences.
		p.output = append(p.output, tabwriter.Escape)
	}

	if debug {
		p.output = append(p.output, fmt.Sprintf("/*%s*/", pos)...) // do not update p.pos!
	}
	p.output = append(p.output, s...)

	// update positions
	nlines := 0
	var li int // index of last newline; valid if nlines > 0
	for i := 0; i < len(s); i++ {
		// Go tokens cannot contain '\f' - no need to look for it
		if s[i] == '\n' {
			nlines++
			li = i
		}
	}
	p.pos.Offset += len(s)
	if nlines > 0 {
		p.pos.Line += nlines
		p.out.Line += nlines
		c := len(s) - li
		p.pos.Column = c
		p.out.Column = c
	} else {
		p.pos.Column += len(s)
		p.out.Column += len(s)
	}

	if isLit {
		p.output = append(p.output, tabwriter.Escape)
	}

	p.last = p.pos
}

// writeCommentPrefix writes the whitespace before a comment.
// If there is any pending whitespace, it consumes as much of
// it as is likely to help position the comment nicely.
// pos is the comment position, next the position of the item
// after all pending comments, prev is the previous comment in
// a group of comments (or nil), and tok is the next token.
//
func (p *printer) writeCommentPrefix(pos, next token.Position, prev, comment *ast.Comment, tok token.Token) {
	if len(p.output) == 0 {
		// the comment is the first item to be printed - don't write any whitespace
		return
	}

	if pos.IsValid() && pos.Filename != p.last.Filename {
		// comment in a different file - separate with newlines
		p.writeByte('\f', maxNewlines)
		return
	}

	if pos.Line == p.last.Line && (prev == nil || prev.Text[1] != '/') {
		// comment on the same line as last item:
		// separate with at least one separator
		hasSep := false
		if prev == nil {
			// first comment of a comment group
			j := 0
			for i, ch := range p.wsbuf {
				switch ch {
				case blank:
					// ignore any blanks before a comment
					p.wsbuf[i] = ignore
					continue
				case vtab:
					// respect existing tabs - important
					// for proper formatting of commented structs
					hasSep = true
					continue
				case indent:
					// apply pending indentation
					continue
				}
				j = i
				break
			}
			p.writeWhitespace(j)
		}
		// make sure there is at least one separator
		if !hasSep {
			sep := byte('\t')
			if pos.Line == next.Line {
				// next item is on the same line as the comment
				// (which must be a /*-style comment): separate
				// with a blank instead of a tab
				sep = ' '
			}
			p.writeByte(sep, 1)
		}

	} else {
		// comment on a different line:
		// separate with at least one line break
		droppedLinebreak := false
		j := 0
		for i, ch := range p.wsbuf {
			switch ch {
			case blank, vtab:
				// ignore any horizontal whitespace before line breaks
				p.wsbuf[i] = ignore
				continue
			case indent:
				// apply pending indentation
				continue
			case unindent:
				// if this is not the last unindent, apply it
				// as it is (likely) belonging to the last
				// construct (e.g., a multi-line expression list)
				// and is not part of closing a block
				if i+1 < len(p.wsbuf) && p.wsbuf[i+1] == unindent {
					continue
				}
				// if the next token is not a closing }, apply the unindent
				// if it appears that the comment is aligned with the
				// token; otherwise assume the unindent is part of a
				// closing block and stop (this scenario appears with
				// comments before a case label where the comments
				// apply to the next case instead of the current one)
				if tok != token.RBRACE && pos.Column == next.Column {
					continue
				}
			case newline, formfeed:
				p.wsbuf[i] = ignore
				droppedLinebreak = prev == nil // record only if first comment of a group
			}
			j = i
			break
		}
		p.writeWhitespace(j)

		// determine number of linebreaks before the comment
		n := 0
		if pos.IsValid() && p.last.IsValid() {
			n = pos.Line - p.last.Line
			if n < 0 { // should never happen
				n = 0
			}
		}

		// at the package scope level only (p.indent == 0),
		// add an extra newline if we dropped one before:
		// this preserves a blank line before documentation
		// comments at the package scope level (issue 2570)
		if p.indent == 0 && droppedLinebreak {
			n++
		}

		// make sure there is at least one line break
		// if the previous comment was a line comment
		if n == 0 && prev != nil && prev.Text[1] == '/' {
			n = 1
		}

		if n > 0 {
			// use formfeeds to break columns before a comment;
			// this is analogous to using formfeeds to separate
			// individual lines of /*-style comments
			p.writeByte('\f', nlimit(n))
		}
	}
}

// Split comment text into lines
// (using strings.Split(text, "\n") is significantly slower for
// this specific purpose, as measured with: go test -bench=Print)
func split(text string) []string {
	// count lines (comment text never ends in a newline)
	n := 1
	for i := 0; i < len(text); i++ {
		if text[i] == '\n' {
			n++
		}
	}

	// split
	lines := make([]string, n)
	n = 0
	i := 0
	for j := 0; j < len(text); j++ {
		if text[j] == '\n' {
			lines[n] = text[i:j] // exclude newline
			i = j + 1            // discard newline
			n++
		}
	}
	lines[n] = text[i:]

	return lines
}

// Returns true if s contains only white space
// (only tabs and blanks can appear in the printer's context).
func isBlank(s string) bool {
	for i := 0; i < len(s); i++ {
		if s[i] > ' ' {
			return false
		}
	}
	return true
}

func commonPrefix(a, b string) string {
	i := 0
	for i < len(a) && i < len(b) && a[i] == b[i] && (a[i] <= ' ' || a[i] == '*') {
		i++
	}
	return a[0:i]
}

func stripCommonPrefix(lines []string) {
	if len(lines) < 2 {
		return // at most one line - nothing to do
	}
	// len(lines) >= 2

	// The heuristic in this function tries to handle a few
	// common patterns of /*-style comments: Comments where
	// the opening /* and closing */ are aligned and the
	// rest of the comment text is aligned and indented with
	// blanks or tabs, cases with a vertical "line of stars"
	// on the left, and cases where the closing */ is on the
	// same line as the last comment text.

	// Compute maximum common white prefix of all but the first,
	// last, and blank lines, and replace blank lines with empty
	// lines (the first line starts with /* and has no prefix).
	// In case of two-line comments, consider the last line for
	// the prefix computation since otherwise the prefix would
	// be empty.
	//
	// Note that the first and last line are never empty (they
	// contain the opening /* and closing */ respectively) and
	// thus they can be ignored by the blank line check.
	var prefix string
	if len(lines) > 2 {
		first := true
		for i, line := range lines[1 : len(lines)-1] {
			switch {
			case isBlank(line):
				lines[1+i] = "" // range starts at line 1
			case first:
				prefix = commonPrefix(line, line)
				first = false
			default:
				prefix = commonPrefix(prefix, line)
			}
		}
	} else { // len(lines) == 2, lines cannot be blank (contain /* and */)
		line := lines[1]
		prefix = commonPrefix(line, line)
	}

	/*
	 * Check for vertical "line of stars" and correct prefix accordingly.
	 */
	lineOfStars := false
	if i := strings.Index(prefix, "*"); i >= 0 {
		// Line of stars present.
		if i > 0 && prefix[i-1] == ' ' {
			i-- // remove trailing blank from prefix so stars remain aligned
		}
		prefix = prefix[0:i]
		lineOfStars = true
	} else {
		// No line of stars present.
		// Determine the white space on the first line after the /*
		// and before the beginning of the comment text, assume two
		// blanks instead of the /* unless the first character after
		// the /* is a tab. If the first comment line is empty but
		// for the opening /*, assume up to 3 blanks or a tab. This
		// whitespace may be found as suffix in the common prefix.
		first := lines[0]
		if isBlank(first[2:]) {
			// no comment text on the first line:
			// reduce prefix by up to 3 blanks or a tab
			// if present - this keeps comment text indented
			// relative to the /* and */'s if it was indented
			// in the first place
			i := len(prefix)
			for n := 0; n < 3 && i > 0 && prefix[i-1] == ' '; n++ {
				i--
			}
			if i == len(prefix) && i > 0 && prefix[i-1] == '\t' {
				i--
			}
			prefix = prefix[0:i]
		} else {
			// comment text on the first line
			suffix := make([]byte, len(first))
			n := 2 // start after opening /*
			for n < len(first) && first[n] <= ' ' {
				suffix[n] = first[n]
				n++
			}
			if n > 2 && suffix[2] == '\t' {
				// assume the '\t' compensates for the /*
				suffix = suffix[2:n]
			} else {
				// otherwise assume two blanks
				suffix[0], suffix[1] = ' ', ' '
				suffix = suffix[0:n]
			}
			// Shorten the computed common prefix by the length of
			// suffix, if it is found as suffix of the prefix.
			if strings.HasSuffix(prefix, string(suffix)) {
				prefix = prefix[0 : len(prefix)-len(suffix)]
			}
		}
	}

	// Handle last line: If it only contains a closing */, align it
	// with the opening /*, otherwise align the text with the other
	// lines.
	last := lines[len(lines)-1]
	closing := "*/"
	i := strings.Index(last, closing) // i >= 0 (closing is always present)
	if isBlank(last[0:i]) {
		// last line only contains closing */
		if lineOfStars {
			closing = " */" // add blank to align final star
		}
		lines[len(lines)-1] = prefix + closing
	} else {
		// last line contains more comment text - assume
		// it is aligned like the other lines and include
		// in prefix computation
		prefix = commonPrefix(prefix, last)
	}

	// Remove the common prefix from all but the first and empty lines.
	for i, line := range lines[1:] {
		if len(line) != 0 {
			lines[1+i] = line[len(prefix):] // range starts at line 1
		}
	}
}

func (p *printer) writeComment(comment *ast.Comment) {
	text := comment.Text
	pos := p.posFor(comment.Pos())

	const linePrefix = "//line "
	if strings.HasPrefix(text, linePrefix) && (!pos.IsValid() || pos.Column == 1) {
		// possibly a line directive
		ldir := strings.TrimSpace(text[len(linePrefix):])
		if i := strings.LastIndex(ldir, ":"); i >= 0 {
			if line, err := strconv.Atoi(ldir[i+1:]); err == nil && line > 0 {
				// The line directive we are about to print changed
				// the Filename and Line number used for subsequent
				// tokens. We have to update our AST-space position
				// accordingly and suspend indentation temporarily.
				indent := p.indent
				p.indent = 0
				defer func() {
					p.pos.Filename = ldir[:i]
					p.pos.Line = line
					p.pos.Column = 1
					p.indent = indent
				}()
			}
		}
	}

	// shortcut common case of //-style comments
	if text[1] == '/' {
		p.writeString(pos, text, true)
		return
	}

	// for /*-style comments, print line by line and let the
	// write function take care of the proper indentation
	lines := split(text)
	stripCommonPrefix(lines)

	// write comment lines, separated by formfeed,
	// without a line break after the last line
	for i, line := range lines {
		if i > 0 {
			p.writeByte('\f', 1)
			pos = p.pos
		}
		if len(line) > 0 {
			p.writeString(pos, line, true)
		}
	}
}

// writeCommentSuffix writes a line break after a comment if indicated
// and processes any leftover indentation information. If a line break
// is needed, the kind of break (newline vs formfeed) depends on the
// pending whitespace. The writeCommentSuffix result indicates if a
// newline was written or if a formfeed was dropped from the whitespace
// buffer.
//
func (p *printer) writeCommentSuffix(needsLinebreak bool) (wroteNewline, droppedFF bool) {
	for i, ch := range p.wsbuf {
		switch ch {
		case blank, vtab:
			// ignore trailing whitespace
			p.wsbuf[i] = ignore
		case indent, unindent:
			// don't lose indentation information
		case newline, formfeed:
			// if we need a line break, keep exactly one
			// but remember if we dropped any formfeeds
			if needsLinebreak {
				needsLinebreak = false
				wroteNewline = true
			} else {
				if ch == formfeed {
					droppedFF = true
				}
				p.wsbuf[i] = ignore
			}
		}
	}
	p.writeWhitespace(len(p.wsbuf))

	// make sure we have a line break
	if needsLinebreak {
		p.writeByte('\n', 1)
		wroteNewline = true
	}

	return
}

// intersperseComments consumes all comments that appear before the next token
// tok and prints it together with the buffered whitespace (i.e., the whitespace
// that needs to be written before the next token). A heuristic is used to mix
// the comments and whitespace. The intersperseComments result indicates if a
// newline was written or if a formfeed was dropped from the whitespace buffer.
//
func (p *printer) intersperseComments(next token.Position, tok token.Token) (wroteNewline, droppedFF bool) {
	var last *ast.Comment
	for p.commentBefore(next) {
		for _, c := range p.comment.List {
			p.writeCommentPrefix(p.posFor(c.Pos()), next, last, c, tok)
			p.writeComment(c)
			last = c
		}
		p.nextComment()
	}

	if last != nil {
		// if the last comment is a /*-style comment and the next item
		// follows on the same line but is not a comma or a "closing"
		// token, add an extra blank for separation
		if last.Text[1] == '*' && p.lineFor(last.Pos()) == next.Line && tok != token.COMMA &&
			tok != token.RPAREN && tok != token.RBRACK && tok != token.RBRACE {
			p.writeByte(' ', 1)
		}
		// ensure that there is a line break after a //-style comment,
		// before a closing '}' unless explicitly disabled, or at eof
		needsLinebreak :=
			last.Text[1] == '/' ||
				tok == token.RBRACE && p.mode&noExtraLinebreak == 0 ||
				tok == token.EOF
		return p.writeCommentSuffix(needsLinebreak)
	}

	// no comment was written - we should never reach here since
	// intersperseComments should not be called in that case
	p.internalError("intersperseComments called without pending comments")
	return
}

// whiteWhitespace writes the first n whitespace entries.
func (p *printer) writeWhitespace(n int) {
	// write entries
	for i := 0; i < n; i++ {
		switch ch := p.wsbuf[i]; ch {
		case ignore:
			// ignore!
		case indent:
			p.indent++
		case unindent:
			p.indent--
			if p.indent < 0 {
				p.internalError("negative indentation:", p.indent)
				p.indent = 0
			}
		case newline, formfeed:
			// A line break immediately followed by a "correcting"
			// unindent is swapped with the unindent - this permits
			// proper label positioning. If a comment is between
			// the line break and the label, the unindent is not
			// part of the comment whitespace prefix and the comment
			// will be positioned correctly indented.
			if i+1 < n && p.wsbuf[i+1] == unindent {
				// Use a formfeed to terminate the current section.
				// Otherwise, a long label name on the next line leading
				// to a wide column may increase the indentation column
				// of lines before the label; effectively leading to wrong
				// indentation.
				p.wsbuf[i], p.wsbuf[i+1] = unindent, formfeed
				i-- // do it again
				continue
			}
			fallthrough
		default:
			p.writeByte(byte(ch), 1)
		}
	}

	// shift remaining entries down
	i := 0
	for ; n < len(p.wsbuf); n++ {
		p.wsbuf[i] = p.wsbuf[n]
		i++
	}
	p.wsbuf = p.wsbuf[0:i]
}

// ----------------------------------------------------------------------------
// Printing interface

// nlines limits n to maxNewlines.
func nlimit(n int) int {
	if n > maxNewlines {
		n = maxNewlines
	}
	return n
}

func mayCombine(prev token.Token, next byte) (b bool) {
	switch prev {
	case token.INT:
		b = next == '.' // 1.
	case token.ADD:
		b = next == '+' // ++
	case token.SUB:
		b = next == '-' // --
	case token.QUO:
		b = next == '*' // /*
	case token.LSS:
		b = next == '-' || next == '<' // <- or <<
	case token.AND:
		b = next == '&' || next == '^' // && or &^
	}
	return
}

// print prints a list of "items" (roughly corresponding to syntactic
// tokens, but also including whitespace and formatting information).
// It is the only print function that should be called directly from
// any of the AST printing functions in nodes.go.
//
// Whitespace is accumulated until a non-whitespace token appears. Any
// comments that need to appear before that token are printed first,
// taking into account the amount and structure of any pending white-
// space for best comment placement. Then, any leftover whitespace is
// printed, followed by the actual token.
//
func (p *printer) print(args ...interface{}) {
	for _, arg := range args {
		// information about the current arg
		var data string
		var isLit bool
		var impliedSemi bool // value for p.impliedSemi after this arg

		switch x := arg.(type) {
		case pmode:
			// toggle printer mode
			p.mode ^= x
			continue

		case whiteSpace:
			if x == ignore {
				// don't add ignore's to the buffer; they
				// may screw up "correcting" unindents (see
				// LabeledStmt)
				continue
			}
			i := len(p.wsbuf)
			if i == cap(p.wsbuf) {
				// Whitespace sequences are very short so this should
				// never happen. Handle gracefully (but possibly with
				// bad comment placement) if it does happen.
				p.writeWhitespace(i)
				i = 0
			}
			p.wsbuf = p.wsbuf[0 : i+1]
			p.wsbuf[i] = x
			if x == newline || x == formfeed {
				// newlines affect the current state (p.impliedSemi)
				// and not the state after printing arg (impliedSemi)
				// because comments can be interspersed before the arg
				// in this case
				p.impliedSemi = false
			}
			p.lastTok = token.ILLEGAL
			continue

		case *ast.Ident:
			data = x.Name
			impliedSemi = true
			p.lastTok = token.IDENT

		case *ast.BasicLit:
			data = x.Value
			isLit = true
			impliedSemi = true
			p.lastTok = x.Kind

		case token.Token:
			s := x.String()
			if mayCombine(p.lastTok, s[0]) {
				// the previous and the current token must be
				// separated by a blank otherwise they combine
				// into a different incorrect token sequence
				// (except for token.INT followed by a '.' this
				// should never happen because it is taken care
				// of via binary expression formatting)
				if len(p.wsbuf) != 0 {
					p.internalError("whitespace buffer not empty")
				}
				p.wsbuf = p.wsbuf[0:1]
				p.wsbuf[0] = ' '
			}
			data = s
			// some keywords followed by a newline imply a semicolon
			switch x {
			case token.BREAK, token.CONTINUE, token.FALLTHROUGH, token.RETURN,
				token.INC, token.DEC, token.RPAREN, token.RBRACK, token.RBRACE:
				impliedSemi = true
			}
			p.lastTok = x

		case token.Pos:
			if x.IsValid() {
				p.pos = p.posFor(x) // accurate position of next item
			}
			continue

		case string:
			// incorrect AST - print error message
			data = x
			isLit = true
			impliedSemi = true
			p.lastTok = token.STRING

		default:
			fmt.Fprintf(os.Stderr, "print: unsupported argument %v (%T)\n", arg, arg)
			panic("go/printer type")
		}
		// data != ""

		next := p.pos // estimated/accurate position of next item
		wroteNewline, droppedFF := p.flush(next, p.lastTok)

		// intersperse extra newlines if present in the source and
		// if they don't cause extra semicolons (don't do this in
		// flush as it will cause extra newlines at the end of a file)
		if !p.impliedSemi {
			n := nlimit(next.Line - p.pos.Line)
			// don't exceed maxNewlines if we already wrote one
			if wroteNewline && n == maxNewlines {
				n = maxNewlines - 1
			}
			if n > 0 {
				ch := byte('\n')
				if droppedFF {
					ch = '\f' // use formfeed since we dropped one before
				}
				p.writeByte(ch, n)
				impliedSemi = false
			}
		}

		p.writeString(next, data, isLit)
		p.impliedSemi = impliedSemi
	}
}

// commentBefore returns true iff the current comment group occurs
// before the next position in the source code and printing it does
// not introduce implicit semicolons.
//
func (p *printer) commentBefore(next token.Position) (result bool) {
	return p.commentOffset < next.Offset && (!p.impliedSemi || !p.commentNewline)
}

// flush prints any pending comments and whitespace occurring textually
// before the position of the next token tok. The flush result indicates
// if a newline was written or if a formfeed was dropped from the whitespace
// buffer.
//
func (p *printer) flush(next token.Position, tok token.Token) (wroteNewline, droppedFF bool) {
	if p.commentBefore(next) {
		// if there are comments before the next item, intersperse them
		wroteNewline, droppedFF = p.intersperseComments(next, tok)
	} else {
		// otherwise, write any leftover whitespace
		p.writeWhitespace(len(p.wsbuf))
	}
	return
}

// getNode returns the ast.CommentGroup associated with n, if any.
func getDoc(n ast.Node) *ast.CommentGroup {
	switch n := n.(type) {
	case *ast.Field:
		return n.Doc
	case *ast.ImportSpec:
		return n.Doc
	case *ast.ValueSpec:
		return n.Doc
	case *ast.TypeSpec:
		return n.Doc
	case *ast.GenDecl:
		return n.Doc
	case *ast.FuncDecl:
		return n.Doc
	case *ast.File:
		return n.Doc
	}
	return nil
}

func (p *printer) printNode(node interface{}) error {
	// unpack *CommentedNode, if any
	var comments []*ast.CommentGroup
	if cnode, ok := node.(*CommentedNode); ok {
		node = cnode.Node
		comments = cnode.Comments
	}

	if comments != nil {
		// commented node - restrict comment list to relevant range
		n, ok := node.(ast.Node)
		if !ok {
			goto unsupported
		}
		beg := n.Pos()
		end := n.End()
		// if the node has associated documentation,
		// include that commentgroup in the range
		// (the comment list is sorted in the order
		// of the comment appearance in the source code)
		if doc := getDoc(n); doc != nil {
			beg = doc.Pos()
		}
		// token.Pos values are global offsets, we can
		// compare them directly
		i := 0
		for i < len(comments) && comments[i].End() < beg {
			i++
		}
		j := i
		for j < len(comments) && comments[j].Pos() < end {
			j++
		}
		if i < j {
			p.comments = comments[i:j]
		}
	} else if n, ok := node.(*ast.File); ok {
		// use ast.File comments, if any
		p.comments = n.Comments
	}

	// if there are no comments, use node comments
	p.useNodeComments = p.comments == nil

	// get comments ready for use
	p.nextComment()

	// format node
	switch n := node.(type) {
	case ast.Expr:
		p.expr(n)
	case ast.Stmt:
		// A labeled statement will un-indent to position the
		// label. Set indent to 1 so we don't get indent "underflow".
		if _, labeledStmt := n.(*ast.LabeledStmt); labeledStmt {
			p.indent = 1
		}
		p.stmt(n, false)
	case ast.Decl:
		p.decl(n)
	case ast.Spec:
		p.spec(n, 1, false)
	case *ast.File:
		p.file(n)
	default:
		goto unsupported
	}

	return nil

unsupported:
	return fmt.Errorf("go/printer: unsupported node type %T", node)
}

// ----------------------------------------------------------------------------
// Trimmer

// A trimmer is an io.Writer filter for stripping tabwriter.Escape
// characters, trailing blanks and tabs, and for converting formfeed
// and vtab characters into newlines and htabs (in case no tabwriter
// is used). Text bracketed by tabwriter.Escape characters is passed
// through unchanged.
//
type trimmer struct {
	output io.Writer
	state  int
	space  []byte
}

// trimmer is implemented as a state machine.
// It can be in one of the following states:
const (
	inSpace  = iota // inside space
	inEscape        // inside text bracketed by tabwriter.Escapes
	inText          // inside text
)

func (p *trimmer) resetSpace() {
	p.state = inSpace
	p.space = p.space[0:0]
}

// Design note: It is tempting to eliminate extra blanks occurring in
//              whitespace in this function as it could simplify some
//              of the blanks logic in the node printing functions.
//              However, this would mess up any formatting done by
//              the tabwriter.

var aNewline = []byte("\n")

func (p *trimmer) Write(data []byte) (n int, err error) {
	// invariants:
	// p.state == inSpace:
	//	p.space is unwritten
	// p.state == inEscape, inText:
	//	data[m:n] is unwritten
	m := 0
	var b byte
	for n, b = range data {
		if b == '\v' {
			b = '\t' // convert to htab
		}
		switch p.state {
		case inSpace:
			switch b {
			case '\t', ' ':
				p.space = append(p.space, b)
			case '\n', '\f':
				p.resetSpace() // discard trailing space
				_, err = p.output.Write(aNewline)
			case tabwriter.Escape:
				_, err = p.output.Write(p.space)
				p.state = inEscape
				m = n + 1 // +1: skip tabwriter.Escape
			default:
				_, err = p.output.Write(p.space)
				p.state = inText
				m = n
			}
		case inEscape:
			if b == tabwriter.Escape {
				_, err = p.output.Write(data[m:n])
				p.resetSpace()
			}
		case inText:
			switch b {
			case '\t', ' ':
				_, err = p.output.Write(data[m:n])
				p.resetSpace()
				p.space = append(p.space, b)
			case '\n', '\f':
				_, err = p.output.Write(data[m:n])
				p.resetSpace()
				_, err = p.output.Write(aNewline)
			case tabwriter.Escape:
				_, err = p.output.Write(data[m:n])
				p.state = inEscape
				m = n + 1 // +1: skip tabwriter.Escape
			}
		default:
			panic("unreachable")
		}
		if err != nil {
			return
		}
	}
	n = len(data)

	switch p.state {
	case inEscape, inText:
		_, err = p.output.Write(data[m:n])
		p.resetSpace()
	}

	return
}

// ----------------------------------------------------------------------------
// Public interface

// A Mode value is a set of flags (or 0). They coontrol printing. 
type Mode uint

const (
	RawFormat Mode = 1 << iota // do not use a tabwriter; if set, UseSpaces is ignored
	TabIndent                  // use tabs for indentation independent of UseSpaces
	UseSpaces                  // use spaces instead of tabs for alignment
	SourcePos                  // emit //line comments to preserve original source positions
)

// A Config node controls the output of Fprint.
type Config struct {
	Mode     Mode // default: 0
	Tabwidth int  // default: 8
}

// fprint implements Fprint and takes a nodesSizes map for setting up the printer state.
func (cfg *Config) fprint(output io.Writer, fset *token.FileSet, node interface{}, nodeSizes map[ast.Node]int) (err error) {
	// print node
	var p printer
	p.init(cfg, fset, nodeSizes)
	if err = p.printNode(node); err != nil {
		return
	}
	// print outstanding comments
	p.impliedSemi = false // EOF acts like a newline
	p.flush(token.Position{Offset: infinity, Line: infinity}, token.EOF)

	// redirect output through a trimmer to eliminate trailing whitespace
	// (Input to a tabwriter must be untrimmed since trailing tabs provide
	// formatting information. The tabwriter could provide trimming
	// functionality but no tabwriter is used when RawFormat is set.)
	output = &trimmer{output: output}

	// redirect output through a tabwriter if necessary
	if cfg.Mode&RawFormat == 0 {
		minwidth := cfg.Tabwidth

		padchar := byte('\t')
		if cfg.Mode&UseSpaces != 0 {
			padchar = ' '
		}

		twmode := tabwriter.DiscardEmptyColumns
		if cfg.Mode&TabIndent != 0 {
			minwidth = 0
			twmode |= tabwriter.TabIndent
		}

		output = tabwriter.NewWriter(output, minwidth, cfg.Tabwidth, 1, padchar, twmode)
	}

	// write printer result via tabwriter/trimmer to output
	if _, err = output.Write(p.output); err != nil {
		return
	}

	// flush tabwriter, if any
	if tw, _ := (output).(*tabwriter.Writer); tw != nil {
		err = tw.Flush()
	}

	return
}

// A CommentedNode bundles an AST node and corresponding comments.
// It may be provided as argument to any of the Fprint functions.
//
type CommentedNode struct {
	Node     interface{} // *ast.File, or ast.Expr, ast.Decl, ast.Spec, or ast.Stmt
	Comments []*ast.CommentGroup
}

// Fprint "pretty-prints" an AST node to output for a given configuration cfg.
// Position information is interpreted relative to the file set fset.
// The node type must be *ast.File, *CommentedNode, or assignment-compatible
// to ast.Expr, ast.Decl, ast.Spec, or ast.Stmt.
//
func (cfg *Config) Fprint(output io.Writer, fset *token.FileSet, node interface{}) error {
	return cfg.fprint(output, fset, node, make(map[ast.Node]int))
}

// Fprint "pretty-prints" an AST node to output.
// It calls Config.Fprint with default settings.
//
func Fprint(output io.Writer, fset *token.FileSet, node interface{}) error {
	return (&Config{Tabwidth: 8}).Fprint(output, fset, node)
}