* tree-ssa-reassoc.c (reassociate_bb): Clarify code slighly.

[official-gcc.git] / libgo / go / 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"
        "unicode"
)

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 (
        noExtraBlank     pmode = 1 << iota // disables extra blank after /*-style comment
        noExtraLinebreak                   // disables extra line break after /*-style comment
)

type commentInfo struct {
        cindex         int               // current comment index
        comment        *ast.CommentGroup // = printer.comments[cindex]; or nil
        commentOffset  int               // = printer.posFor(printer.comments[cindex].List[0].Pos()).Offset; or infinity
        commentNewline bool              // true if the comment group contains newlines
}

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

        // Current state
        output      []byte       // raw printer result
        indent      int          // current indentation
        level       int          // level == 0: outside composite literal; level > 0: inside composite literal
        mode        pmode        // current printer mode
        impliedSemi bool         // if set, a linebreak implies a semicolon
        lastTok     token.Token  // last token printed (token.ILLEGAL if it's whitespace)
        prevOpen    token.Token  // previous non-brace "open" token (, [, or token.ILLEGAL
        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 directives 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
        linePtr *int           // if set, record out.Line for the next token in *linePtr

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

        // Information about p.comments[p.cindex]; set up by nextComment.
        commentInfo

        // 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
}

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

// 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
}

// commentBefore reports whether 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) bool {
        return p.commentOffset < next.Offset && (!p.impliedSemi || !p.commentNewline)
}

// commentSizeBefore returns the estimated size of the
// comments on the same line before the next position.
//
func (p *printer) commentSizeBefore(next token.Position) int {
        // save/restore current p.commentInfo (p.nextComment() modifies it)
        defer func(info commentInfo) {
                p.commentInfo = info
        }(p.commentInfo)

        size := 0
        for p.commentBefore(next) {
                for _, c := range p.comment.List {
                        size += len(c.Text)
                }
                p.nextComment()
        }
        return size
}

// recordLine records the output line number for the next non-whitespace
// token in *linePtr. It is used to compute an accurate line number for a
// formatted construct, independent of pending (not yet emitted) whitespace
// or comments.
//
func (p *printer) recordLine(linePtr *int) {
        p.linePtr = linePtr
}

// linesFrom returns the number of output lines between the current
// output line and the line argument, ignoring any pending (not yet
// emitted) whitespace or comments. It is used to compute an accurate
// size (in number of lines) for a formatted construct.
//
func (p *printer) linesFrom(line int) int {
        return p.out.Line - line
}

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
}

// writeLineDirective writes a //line directive if necessary.
func (p *printer) writeLineDirective(pos token.Position) {
        if 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 directive
                p.out.Filename = pos.Filename
                p.out.Line = pos.Line
        }
}

// writeIndent writes indentation.
func (p *printer) writeIndent() {
        // use "hard" htabs - indentation columns
        // must not be discarded by the tabwriter
        n := p.Config.Indent + p.indent // include base indentation
        for i := 0; i < n; i++ {
                p.output = append(p.output, '\t')
        }

        // update positions
        p.pos.Offset += n
        p.pos.Column += n
        p.out.Column += n
}

// writeByte writes ch n times to p.output and updates p.pos.
// Only used to write formatting (white space) characters.
func (p *printer) writeByte(ch byte, n int) {
        if p.out.Column == 1 {
                // no need to write line directives before white space
                p.writeIndent()
        }

        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 {
                if p.Config.Mode&SourcePos != 0 {
                        p.writeLineDirective(pos)
                }
                p.writeIndent()
        }

        if pos.IsValid() {
                // update p.pos (if pos is invalid, continue with existing p.pos)
                // Note: Must do this after handling line beginnings because
                // writeIndent updates p.pos if there's indentation, but p.pos
                // is the position of s.
                p.pos = pos
        }

        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 *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))
                }
        }
}

// 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
}

// commonPrefix returns the common prefix of a and b.
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]
}

// trimRight returns s with trailing whitespace removed.
func trimRight(s string) string {
        return strings.TrimRightFunc(s, unicode.IsSpace)
}

// stripCommonPrefix removes a common prefix from /*-style comment lines (unless no
// comment line is indented, all but the first line have some form of space prefix).
// The prefix is computed using heuristics such that is likely that the comment
// contents are nicely laid out after re-printing each line using the printer's
// current indentation.
//
func stripCommonPrefix(lines []string) {
        if len(lines) <= 1 {
                return // at most one line - nothing to do
        }
        // len(lines) > 1

        // 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 cases where only the first and last lines are not blank,
        // such as two-line comments, or comments where all inner lines
        // are blank, 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.
        prefix := ""
        prefixSet := false
        if len(lines) > 2 {
                for i, line := range lines[1 : len(lines)-1] {
                        if isBlank(line) {
                                lines[1+i] = "" // range starts with lines[1]
                        } else {
                                if !prefixSet {
                                        prefix = line
                                        prefixSet = true
                                }
                                prefix = commonPrefix(prefix, line)
                        }

                }
        }
        // If we don't have a prefix yet, consider the last line.
        if !prefixSet {
                line := lines[len(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.
                        prefix = strings.TrimSuffix(prefix, string(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 {
                if i > 0 && line != "" {
                        lines[i] = line[len(prefix):]
                }
        }
}

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, trimRight(text), true)
                return
        }

        // for /*-style comments, print line by line and let the
        // write function take care of the proper indentation
        lines := strings.Split(text, "\n")

        // The comment started in the first column but is going
        // to be indented. For an idempotent result, add indentation
        // to all lines such that they look like they were indented
        // before - this will make sure the common prefix computation
        // is the same independent of how many times formatting is
        // applied (was issue 1835).
        if pos.IsValid() && pos.Column == 1 && p.indent > 0 {
                for i, line := range lines[1:] {
                        lines[1+i] = "   " + line
                }
        }

        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, trimRight(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
}

// containsLinebreak reports whether the whitespace buffer contains any line breaks.
func (p *printer) containsLinebreak() bool {
        for _, ch := range p.wsbuf {
                if ch == newline || ch == formfeed {
                        return true
                }
        }
        return false
}

// 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, 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, and not a "closing"
                // token immediately following its corresponding "opening" token,
                // add an extra separator unless explicitly disabled. Use a blank
                // as separator unless we have pending linebreaks, they are not
                // disabled, and we are outside a composite literal, in which case
                // we want a linebreak (issue 15137).
                // TODO(gri) This has become overly complicated. We should be able
                // to track whether we're inside an expression or statement and
                // use that information to decide more directly.
                needsLinebreak := false
                if p.mode&noExtraBlank == 0 &&
                        last.Text[1] == '*' && p.lineFor(last.Pos()) == next.Line &&
                        tok != token.COMMA &&
                        (tok != token.RPAREN || p.prevOpen == token.LPAREN) &&
                        (tok != token.RBRACK || p.prevOpen == token.LBRACK) {
                        if p.containsLinebreak() && p.mode&noExtraLinebreak == 0 && p.level == 0 {
                                needsLinebreak = true
                        } else {
                                p.writeByte(' ', 1)
                        }
                }
                // Ensure that there is a line break after a //-style comment,
                // before EOF, and before a closing '}' unless explicitly disabled.
                if last.Text[1] == '/' ||
                        tok == token.EOF ||
                        tok == token.RBRACE && p.mode&noExtraLinebreak == 0 {
                        needsLinebreak = true
                }
                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
        l := copy(p.wsbuf, p.wsbuf[n:])
        p.wsbuf = p.wsbuf[:l]
}

// ----------------------------------------------------------------------------
// 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

                // record previous opening token, if any
                switch p.lastTok {
                case token.ILLEGAL:
                        // ignore (white space)
                case token.LPAREN, token.LBRACK:
                        p.prevOpen = p.lastTok
                default:
                        // other tokens followed any opening token
                        p.prevOpen = token.ILLEGAL
                }

                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
                        }
                }

                // the next token starts now - record its line number if requested
                if p.linePtr != nil {
                        *p.linePtr = p.out.Line
                        p.linePtr = nil
                }

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

// 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 getLastComment(n ast.Node) *ast.CommentGroup {
        switch n := n.(type) {
        case *ast.Field:
                return n.Comment
        case *ast.ImportSpec:
                return n.Comment
        case *ast.ValueSpec:
                return n.Comment
        case *ast.TypeSpec:
                return n.Comment
        case *ast.GenDecl:
                if len(n.Specs) > 0 {
                        return getLastComment(n.Specs[len(n.Specs)-1])
                }
        case *ast.File:
                if len(n.Comments) > 0 {
                        return n.Comments[len(n.Comments)-1]
                }
        }
        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()
                }
                if com := getLastComment(n); com != nil {
                        if e := com.End(); e > end {
                                end = e
                        }
                }
                // 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 p.indent to 1 so we don't get indent "underflow".
                if _, ok := n.(*ast.LabeledStmt); ok {
                        p.indent = 1
                }
                p.stmt(n, false)
        case ast.Decl:
                p.decl(n)
        case ast.Spec:
                p.spec(n, 1, false)
        case []ast.Stmt:
                // A labeled statement will un-indent to position the label.
                // Set p.indent to 1 so we don't get indent "underflow".
                for _, s := range n {
                        if _, ok := s.(*ast.LabeledStmt); ok {
                                p.indent = 1
                        }
                }
                p.stmtList(n, 0, false)
        case []ast.Decl:
                p.declList(n)
        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()
                                if err == nil {
                                        _, 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 control 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 directives to preserve original source positions
)

// A Config node controls the output of Fprint.
type Config struct {
        Mode     Mode // default: 0
        Tabwidth int  // default: 8
        Indent   int  // default: 0 (all code is indented at least by this much)
}

// 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, []ast.Decl, []ast.Stmt,
// 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.
// Note that gofmt uses tabs for indentation but spaces for alignment;
// use format.Node (package go/format) for output that matches gofmt.
//
func Fprint(output io.Writer, fset *token.FileSet, node interface{}) error {
        return (&Config{Tabwidth: 8}).Fprint(output, fset, node)
}