go/printer, gofmt: don't write too many newlines

In some rare cases, gofmt would accept more than the maximum
number of empty lines (1) between source code snippets.

The actual change is in printer.go, lines 773-775; the rest
is some minor restructuring.

Applied gofmt -w src misc .

Fixes #2387.

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/5496047

src/cmd/cgo/godefs.go

@@ -2,7 +2,6 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-
 package main
 
 import (

src/pkg/bytes/buffer_test.go

@@ -16,7 +16,6 @@ const N = 10000  // make this bigger for a larger (and slower) test
 var data string  // test data for write tests
 var bytes []byte // test data; same as data but as a slice.
 
-
 func init() {
 	bytes = make([]byte, N)
 	for i := 0; i < N; i++ {

src/pkg/go/printer/nodes.go

@@ -39,7 +39,10 @@ import (
 //            future (not yet interspersed) comments in this function.
 //
 func (p *printer) linebreak(line, min int, ws whiteSpace, newSection bool) (printedBreak bool) {
-	n := p.nlines(line-p.pos.Line, min)
+	n := nlimit(line - p.pos.Line)
+	if n < min {
+		n = min
+	}
 	if n > 0 {
 		p.print(ws)
 		if newSection {

src/pkg/go/printer/printer.go

@@ -18,8 +18,11 @@ import (
 	"text/tabwriter"
 )
 
-const debug = false // enable for debugging
-const infinity = 1 << 30
+const (
+	maxNewlines = 2     // max. number of newlines between source text
+	debug       = false // enable for debugging
+	infinity    = 1 << 30
+)
 
 type whiteSpace byte
 
@@ -89,21 +92,7 @@ func (p *printer) internalError(msg ...interface{}) {
 	}
 }
 
-// nlines returns the adjusted number of linebreaks given the desired number
-// of breaks n such that min <= result <= max.
-//
-func (p *printer) nlines(n, min int) int {
-	const max = 2 // max. number of newlines
-	switch {
-	case n < min:
-		return min
-	case n > max:
-		return max
-	}
-	return n
-}
-
-// writeByte writes a single byte to p.output and updates p.pos.
+// writeByte writes ch to p.output and updates p.pos.
 func (p *printer) writeByte(ch byte) {
 	p.output.WriteByte(ch)
 	p.pos.Offset++
@@ -128,13 +117,11 @@ func (p *printer) writeByte(ch byte) {
 	}
 }
 
-// writeNewlines writes up to n newlines to p.output and updates p.pos.
-// The actual number of newlines written is limited by nlines.
-// nl must be one of '\n' or '\f'.
-//
-func (p *printer) writeNewlines(n int, nl byte) {
-	for n = p.nlines(n, 0); n > 0; n-- {
-		p.writeByte(nl)
+// writeByteN writes ch n times to p.output and updates p.pos.
+func (p *printer) writeByteN(ch byte, n int) {
+	for n > 0 {
+		p.writeByte(ch)
+		n--
 	}
 }
 
@@ -223,8 +210,8 @@ func (p *printer) writeCommentPrefix(pos, next token.Position, prev, comment *as
 	}
 
 	if pos.IsValid() && pos.Filename != p.last.Filename {
-		// comment in a different file - separate with newlines (writeNewlines will limit the number)
-		p.writeNewlines(10, '\f')
+		// comment in a different file - separate with newlines
+		p.writeByteN('\f', maxNewlines)
 		return
 	}
 
@@ -318,7 +305,7 @@ func (p *printer) writeCommentPrefix(pos, next token.Position, prev, comment *as
 			n = 1
 		}
 		if n > 0 {
-			p.writeNewlines(n, '\f')
+			p.writeByteN('\f', nlimit(n))
 		}
 		p.indent = indent
 	}
@@ -550,10 +537,11 @@ func (p *printer) writeComment(comment *ast.Comment) {
 // 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. writeCommentSuffix returns true if a pending
-// formfeed was dropped from the whitespace buffer.
+// 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) (droppedFF bool) {
+func (p *printer) writeCommentSuffix(needsLinebreak bool) (wroteNewline, droppedFF bool) {
 	for i, ch := range p.wsbuf {
 		switch ch {
 		case blank, vtab:
@@ -566,6 +554,7 @@ func (p *printer) writeCommentSuffix(needsLinebreak bool) (droppedFF bool) {
 			// but remember if we dropped any formfeeds
 			if needsLinebreak {
 				needsLinebreak = false
+				wroteNewline = true
 			} else {
 				if ch == formfeed {
 					droppedFF = true
@@ -579,6 +568,7 @@ func (p *printer) writeCommentSuffix(needsLinebreak bool) (droppedFF bool) {
 	// make sure we have a line break
 	if needsLinebreak {
 		p.writeByte('\n')
+		wroteNewline = true
 	}
 
 	return
@@ -587,10 +577,10 @@ func (p *printer) writeCommentSuffix(needsLinebreak bool) (droppedFF bool) {
 // 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. intersperseComments returns true if a pending
-// formfeed was dropped from the whitespace buffer.
+// 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) (droppedFF bool) {
+func (p *printer) intersperseComments(next token.Position, tok token.Token) (wroteNewline, droppedFF bool) {
 	var last *ast.Comment
 	for ; p.commentBefore(next); p.cindex++ {
 		for _, c := range p.comments[p.cindex].List {
@@ -618,7 +608,7 @@ func (p *printer) intersperseComments(next token.Position, tok token.Token) (dro
 	// 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 false
+	return
 }
 
 // whiteWhitespace writes the first n whitespace entries.
@@ -671,6 +661,14 @@ func (p *printer) writeWhitespace(n int) {
 // ----------------------------------------------------------------------------
 // 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:
@@ -765,17 +763,22 @@ func (p *printer) print(args ...interface{}) {
 		p.pos = next
 
 		if data != "" {
-			nl := byte('\n')
-			if p.flush(next, tok) {
-				nl = '\f' // dropped formfeed before
-			}
+			wroteNewline, droppedFF := p.flush(next, tok)
 
 			// intersperse extra newlines if present in the source
 			// (don't do this in flush as it will cause extra newlines
-			// at the end of a file) - use formfeeds if we dropped one
-			// before
-			if n := next.Line - p.pos.Line; n > 0 {
-				p.writeNewlines(n, nl)
+			// at the end of a file)
+			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.writeByteN(ch, n)
 			}
 
 			p.writeItem(next, data, isLit)
@@ -790,16 +793,15 @@ func (p *printer) commentBefore(next token.Position) bool {
 	return p.cindex < len(p.comments) && p.fset.Position(p.comments[p.cindex].List[0].Pos()).Offset < next.Offset
 }
 
-// Flush prints any pending comments and whitespace occurring
-// textually before the position of the next token tok. Flush
-// returns true if a pending formfeed character was dropped
-// from the whitespace buffer as a result of interspersing
-// comments.
+// 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) (droppedFF bool) {
+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
-		droppedFF = p.intersperseComments(next, tok)
+		wroteNewline, droppedFF = p.intersperseComments(next, tok)
 	} else {
 		// otherwise, write any leftover whitespace
 		p.writeWhitespace(len(p.wsbuf))

src/pkg/go/printer/testdata/statements.golden

@@ -271,7 +271,6 @@ func _() {
 	// Known bug: The first use call may have more than one empty line before
 	//            (see go/printer/nodes.go, func linebreak).
 
-
 	use(x)
 
 	if x < x {
@@ -386,7 +385,6 @@ L:	// A comment on the same line as the label, followed by a single empty line.
 	// Known bug: There may be more than one empty line before MoreCode()
 	//            (see go/printer/nodes.go, func linebreak).
 
-
 	MoreCode()
 }