go/printer: idempotent comment formatting

Also:

- Refactored testing framework to permit easier
idempotency testing.

- Applied gofmt -w src misc

This CL depends on CL 6639044 being applied first.

Formatting is not idempotent for all files: In those
files the comment position has changed (due to missing
precise location information) and/or the comment formatting
cannot/is not aware of independent code re-formatting.
In general it is very hard to make format idempotent when
running it in one pass only. Leaving that aside for now.

Fixes #1835.

R=r, rsc
CC=golang-dev
https://golang.org/cl/6624051

src/pkg/go/printer/printer.go

@@ -449,11 +449,17 @@ func commonPrefix(a, b string) string {
 	return a[0:i]
 }
 
+// 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 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) < 2 {
+	if len(lines) <= 1 {
 		return // at most one line - nothing to do
 	}
-	// len(lines) >= 2
+	// len(lines) > 1
 
 	// The heuristic in this function tries to handle a few
 	// common patterns of /*-style comments: Comments where
@@ -479,7 +485,7 @@ func stripCommonPrefix(lines []string) {
 		for i, line := range lines[1 : len(lines)-1] {
 			switch {
 			case isBlank(line):
-				lines[1+i] = "" // range starts at line 1
+				lines[1+i] = "" // range starts with lines[1]
 			case first:
 				prefix = commonPrefix(line, line)
 				first = false
@@ -570,9 +576,9 @@ func stripCommonPrefix(lines []string) {
 	}
 
 	// 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
+	for i, line := range lines {
+		if i > 0 && line != "" {
+			lines[i] = line[len(prefix):]
 		}
 	}
 }
@@ -612,6 +618,19 @@ func (p *printer) writeComment(comment *ast.Comment) {
 	// for /*-style comments, print line by line and let the
 	// write function take care of the proper indentation
 	lines := split(text)
+
+	// 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,
@@ -1140,7 +1159,7 @@ func (p *trimmer) Write(data []byte) (n int, err error) {
 // ----------------------------------------------------------------------------
 // Public interface
 
-// A Mode value is a set of flags (or 0). They coontrol printing. 
+// A Mode value is a set of flags (or 0). They control printing. 
 type Mode uint
 
 const (

src/pkg/go/printer/printer_test.go

@@ -6,7 +6,9 @@ package printer
 
 import (
 	"bytes"
+	"errors"
 	"flag"
+	"fmt"
 	"go/ast"
 	"go/parser"
 	"go/token"
@@ -25,33 +27,28 @@ var update = flag.Bool("update", false, "update golden files")
 
 var fset = token.NewFileSet()
 
-func lineString(text []byte, i int) string {
-	i0 := i
-	for i < len(text) && text[i] != '\n' {
-		i++
-	}
-	return string(text[i0:i])
-}
-
 type checkMode uint
 
 const (
 	export checkMode = 1 << iota
 	rawFormat
+	idempotent
 )
 
-func runcheck(t *testing.T, source, golden string, mode checkMode) {
-	// parse source
-	prog, err := parser.ParseFile(fset, source, nil, parser.ParseComments)
+// format parses src, prints the corresponding AST, verifies the resulting
+// src is syntactically correct, and returns the resulting src or an error
+// if any.
+func format(src []byte, mode checkMode) ([]byte, error) {
+	// parse src
+	f, err := parser.ParseFile(fset, "", src, parser.ParseComments)
 	if err != nil {
-		t.Error(err)
-		return
+		return nil, fmt.Errorf("parse: %s\n%s", err, src)
 	}
 
 	// filter exports if necessary
 	if mode&export != 0 {
-		ast.FileExports(prog) // ignore result
-		prog.Comments = nil   // don't print comments that are not in AST
+		ast.FileExports(f) // ignore result
+		f.Comments = nil   // don't print comments that are not in AST
 	}
 
 	// determine printer configuration
@@ -60,17 +57,72 @@ func runcheck(t *testing.T, source, golden string, mode checkMode) {
 		cfg.Mode |= RawFormat
 	}
 
-	// format source
+	// print AST
 	var buf bytes.Buffer
-	if err := cfg.Fprint(&buf, fset, prog); err != nil {
-		t.Error(err)
+	if err := cfg.Fprint(&buf, fset, f); err != nil {
+		return nil, fmt.Errorf("print: %s", err)
 	}
-	res := buf.Bytes()
 
-	// formatted source must be valid
+	// make sure formated output is syntactically correct
+	res := buf.Bytes()
 	if _, err := parser.ParseFile(fset, "", res, 0); err != nil {
+		return nil, fmt.Errorf("re-parse: %s\n%s", err, buf.Bytes())
+	}
+
+	return res, nil
+}
+
+// lineAt returns the line in text starting at offset offs.
+func lineAt(text []byte, offs int) []byte {
+	i := offs
+	for i < len(text) && text[i] != '\n' {
+		i++
+	}
+	return text[offs:i]
+}
+
+// diff compares a and b.
+func diff(aname, bname string, a, b []byte) error {
+	var buf bytes.Buffer // holding long error message
+
+	// compare lengths
+	if len(a) != len(b) {
+		fmt.Fprintf(&buf, "\nlength changed: len(%s) = %d, len(%s) = %d", aname, len(a), bname, len(b))
+	}
+
+	// compare contents
+	line := 1
+	offs := 1
+	for i := 0; i < len(a) && i < len(b); i++ {
+		ch := a[i]
+		if ch != b[i] {
+			fmt.Fprintf(&buf, "\n%s:%d:%d: %s", aname, line, i-offs+1, lineAt(a, offs))
+			fmt.Fprintf(&buf, "\n%s:%d:%d: %s", bname, line, i-offs+1, lineAt(b, offs))
+			fmt.Fprintf(&buf, "\n\n")
+			break
+		}
+		if ch == '\n' {
+			line++
+			offs = i + 1
+		}
+	}
+
+	if buf.Len() > 0 {
+		return errors.New(buf.String())
+	}
+	return nil
+}
+
+func runcheck(t *testing.T, source, golden string, mode checkMode) {
+	src, err := ioutil.ReadFile(source)
+	if err != nil {
+		t.Error(err)
+		return
+	}
+
+	res, err := format(src, mode)
+	if err != nil {
 		t.Error(err)
-		t.Logf("\n%s", res)
 		return
 	}
 
@@ -89,23 +141,19 @@ func runcheck(t *testing.T, source, golden string, mode checkMode) {
 		return
 	}
 
-	// compare lengths
-	if len(res) != len(gld) {
-		t.Errorf("len = %d, expected %d (= len(%s))", len(res), len(gld), golden)
+	// formatted source and golden must be the same
+	if err := diff(source, golden, res, gld); err != nil {
+		t.Error(err)
+		return
 	}
 
-	// compare contents
-	for i, line, offs := 0, 1, 0; i < len(res) && i < len(gld); i++ {
-		ch := res[i]
-		if ch != gld[i] {
-			t.Errorf("%s:%d:%d: %s", source, line, i-offs+1, lineString(res, offs))
-			t.Errorf("%s:%d:%d: %s", golden, line, i-offs+1, lineString(gld, offs))
-			t.Error()
-			return
-		}
-		if ch == '\n' {
-			line++
-			offs = i + 1
+	if mode&idempotent != 0 {
+		// formatting golden must be idempotent
+		// (This is very difficult to achieve in general and for now
+		// it is only checked for files explicitly marked as such.)
+		res, err = format(gld, mode)
+		if err := diff(golden, fmt.Sprintf("format(%s)", golden), gld, res); err != nil {
+			t.Errorf("golden is not idempotent: %s", err)
 		}
 	}
 }
@@ -142,15 +190,16 @@ type entry struct {
 
 // Use go test -update to create/update the respective golden files.
 var data = []entry{
-	{"empty.input", "empty.golden", 0},
+	{"empty.input", "empty.golden", idempotent},
 	{"comments.input", "comments.golden", 0},
 	{"comments.input", "comments.x", export},
-	{"linebreaks.input", "linebreaks.golden", 0},
-	{"expressions.input", "expressions.golden", 0},
-	{"expressions.input", "expressions.raw", rawFormat},
+	{"comments2.input", "comments2.golden", idempotent},
+	{"linebreaks.input", "linebreaks.golden", idempotent},
+	{"expressions.input", "expressions.golden", idempotent},
+	{"expressions.input", "expressions.raw", rawFormat | idempotent},
 	{"declarations.input", "declarations.golden", 0},
 	{"statements.input", "statements.golden", 0},
-	{"slow.input", "slow.golden", 0},
+	{"slow.input", "slow.golden", idempotent},
 }
 
 func TestFiles(t *testing.T) {
@@ -248,7 +297,7 @@ func testComment(t *testing.T, f *ast.File, srclen int, comment *ast.Comment) {
 	}
 }
 
-// Verify that the printer produces always produces a correct program
+// Verify that the printer produces a correct program
 // even if the position information of comments introducing newlines
 // is incorrect.
 func TestBadComments(t *testing.T) {
@@ -421,21 +470,8 @@ func TestX(t *testing.T) {
 package p
 func _() {}
 `
-	// parse original
-	f, err := parser.ParseFile(fset, "src", src, parser.ParseComments)
+	_, err := format([]byte(src), 0)
 	if err != nil {
-		t.Fatal(err)
-	}
-
-	// pretty-print original
-	var buf bytes.Buffer
-	if err = (&Config{Mode: UseSpaces, Tabwidth: 8}).Fprint(&buf, fset, f); err != nil {
-		t.Fatal(err)
-	}
-
-	// parse pretty printed original
-	if _, err := parser.ParseFile(fset, "", buf.Bytes(), 0); err != nil {
-		t.Fatalf("%s\n%s", err, buf.Bytes())
+		t.Error(err)
 	}
-
 }

src/pkg/go/printer/testdata/comments2.golden

+// Copyright 2012 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.
+
+// This is a package for testing comment placement by go/printer.
+//
+package main
+
+// Test cases for idempotent comment formatting (was issue 1835).
+/*
+c1a
+*/
+/*
+   c1b
+*/
+/* foo
+c1c
+*/
+/* foo
+   c1d
+*/
+/*
+c1e
+foo */
+/*
+   c1f
+   foo */
+
+func f() {
+	/*
+	   c2a
+	*/
+	/*
+	   c2b
+	*/
+	/* foo
+	   c2c
+	*/
+	/* foo
+	   c2d
+	*/
+	/*
+	   c2e
+	   foo */
+	/*
+	   c2f
+	   foo */
+}
+
+func g() {
+	/*
+	   c3a
+	*/
+	/*
+	   c3b
+	*/
+	/* foo
+	   c3c
+	*/
+	/* foo
+	   c3d
+	*/
+	/*
+	   c3e
+	   foo */
+	/*
+	   c3f
+	   foo */
+}
+
+// Test case taken literally from issue 1835.
+func main() {
+	/*
+	   prints test 5 times
+	*/
+	for i := 0; i < 5; i++ {
+		println("test")
+	}
+}

src/pkg/go/printer/testdata/comments2.input

+// Copyright 2012 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.
+
+// This is a package for testing comment placement by go/printer.
+//
+package main
+
+// Test cases for idempotent comment formatting (was issue 1835).
+/*
+c1a
+*/
+/*
+   c1b
+*/
+/* foo
+c1c
+*/
+/* foo
+   c1d
+*/
+/*
+c1e
+foo */
+/*
+   c1f
+   foo */
+
+func f() {
+/*
+c2a
+*/
+/*
+   c2b
+*/
+/* foo
+c2c
+*/
+/* foo
+   c2d
+*/
+/*
+c2e
+foo */
+/*
+   c2f
+   foo */
+}
+
+func g() {
+/*
+c3a
+*/
+/*
+   c3b
+*/
+/* foo
+c3c
+*/
+/* foo
+   c3d
+*/
+/*
+c3e
+foo */
+/*
+   c3f
+   foo */
+}
+
+// Test case taken literally from issue 1835.
+func main() {
+/*
+prints test 5 times
+*/
+   for i := 0; i < 5; i++ {
+      println("test")
+   }
+}
\ No newline at end of file