go/ast: fix SortImports to handle block comments (take 2)

This is a 2nd attempt at fixing CL 162337 which had an off-by-one error.
We were unconditionally getting the position of the start of the next line
from the last import without checking whether it is the end of the file or not.

Fix the code to check for that and move the testcase added in CL 190523
to the end of the file for it to trigger the issue properly.

Fixes #18929

Change-Id: I59e77256e256570b160fea6a17bce9ef49e810df
Reviewed-on: https://go-review.googlesource.com/c/go/+/190480
Run-TryBot: Agniva De Sarker <agniva.quicksilver@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>

src/cmd/gofmt/testdata/import.golden

+// package comment
 package main
 
 import (
@@ -8,11 +9,6 @@ import (
 	"math"
 )
 
-import (
-	"fmt"
-	"math"
-)
-
 import (
 	"fmt"
 
@@ -25,6 +21,10 @@ import (
 	"io"
 )
 
+// We reset the line numbering to test that
+// the formatting works independent of line directives
+//line :19
+
 import (
 	"errors"
 	"fmt"
@@ -129,3 +129,66 @@ import (
 
 	"dedup_by_group"
 )
+
+import (
+	"fmt" // for Printf
+	/* comment */ io1 "io"
+	/* comment */ io2 "io"
+	/* comment */ "log"
+)
+
+import (
+	"fmt"
+	/* comment */ io1 "io"
+	/* comment */ io2 "io" // hello
+	"math"                 /* right side */
+	// end
+)
+
+import (
+	"errors" // for New
+	"fmt"
+	/* comment */ io1 "io" /* before */ // after
+	io2 "io"               // another
+	// end
+)
+
+import (
+	"errors" // for New
+	/* left */ "fmt" /* right */
+	"log"            // for Fatal
+	/* left */ "math" /* right */
+)
+
+import /* why */ /* comment here? */ (
+	/* comment */ "fmt"
+	"math"
+)
+
+// Reset it again
+//line :100
+
+// Dedup with different import styles
+import (
+	"path"
+	. "path"
+	_ "path"
+	pathpkg "path"
+)
+
+/* comment */
+import (
+	"fmt"
+	"math" // for Abs
+	// This is a new run
+	"errors"
+	"fmt"
+)
+
+// End an import declaration in the same line
+// as the last import. See golang.org/issue/33538.
+// Note: Must be the last (or 2nd last) line of the file.
+import (
+	"fmt"
+	"math"
+)

src/cmd/gofmt/testdata/import.input

+// package comment
 package main
 
 import (
@@ -8,9 +9,6 @@ import (
 	"io"
 )
 
-import("fmt"
-"math")
-
 import (
 	"fmt"
 
@@ -23,6 +21,10 @@ import (
 	"io"
 )
 
+// We reset the line numbering to test that
+// the formatting works independent of line directives
+//line :19
+
 import (
 	"fmt"
 	"math"
@@ -132,3 +134,66 @@ import (
 
 	"dedup_by_group"
 )
+
+import (
+	/* comment */ io1 "io"
+	"fmt" // for Printf
+	/* comment */ "log"
+	/* comment */ io2 "io"
+)
+
+import (
+	/* comment */ io2 "io" // hello
+	/* comment */ io1 "io"
+	"math" /* right side */
+	"fmt"
+	// end
+)
+
+import (
+	/* comment */ io1 "io" /* before */ // after
+	"fmt"
+	"errors" // for New
+	io2 "io" // another
+	// end
+)
+
+import (
+	/* left */ "fmt" /* right */
+	"errors" // for New
+	/* left */ "math" /* right */
+	"log" // for Fatal
+)
+
+import /* why */ /* comment here? */ (
+	/* comment */ "fmt"
+	"math"
+)
+
+// Reset it again
+//line :100
+
+// Dedup with different import styles
+import (
+	"path"
+	. "path"
+	_ "path"
+	"path"
+	pathpkg "path"
+)
+
+/* comment */
+import (
+	"math" // for Abs
+	"fmt"
+	// This is a new run
+	"errors"
+	"fmt"
+	"errors"
+)
+
+// End an import declaration in the same line
+// as the last import. See golang.org/issue/33538.
+// Note: Must be the last (or 2nd last) line of the file.
+import("fmt"
+"math")
\ No newline at end of file

src/go/ast/import.go

@@ -30,7 +30,7 @@ func SortImports(fset *token.FileSet, f *File) {
 		i := 0
 		specs := d.Specs[:0]
 		for j, s := range d.Specs {
-			if j > i && fset.Position(s.Pos()).Line > 1+fset.Position(d.Specs[j-1].End()).Line {
+			if j > i && lineAt(fset, s.Pos()) > 1+lineAt(fset, d.Specs[j-1].End()) {
 				// j begins a new run. End this one.
 				specs = append(specs, sortSpecs(fset, f, d.Specs[i:j])...)
 				i = j
@@ -42,8 +42,8 @@ func SortImports(fset *token.FileSet, f *File) {
 		// Deduping can leave a blank line before the rparen; clean that up.
 		if len(d.Specs) > 0 {
 			lastSpec := d.Specs[len(d.Specs)-1]
-			lastLine := fset.Position(lastSpec.Pos()).Line
-			rParenLine := fset.Position(d.Rparen).Line
+			lastLine := lineAt(fset, lastSpec.Pos())
+			rParenLine := lineAt(fset, d.Rparen)
 			for rParenLine > lastLine+1 {
 				rParenLine--
 				fset.File(d.Rparen).MergeLine(rParenLine)
@@ -52,6 +52,10 @@ func SortImports(fset *token.FileSet, f *File) {
 	}
 }
 
+func lineAt(fset *token.FileSet, pos token.Pos) int {
+	return fset.PositionFor(pos, false).Line
+}
+
 func importPath(s Spec) string {
 	t, err := strconv.Unquote(s.(*ImportSpec).Path.Value)
 	if err == nil {
@@ -89,6 +93,11 @@ type posSpan struct {
 	End   token.Pos
 }
 
+type cgPos struct {
+	left bool // true if comment is to the left of the spec, false otherwise.
+	cg   *CommentGroup
+}
+
 func sortSpecs(fset *token.FileSet, f *File, specs []Spec) []Spec {
 	// Can't short-circuit here even if specs are already sorted,
 	// since they might yet need deduplication.
@@ -104,39 +113,64 @@ func sortSpecs(fset *token.FileSet, f *File, specs []Spec) []Spec {
 	}
 
 	// Identify comments in this range.
-	// Any comment from pos[0].Start to the final line counts.
-	lastLine := fset.Position(pos[len(pos)-1].End).Line
-	cstart := len(f.Comments)
-	cend := len(f.Comments)
+	begSpecs := pos[0].Start
+	endSpecs := pos[len(pos)-1].End
+	beg := fset.File(begSpecs).LineStart(lineAt(fset, begSpecs))
+	endLine := lineAt(fset, endSpecs)
+	endFile := fset.File(endSpecs)
+	var end token.Pos
+	if endLine == endFile.LineCount() {
+		end = endSpecs
+	} else {
+		end = endFile.LineStart(endLine + 1) // beginning of next line
+	}
+	first := len(f.Comments)
+	last := -1
 	for i, g := range f.Comments {
-		if g.Pos() < pos[0].Start {
-			continue
+		if g.End() >= end {
+			break
 		}
-		if i < cstart {
-			cstart = i
+		// g.End() < end
+		if beg <= g.Pos() {
+			// comment is within the range [beg, end[ of import declarations
+			if i < first {
+				first = i
+			}
+			if i > last {
+				last = i
 			}
-		if fset.Position(g.End()).Line > lastLine {
-			cend = i
-			break
 		}
 	}
-	comments := f.Comments[cstart:cend]
 
-	// Assign each comment to the import spec preceding it.
-	importComments := map[*ImportSpec][]*CommentGroup{}
+	var comments []*CommentGroup
+	if last >= 0 {
+		comments = f.Comments[first : last+1]
+	}
+
+	// Assign each comment to the import spec on the same line.
+	importComments := map[*ImportSpec][]cgPos{}
 	specIndex := 0
 	for _, g := range comments {
 		for specIndex+1 < len(specs) && pos[specIndex+1].Start <= g.Pos() {
 			specIndex++
 		}
+		var left bool
+		// A block comment can appear before the first import spec.
+		if specIndex == 0 && pos[specIndex].Start > g.Pos() {
+			left = true
+		} else if specIndex+1 < len(specs) && // Or it can appear on the left of an import spec.
+			lineAt(fset, pos[specIndex].Start)+1 == lineAt(fset, g.Pos()) {
+			specIndex++
+			left = true
+		}
 		s := specs[specIndex].(*ImportSpec)
-		importComments[s] = append(importComments[s], g)
+		importComments[s] = append(importComments[s], cgPos{left: left, cg: g})
 	}
 
 	// Sort the import specs by import path.
 	// Remove duplicates, when possible without data loss.
 	// Reassign the import paths to have the same position sequence.
-	// Reassign each comment to abut the end of its spec.
+	// Reassign each comment to the spec on the same line.
 	// Sort the comments by new position.
 	sort.Slice(specs, func(i, j int) bool {
 		ipath := importPath(specs[i])
@@ -160,7 +194,7 @@ func sortSpecs(fset *token.FileSet, f *File, specs []Spec) []Spec {
 			deduped = append(deduped, s)
 		} else {
 			p := s.Pos()
-			fset.File(p).MergeLine(fset.Position(p).Line)
+			fset.File(p).MergeLine(lineAt(fset, p))
 		}
 	}
 	specs = deduped
@@ -174,11 +208,19 @@ func sortSpecs(fset *token.FileSet, f *File, specs []Spec) []Spec {
 		s.Path.ValuePos = pos[i].Start
 		s.EndPos = pos[i].End
 		for _, g := range importComments[s] {
-			for _, c := range g.List {
+			for _, c := range g.cg.List {
+				if g.left {
+					c.Slash = pos[i].Start - 1
+				} else {
+					// An import spec can have both block comment and a line comment
+					// to its right. In that case, both of them will have the same pos.
+					// But while formatting the AST, the line comment gets moved to
+					// after the block comment.
 					c.Slash = pos[i].End
 				}
 			}
 		}
+	}
 
 	sort.Slice(comments, func(i, j int) bool {
 		return comments[i].Pos() < comments[j].Pos()