cmd/fillstruct: add option to select by line number.

Fixes #3.

cmd/fillstruct/Readme.md

@@ -56,3 +56,4 @@ Flags:
 	-file:     filename
 	-modified: read an archive of modified files from stdin
 	-offset:   byte offset of the struct literal
+	-line:     line number of the struct literal, can be used instead of -offset

cmd/fillstruct/main.go

@@ -51,6 +51,8 @@
 //
 // -offset:   byte offset of the struct literal
 //
+// -line:     line number of the struct literal, can be used instead of -offset
+//
 package main
 
 import (
@@ -338,10 +340,11 @@ func main() {
 		filename = flag.String("file", "", "filename")
 		modified = flag.Bool("modified", false, "read an archive of modified files from stdin")
 		offset   = flag.Int("offset", 0, "byte offset of the struct literal")
+		line     = flag.Int("line", 0, "line number of the struct literal, can be used instead of -offset")
 	)
 	flag.Parse()
 
-	if *offset == 0 || *filename == "" {
+	if (*offset == 0 && *line == 0) || *filename == "" {
 		flag.PrintDefaults()
 		os.Exit(1)
 	}
@@ -357,6 +360,18 @@ func main() {
 	}
 	pkg := lprog.InitialPackages()[0]
 
+	// --- selection by line
+
+	if *line > 0 {
+		err = byLine(lprog, path, pkg, *line)
+		if err != nil {
+			log.Fatal(err)
+		}
+		return
+	}
+
+	// --- selection by offset
+
 	f, pos, err := findPos(lprog, path, *offset)
 	if err != nil {
 		log.Fatal(err)
@@ -422,6 +437,48 @@ func findPos(lprog *loader.Program, filename string, off int) (*ast.File, token.
 	return nil, 0, fmt.Errorf("could not find file %q", filename)
 }
 
+func byLine(lprog *loader.Program, filename string, pkg *loader.PackageInfo, line int) (err error) {
+	var f *ast.File
+	for _, af := range lprog.InitialPackages()[0].Files {
+		if file := lprog.Fset.File(af.Pos()); file.Name() == filename {
+			f = af
+		}
+	}
+	if f == nil {
+		return fmt.Errorf("could not find file %q", filename)
+	}
+
+	ast.Inspect(f, func(n ast.Node) bool {
+		lit, ok := n.(*ast.CompositeLit)
+		if !ok {
+			return true
+		}
+		startLine := lprog.Fset.Position(lit.Pos()).Line
+		endLine := lprog.Fset.Position(lit.End()).Line
+		if !(startLine <= line && line <= endLine) {
+			return true
+		}
+
+		name, _ := pkg.Types[lit].Type.(*types.Named)
+		typ, ok := pkg.Types[lit].Type.Underlying().(*types.Struct)
+		if !ok {
+			err = errors.New("no struct literal found at selection")
+			return false
+		}
+
+		startOff := lprog.Fset.Position(lit.Pos()).Offset
+		endOff := lprog.Fset.Position(lit.End()).Offset
+
+		newlit, lines := zeroValue(pkg.Pkg, lit, typ, name)
+		if err = print(newlit, lines, startOff, endOff); err != nil {
+			return false
+		}
+
+		return false
+	})
+	return err
+}
+
 func findCompositeLit(f *ast.File, info types.Info, pos token.Pos) (*ast.CompositeLit, *types.Struct, *types.Named, error) {
 	path, _ := astutil.PathEnclosingInterval(f, pos, pos)
 	for _, n := range path {