[sbutil] Use filname field in json when packing

- Use the filename provided in the json file when writing out a .studybox
  file.  By default, it will not overwrite an existing file.
- Passing --force will overwrite an existing file.
- Passing --output will override the filename in the json.

cmd/sbutil.go

@@ -5,6 +5,8 @@ import (
 	"os"
 	"path/filepath"
 	"strings"
+	"errors"
+	"io/fs"
 
 	"github.com/alexflint/go-arg"
 
@@ -17,7 +19,9 @@ type Arguments struct {
 }
 
 type ArgPack struct {
-	Input string `arg:"positional,required"`
+	Input  string `arg:"positional,required"`
+	Force  bool   `arg:"--force"`
+	Output string `arg:"--output,-o"`
 }
 
 type ArgUnPack struct {
@@ -73,9 +77,17 @@ func pack(args *ArgPack) error {
 
 	// TODO: put this in the json file?
 
-	outname := args.Input[:len(args.Input)-len(".json")]+".studybox"
+	if args.Output != "" {
-	fmt.Println(outname)
+		sb.Filename = args.Output
-	err = sb.Write(outname)
+	}
+
+	// outname := args.Input[:len(args.Input)-len(".json")]+".studybox"
+	if exists(sb.Filename) && !args.Force {
+		return fmt.Errorf("%s already exists or cannot be written to", sb.Filename)
+	}
+
+	fmt.Println(sb.Filename)
+	err = sb.Write(sb.Filename)
 	if err != nil {
 		return err
 	}
@@ -117,6 +129,19 @@ func unpack(args *ArgUnPack) error {
 	return nil
 }
 
+func exists(filename string) bool {
+	_, err := os.Stat(filename)
+	if err == nil {
+		return true
+	}
+
+	if errors.Is(err, fs.ErrNotExist) {
+		return false
+	}
+
+	return true
+}
+
 //func main_old() {
 //	if len(os.Args) < 3 {
 //		fmt.Println("Missing command")

rom/import.go

@@ -16,7 +16,7 @@ func Import(filename string) (*StudyBox, error) {
 		return nil, err
 	}
 
-	sbj := &StudyBoxJson{}
+	sbj := &StudyBoxJson{ Filename: filename }
 	err = json.Unmarshal(raw, sbj)
 	if err != nil {
 		return nil, fmt.Errorf("Unable to unmarshal json: %v", err)
@@ -28,6 +28,7 @@ func Import(filename string) (*StudyBox, error) {
 	}
 
 	sb := &StudyBox{
+		Filename: sbj.Filename,
 		Data:  &TapeData{Pages: []*Page{}},
 		Audio: audio,
 	}

rom/rom.go

@@ -8,6 +8,7 @@ import (
 )
 
 type StudyBox struct {
+	Filename string
 	Data  *TapeData
 	Audio *TapeAudio
 }