Fix FilesFromDisk and make options optional

Author: mholt

README.md

@@ -63,16 +63,18 @@ $ go get github.com/mholt/archiver/v4
 
 Creating archives can be done entirely without needing a real disk or storage device since all you need is a list of [`File` structs](https://pkg.go.dev/github.com/mholt/archiver/v4#File) to pass in.
 
-However, creating archives from files on disk is very common, so you can use the `FilesFromDisk()` function to help you map filenames on disk to their paths in the archive. Then create and customize the format type.
+However, creating archives from files on disk is very common, so you can use the [`FilesFromDisk()` function](https://pkg.go.dev/github.com/mholt/archiver/v4#FilesFromDisk) to help you map filenames on disk to their paths in the archive. Then create and customize the format type.
 
-In this example, we add 2 files and a directory (which includes its contents recursively) to a .tar.gz file:
+In this example, we add 4 files and a directory (which includes its contents recursively) to a .tar.gz file:
 
 ```go
 // map files on disk to their paths in the archive
-files, err := archiver.FilesFromDisk(map[string]string{
+files, err := archiver.FilesFromDisk(nil, map[string]string{
 	"/path/on/disk/file1.txt": "file1.txt",
 	"/path/on/disk/file2.txt": "subfolder/file2.txt",
-	"/path/on/disk/folder":    "",
+	"/path/on/disk/file3.txt": "",              // put in root of archive as file3.txt
+	"/path/on/disk/file4.txt": "subfolder/",    // put in subfolder as file4.txt
+	"/path/on/disk/folder":    "Custom Folder", // contents added recursively
 })
 if err != nil {
 	return err
@@ -99,6 +101,8 @@ if err != nil {
 }
 ```
 
+The first parameter to `FilesFromDisk()` is an optional options struct, allowing you to customize how files are added.
+
 ### Extract archive
 
 Extracting an archive, extracting _from_ an archive, and walking an archive are all the same function.

archiver.go

@@ -45,18 +45,23 @@ func (f File) Stat() (fs.FileInfo, error) { return f.FileInfo, nil }
 // given filenames map. The keys are the names on disk, and the values are
 // their associated names in the archive. For convenience, empty values are
 // interpreted as the base name of the file (sans path) in the root of the
-// archive. Keys that specify directories on disk will be walked and added
-// to the archive recursively, rooted at the named directory.
+// archive; and values that end in a slash will use the bae name of the file
+// in that folder of the archive. Keys that specify directories on disk will
+// be walked and added to the archive recursively, rooted at the named
+// directory.
 //
 // File gathering will adhere to the settings specified in options.
 //
 // This function is primarily used when preparing a list of files to add to
 // an archive.
-func FilesFromDisk(options FromDiskOptions, filenames map[string]string) ([]File, error) {
+func FilesFromDisk(options *FromDiskOptions, filenames map[string]string) ([]File, error) {
 	var files []File
 	for rootOnDisk, rootInArchive := range filenames {
 		if rootInArchive == "" {
-			rootInArchive = filepath.Base(rootInArchive)
+			rootInArchive = filepath.Base(rootOnDisk)
+		}
+		if strings.HasSuffix(rootInArchive, "/") {
+			rootInArchive += filepath.Base(rootOnDisk)
 		}
 
 		filepath.WalkDir(rootOnDisk, func(filename string, d fs.DirEntry, err error) error {
@@ -73,7 +78,7 @@ func FilesFromDisk(options FromDiskOptions, filenames map[string]string) ([]File
 			var linkTarget string
 
 			if isSymlink(info) {
-				if options.FollowSymlinks {
+				if options != nil && options.FollowSymlinks {
 					// dereference symlinks
 					filename, err = os.Readlink(filename)
 					if err != nil {