group DCF files that share a file number (closes #3)

Files in one DCF directory that share a file number are one file group,
as section 4.3.2.2 of the specification says. `Directory.FileGroups`
lists them, each with its numbers, its members and its primary member. A
lone file is a group of one. A group reports its member count and total
size.

Section 6.2.1.3 gives a DCF basic file priority over a DCF extended image
file. It says nothing about a DCF optional file, so ranking every `JPG`
first is a choice. A `THM` comes last.

`isImageFile` now accepts `.tif`, so a `TIF` counts as an image.
`Store.Images`, `Store.Videos` and the totals still count each file on
its own.

Model: opus-5-5

README.md

@@ -109,14 +109,16 @@ All the code is in the `pkg/dcf` package. The repo has no commands.
   each one.
 - `helpers.go` finds the mount points that hold a DCF store.
 - `parse.go` reads one DCF store through an `fs.FS` rooted at its mount point.
+- `filegroup.go` sorts the files of one DCF directory into file groups.
 
 The main types are:
 
 - `Store` is one DCF store on one mounted filesystem.
-- `Directory` is one DCF directory. It has a directory number, a name and the
-  objects inside it.
+- `Directory` is one DCF directory. It has a directory number, a name, the
+  objects inside it and their file groups.
 - `Object` is one file in a store. A file in a DCF directory also has a
   directory number and a file number.
+- `FileGroup` is one file group in a DCF directory.
 - `Image` and `Video` embed `Object`. The file extension decides which one a
   file becomes. A file with any other extension, such as a `THM` thumbnail, is
   neither.
@@ -149,11 +151,37 @@ The naming rules do not apply under `PRIVATE/M4ROOT`. The images and videos
 there are in `Store.Images` and `Store.Videos`, and their DCF numbers are 0.
 `dcf` skips any symlink there.
 
+A file group is made of the DCF files in one DCF directory that share a file
+number. The specification calls it a DCF object. A RAW file and its JPEG form a
+file group, and so do a video and its `THM` thumbnail.
+
+- The first four characters and the extension can differ within a group.
+- A DCF file that shares its number with no other file is a group of one.
+- Files in different DCF directories are never in the same group.
+- Only DCF files are grouped. Files in `Store.NonConforming` and under
+  `PRIVATE/M4ROOT` are in no group.
+
+`Directory.FileGroups` lists the groups in file number order. Each `FileGroup`
+has:
+
+- `DirectoryNumber` and `FileNumber`
+- `Members`, which is every file in the group
+- `Primary`, which is the member that stands for the whole group
+- `MembersCount`, which returns the number of members
+- `TotalSize`, which returns the total size of the members in bytes
+
+The primary member is the first of these that the group holds:
+
+1. a `JPG` file
+2. any other file in `Store.Images`
+3. a file in `Store.Videos`
+4. any other file except a `THM`
+5. a `THM`
+
+`Store.Images` and `Store.Videos` list each file on its own, whatever its group.
+
 ## TODO
 
-- Handle DCF file groups, where multiple objects share the same index number.
-  `dcf` does not handle these yet. If you need this, send me a link to a zip or
-  tar of a memory card that uses them, and I'll see what I can do.
 - Take a `context.Context` to cancel the walk of a store.
 - Return a plain slice from `GetDCFStores`, not a pointer to one.
 - Copy files off a card, and verify each copy against its SHA256 hash.