get cards with GetStores and GetStore, read them through fs.FS (closes #4)

GetStores replaces GetDCFStores: it takes a context, has no count
argument and returns a plain slice. GetStore returns the one mounted
card, or ErrNoStore or ErrMultipleStores.

Each Object keeps the fs.FS its card was read through, and Hash reads
through it, so tests can use fstest.MapFS. A cancelled context stops
the read before the next directory. Library errors wrap with %w.

Model: opus-5-5
Co-authored-by: clawbot <sneak+clawbot@sneak.cloud>

README.md

@@ -29,6 +29,7 @@ Then list the DCF stores on the mounted filesystems:
 package main
 
 import (
+	"context"
 	"fmt"
 	"log"
 
@@ -36,17 +37,21 @@ import (
 )
 
 func main() {
-	stores, err := dcf.GetDCFStores(0)
+	stores, err := dcf.GetStores(context.Background())
 	if err != nil {
 		log.Fatal(err)
 	}
 
-	for _, store := range *stores {
+	for _, store := range stores {
 		fmt.Println(store.String())
 	}
 }
 ```
 
+Call `dcf.GetStore` instead when you expect exactly one DCF store. It returns
+`dcf.ErrNoStore` when none is mounted, and `dcf.ErrMultipleStores` when more
+than one is.
+
 To work on the library itself, you need docker and git:
 
 ```sh
@@ -105,34 +110,89 @@ overengineering and reusable code.
 
 All the code is in the `pkg/dcf` package. The repo has no commands.
 
-- `dcf.go` holds the public API. `GetDCFStores` finds the DCF stores and lists
-  the files on them.
+- `dcf.go` holds the public API. `GetStores` finds the DCF stores and reads each
+  one. `GetStore` returns the one DCF store when exactly one is mounted.
 - `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.
-- `Object` is one file in a store.
+- `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. `Object.Hash` reads the file through the
+  `fs.FS` of its store. An `Object` you build yourself has none, so its `Hash`
+  returns an error.
+- `FileGroup` is one file group in a DCF directory.
 - `Image` and `Video` embed `Object`. The file extension decides which one a
-  file becomes.
+  file becomes. A file with any other extension, such as a `THM` thumbnail, is
+  neither.
 
-A mounted filesystem is a DCF store when its root holds a `DCIM` or `M4ROOT`
-directory. `dcf` never mounts a filesystem.
+A mounted filesystem is a DCF store when its root holds a `DCIM` or
+`PRIVATE/M4ROOT` directory. It needs only one of them. `dcf` never mounts a
+filesystem. Media anywhere else on the filesystem is not part of the store.
+
+`dcf` never follows a symlink.
+
+`GetStores` and `GetStore` take a `context.Context`. When it is cancelled, the
+read stops before the next directory. The returned error matches
+`context.Canceled`.
+
+`dcf` applies the naming rules of the DCF specification, CIPA DC-009-2010, under
+`DCIM`:
+
+- A DCF directory sits directly under `DCIM`. Its name is a directory number
+  from 100 to 999, then five characters, such as `100MSDCF`.
+- A DCF file sits directly in a DCF directory. Its name is four characters, then
+  a file number from 0001 to 9999, then a dot and a three-character extension,
+  such as `DSC00001.ARW`.
+- The characters allowed are `A` to `Z`, `0` to `9` and `_`. Lower-case letters
+  count as upper-case.
+- Directories that share a directory number are not DCF directories.
+- `JPG` files that share a file number in one directory are not DCF files.
+
+`Store.NonConforming` lists the path of everything else under `DCIM`. It
+includes everything inside a directory that breaks the rules. It also includes
+every symlink, whatever its name. These entries have no DCF numbers, and they
+are not counted as images or videos.
+
+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.
-- Parse the DCF directory and file names, rather than listing every media file
-  on the filesystem.
-- Look for `PRIVATE/M4ROOT` rather than `M4ROOT` when detecting a DCF store.
-- Return exactly the number of stores asked for. `GetDCFStores` returns one too
-  few when asked for two or more.
-- Read stores through `fs.FS`, with a `context.Context` to cancel a walk.
-- Return a plain slice from `GetDCFStores`, not a pointer to one.
-- Test the walk of a store and the detection of mount points. They only run
-  against real mounted media today.
 - Copy files off a card, and verify each copy against its SHA256 hash.
 
 ## License

pkg/dcf/dcf.go

@@ -1,13 +1,19 @@
 // Package dcf reads DCF cards: memory cards laid out to the "Design rule for
 // Camera File system" specification (JEITA CP-3461). It finds the cards among
-// the mounted filesystems and lists the images and videos on each one.
+// the mounted filesystems and parses the DCF directories and files on each one
+// into their directory numbers and file numbers. It lists the images and videos
+// on each card, and everything under DCIM that is not a DCF directory or file. It
+// groups the files in each DCF directory that share a file number.
 package dcf
 
 import (
+	"context"
 	"crypto/sha256"
 	"encoding/hex"
+	"errors"
 	"fmt"
 	"io"
+	"io/fs"
 	"os"
 	"path/filepath"
 	"slices"
@@ -16,12 +22,58 @@ import (
 	"github.com/dustin/go-humanize"
 )
 
+var (
+	// ErrNoStore is the error from GetStore when no DCF store is mounted.
+	ErrNoStore = errors.New("no DCF store is mounted")
+	// ErrMultipleStores is the error from GetStore when more than one DCF store
+	// is mounted.
+	ErrMultipleStores = errors.New("more than one DCF store is mounted")
+)
+
 // Store represents a DCF store, which is a directory structure that contains
 // images and videos in the root of a mounted filesystem.
 type Store struct {
 	RootDirectory string
-	Images        []*Image
-	Videos        []*Video
+	// Directories holds the DCF directories under DCIM.
+	Directories []*Directory
+	// Images and Videos hold the images and videos among the objects in
+	// Directories, and those under PRIVATE/M4ROOT.
+	Images []*Image
+	Videos []*Video
+	// NonConforming holds the path, relative to RootDirectory, of everything
+	// under DCIM that is not a DCF directory or a DCF file. It includes every
+	// symlink, and everything inside a directory that breaks the DCF naming
+	// rules. A directory whose name is not valid UTF-8 is listed without what is
+	// inside it, because fs.FS refuses to open one.
+	NonConforming []string
+}
+
+// Directory represents a DCF directory: a directory directly under DCIM whose
+// name follows the DCF naming rules.
+type Directory struct {
+	// Number is the directory number, from 100 to 999.
+	Number int
+	// Name is the name of the directory as it appears on the card.
+	Name string
+	// Objects holds every DCF file in the directory, including those that are
+	// neither images nor videos.
+	Objects []*Object
+	// FileGroups holds the same files, grouped by file number, in file number
+	// order.
+	FileGroups []*FileGroup
+}
+
+// FileGroup represents one file group: the files in one DCF directory that share
+// a file number. The DCF specification calls this a DCF object. A file that
+// shares its number with no other file is a group of one.
+type FileGroup struct {
+	DirectoryNumber int
+	FileNumber      int
+	// Members holds every file in the group, including Primary, in name order.
+	// Two members can have the same extension.
+	Members []*Object
+	// Primary is the member that stands for the whole group.
+	Primary *Object
 }
 
 // Object represents a file in a DCF store. It is embedded in Image and Video
@@ -31,6 +83,13 @@ type Object struct {
 	Path         string
 	Size         int64
 	Extension    string
+	// DirectoryNumber and FileNumber are the DCF numbers of a file in a DCF
+	// directory. Both are 0 for a file under PRIVATE/M4ROOT.
+	DirectoryNumber int
+	FileNumber      int
+	// fsys is the fs.FS the store was read through. Hash reads the file through
+	// it.
+	fsys fs.FS
 }
 
 // Image represents an image file on a DCF store.
@@ -86,6 +145,21 @@ func (d *Store) TotalSize() int64 {
 	return d.TotalImageSize() + d.TotalVideoSize()
 }
 
+// MembersCount returns the number of files in the file group.
+func (g *FileGroup) MembersCount() int {
+	return len(g.Members)
+}
+
+// TotalSize returns the total size in bytes of the files in the file group.
+func (g *FileGroup) TotalSize() int64 {
+	totalSize := int64(0)
+	for _, member := range g.Members {
+		totalSize += member.Size
+	}
+
+	return totalSize
+}
+
 func (d *Store) String() string {
 	// A caller can set a negative Size, and uint64 would make that huge.
 	totalSize := uint64(max(d.TotalSize(), 0))
@@ -113,15 +187,17 @@ func (i *Image) String() string {
 	)
 }
 
-// Hash returns the SHA256 hash of the file as a hex string.
+// Hash returns the SHA256 hash of the file as a hex string. It reads the file
+// through the fs.FS of its store. An Object built by hand has none, so Hash
+// returns an error that matches fs.ErrInvalid.
 func (d *Object) Hash() (string, error) {
-	return pathToSHA256(d.FullFilePath())
-}
+	if d.fsys == nil {
+		return "", fmt.Errorf("hashing %s: %w", d.FullFilePath(), fs.ErrInvalid)
+	}
 
-func pathToSHA256(path string) (string, error) {
-	file, err := os.Open(filepath.Clean(path))
+	file, err := d.fsys.Open(d.Path)
 	if err != nil {
-		return "", err
+		return "", fmt.Errorf("hashing %s: %w", d.FullFilePath(), err)
 	}
 
 	defer func() { _ = file.Close() }()
@@ -130,60 +206,59 @@ func pathToSHA256(path string) (string, error) {
 
 	_, err = io.Copy(hash, file)
 	if err != nil {
-		return "", err
+		return "", fmt.Errorf("hashing %s: %w", d.FullFilePath(), err)
 	}
 
 	return hex.EncodeToString(hash.Sum(nil)), nil
 }
 
-// GetDCFStores returns a list of DCF stores found on the system in the root of
-// any mounted filesystems. The integer argument is the number of DCF stores to
-// return. If the argument is 0, all DCF stores are returned. It does not mount
-// filesystems or search outside of filesystem root directories. It does,
-// however, walk the filesystems to find images and videos and populate the
-// returned Stores.
-func GetDCFStores(requestedCount int) (*[]Store, error) {
-	dcfStorePaths, err := findDCFMountPoints(requestedCount)
+// GetStores finds the DCF stores in the root of the mounted filesystems and reads
+// each one. It does not mount filesystems or search outside of filesystem root
+// directories. It reads the DCIM and PRIVATE/M4ROOT directories of each
+// filesystem to populate the returned Stores. It never follows a symlink. If a
+// card cannot be read, it returns an error that names the card's mount point.
+// When ctx is cancelled, the read stops before the next directory, and the error
+// matches ctx.Err().
+func GetStores(ctx context.Context) ([]*Store, error) {
+	dcfStorePaths, err := findDCFMountPoints(ctx)
 	if err != nil {
 		return nil, err
 	}
 
-	dcfStores := []Store{}
+	return parseStores(ctx, dcfStorePaths)
+}
 
-	for _, dcfStorePath := range dcfStorePaths {
-		dcfStore := Store{
-			RootDirectory: dcfStorePath,
-			Images:        make([]*Image, 0),
-			Videos:        make([]*Video, 0),
-		}
+// GetStore returns the one DCF store when exactly one is mounted. It returns
+// ErrNoStore when none is mounted, and ErrMultipleStores when more than one is.
+// It reads every card first, the way GetStores does, so a card that cannot be
+// read returns that error instead.
+func GetStore(ctx context.Context) (*Store, error) {
+	stores, err := GetStores(ctx)
+	if err != nil {
+		return nil, err
+	}
 
-		addFile := func(path string, info os.FileInfo, err error) error {
-			if err != nil {
-				return err
-			}
+	return onlyStore(stores)
+}
 
-			if !info.IsDir() && isImageFile(path) {
-				image := Image{}
-				image.Path = strings.TrimPrefix(path, dcfStorePath+"/")
-				image.Size = info.Size()
-				image.Extension = filepath.Ext(path)
-				image.DCFStoreRoot = dcfStorePath
-				dcfStore.Images = append(dcfStore.Images, &image)
-			}
+// onlyStore returns the store when stores holds exactly one.
+func onlyStore(stores []*Store) (*Store, error) {
+	switch len(stores) {
+	case 0:
+		return nil, ErrNoStore
+	case 1:
+		return stores[0], nil
+	default:
+		return nil, ErrMultipleStores
+	}
+}
 
-			if !info.IsDir() && isVideoFile(path) {
-				video := Video{}
-				video.Path = strings.TrimPrefix(path, dcfStorePath+"/")
-				video.Size = info.Size()
-				video.Extension = filepath.Ext(path)
-				video.DCFStoreRoot = dcfStorePath
-				dcfStore.Videos = append(dcfStore.Videos, &video)
-			}
+// parseStores reads the card at each of the given mount points.
+func parseStores(ctx context.Context, mountpoints []string) ([]*Store, error) {
+	dcfStores := []*Store{}
 
-			return nil
-		}
-
-		err := filepath.Walk(dcfStorePath, addFile)
+	for _, mountpoint := range mountpoints {
+		dcfStore, err := parseStore(ctx, os.DirFS(mountpoint), mountpoint)
 		if err != nil {
 			return nil, err
 		}
@@ -191,12 +266,12 @@ func GetDCFStores(requestedCount int) (*[]Store, error) {
 		dcfStores = append(dcfStores, dcfStore)
 	}
 
-	return &dcfStores, nil
+	return dcfStores, nil
 }
 
 func isImageFile(path string) bool {
 	imageExtensions := []string{
-		".jpg", ".jpeg", ".png", ".gif", ".bmp", ".tiff", ".raw", ".arw",
+		".jpg", ".jpeg", ".png", ".gif", ".bmp", ".tif", ".tiff", ".raw", ".arw",
 	}
 
 	return slices.Contains(imageExtensions, strings.ToLower(filepath.Ext(path)))

pkg/dcf/dcf_internal_test.go

@@ -1,6 +1,144 @@
 package dcf
 
-import "testing"
+import (
+	"context"
+	"errors"
+	"io/fs"
+	"path/filepath"
+	"strings"
+	"testing"
+	"testing/fstest"
+)
+
+func TestParseStoresReadsEachMountPoint(t *testing.T) {
+	t.Parallel()
+
+	first := newCardOnDisk(t, "DCIM/100ABCDE/ABCD0001.JPG")
+	second := newCardOnDisk(t, "PRIVATE/M4ROOT/CLIP/C0001.MP4")
+
+	stores, err := parseStores(t.Context(), []string{first, second})
+	if err != nil {
+		t.Fatalf("parseStores: %v", err)
+	}
+
+	if len(stores) != 2 {
+		t.Fatalf("parseStores returned %d stores, want 2", len(stores))
+	}
+
+	cases := map[string]Object{
+		filepath.Join(first, "DCIM/100ABCDE/ABCD0001.JPG"):     stores[0].Images[0].Object,
+		filepath.Join(second, "PRIVATE/M4ROOT/CLIP/C0001.MP4"): stores[1].Videos[0].Object,
+	}
+
+	for want, object := range cases {
+		if got := object.FullFilePath(); got != want {
+			t.Errorf("FullFilePath() = %q, want %q", got, want)
+		}
+
+		got, err := object.Hash()
+		if err != nil {
+			t.Fatalf("Hash() returned error: %v", err)
+		}
+
+		// The published SHA-256 test vector for "abc".
+		wantHash := "ba7816bf8f01cfea414140de5dae2223b00361a396177a9cb410ff61f20015ad"
+		if got != wantHash {
+			t.Errorf("Hash() = %q, want %q", got, wantHash)
+		}
+	}
+}
+
+func TestParseStoresReturnsErrorNamingTheCard(t *testing.T) {
+	t.Parallel()
+
+	// Every lookup under a plain file fails.
+	mountpoint := filepath.Join(newCardOnDisk(t, "FILE"), "FILE")
+
+	_, err := parseStores(t.Context(), []string{mountpoint})
+	if err == nil || !strings.Contains(err.Error(), mountpoint) {
+		t.Fatalf("parseStores error = %v, want one naming %s", err, mountpoint)
+	}
+}
+
+func TestParseStoresReturnsErrorWhenCancelled(t *testing.T) {
+	t.Parallel()
+
+	ctx, cancel := context.WithCancel(t.Context())
+	cancel()
+
+	mountpoint := newCardOnDisk(t, "DCIM/100ABCDE/ABCD0001.JPG")
+
+	_, err := parseStores(ctx, []string{mountpoint})
+	if !errors.Is(err, context.Canceled) {
+		t.Fatalf("parseStores error = %v, want %v", err, context.Canceled)
+	}
+}
+
+func TestOnlyStore(t *testing.T) {
+	t.Parallel()
+
+	first := newMountPoint(t, dcimDirectory)
+	second := newMountPoint(t, m4RootDirectory)
+
+	stores, err := parseStores(t.Context(), []string{first, second})
+	if err != nil {
+		t.Fatalf("parseStores: %v", err)
+	}
+
+	_, err = onlyStore(stores[:0])
+	if !errors.Is(err, ErrNoStore) {
+		t.Errorf("onlyStore of no cards: error = %v, want %v", err, ErrNoStore)
+	}
+
+	store, err := onlyStore(stores[:1])
+	if err != nil {
+		t.Fatalf("onlyStore of one card: %v", err)
+	}
+
+	if store.RootDirectory != first {
+		t.Errorf("onlyStore returned the card at %s, want %s",
+			store.RootDirectory, first)
+	}
+
+	_, err = onlyStore(stores)
+	if !errors.Is(err, ErrMultipleStores) {
+		t.Errorf("onlyStore of two cards: error = %v, want %v",
+			err, ErrMultipleStores)
+	}
+}
+
+func TestObjectHashReadsThroughTheCard(t *testing.T) {
+	t.Parallel()
+
+	card := fstest.MapFS{"DCIM/100ABCDE/ABCD0001.JPG": {Data: []byte("abc")}}
+
+	got, err := mustParseStore(t, card).Images[0].Hash()
+	if err != nil {
+		t.Fatalf("Hash() returned error: %v", err)
+	}
+
+	// The published SHA-256 test vector for "abc".
+	want := "ba7816bf8f01cfea414140de5dae2223b00361a396177a9cb410ff61f20015ad"
+	if got != want {
+		t.Fatalf("Hash() = %q, want %q", got, want)
+	}
+}
+
+func TestObjectHashOfMissingFileReturnsError(t *testing.T) {
+	t.Parallel()
+
+	const filePath = "DCIM/100ABCDE/ABCD0001.JPG"
+
+	card := newCard(filePath)
+	image := mustParseStore(t, card).Images[0]
+
+	delete(card, filePath)
+
+	_, err := image.Hash()
+	if !errors.Is(err, fs.ErrNotExist) {
+		t.Fatalf("Hash() error = %v, want %v", err, fs.ErrNotExist)
+	}
+}
 
 func TestIsImageFile(t *testing.T) {
 	t.Parallel()
@@ -8,6 +146,8 @@ func TestIsImageFile(t *testing.T) {
 	cases := map[string]bool{
 		"DCIM/100MSDCF/DSC00001.jpg":    true,
 		"DCIM/100MSDCF/DSC00001.ARW":    true,
+		"DCIM/100MSDCF/DSC00001.TIF":    true,
+		"DCIM/100MSDCF/DSC00001.tif":    true,
 		"PRIVATE/M4ROOT/CLIP/C0001.MP4": false,
 		"DCIM/100MSDCF/notes.txt":       false,
 		"DCIM/100MSDCF/noextension":     false,

pkg/dcf/dcf_test.go

@@ -1,8 +1,9 @@
 package dcf_test
 
 import (
-	"os"
-	"path/filepath"
+	"context"
+	"errors"
+	"io/fs"
 	"testing"
 
 	"sneak.berlin/go/dcf/pkg/dcf"
@@ -125,37 +126,30 @@ func TestVideoString(t *testing.T) {
 	}
 }
 
-func TestObjectHashIsSHA256OfFile(t *testing.T) {
+func TestObjectHashOfObjectBuiltByHandReturnsError(t *testing.T) {
 	t.Parallel()
 
-	root := t.TempDir()
-
-	err := os.WriteFile(filepath.Join(root, "a.jpg"), []byte("abc"), 0o600)
-	if err != nil {
-		t.Fatalf("WriteFile: %v", err)
-	}
-
-	object := dcf.Object{DCFStoreRoot: root, Path: "a.jpg"}
-
-	got, err := object.Hash()
-	if err != nil {
-		t.Fatalf("Hash() returned error: %v", err)
-	}
-
-	// The published SHA-256 test vector for "abc".
-	want := "ba7816bf8f01cfea414140de5dae2223b00361a396177a9cb410ff61f20015ad"
-	if got != want {
-		t.Fatalf("Hash() = %q, want %q", got, want)
-	}
-}
-
-func TestObjectHashOfMissingFileReturnsError(t *testing.T) {
-	t.Parallel()
-
-	object := dcf.Object{DCFStoreRoot: t.TempDir(), Path: "absent.jpg"}
+	object := dcf.Object{DCFStoreRoot: cardRoot, Path: "a.jpg"}
 
 	_, err := object.Hash()
-	if err == nil {
-		t.Fatal("Hash() of a missing file returned no error")
+	if !errors.Is(err, fs.ErrInvalid) {
+		t.Fatalf("Hash() error = %v, want %v", err, fs.ErrInvalid)
+	}
+}
+
+func TestGetStoresAndGetStoreReturnCancellation(t *testing.T) {
+	t.Parallel()
+
+	ctx, cancel := context.WithCancel(t.Context())
+	cancel()
+
+	_, err := dcf.GetStores(ctx)
+	if !errors.Is(err, context.Canceled) {
+		t.Errorf("GetStores error = %v, want %v", err, context.Canceled)
+	}
+
+	_, err = dcf.GetStore(ctx)
+	if !errors.Is(err, context.Canceled) {
+		t.Errorf("GetStore error = %v, want %v", err, context.Canceled)
 	}
 }

pkg/dcf/filegroup.go

@@ -0,0 +1,67 @@
+package dcf
+
+import (
+	"cmp"
+	"path"
+	"slices"
+	"strings"
+)
+
+// newFileGroups sorts the objects of one DCF directory into file groups, in file
+// number order. Section 4.3.2.2 of the specification says the files in a DCF
+// directory that share a file number are one group. Their first four characters
+// and their extensions can differ.
+func newFileGroups(objects []*Object) []*FileGroup {
+	var groups []*FileGroup
+
+	groupWithNumber := map[int]*FileGroup{}
+
+	for _, object := range objects {
+		group, found := groupWithNumber[object.FileNumber]
+		if !found {
+			group = &FileGroup{
+				DirectoryNumber: object.DirectoryNumber,
+				FileNumber:      object.FileNumber,
+			}
+			groupWithNumber[object.FileNumber] = group
+			groups = append(groups, group)
+		}
+
+		group.Members = append(group.Members, object)
+	}
+
+	for _, group := range groups {
+		group.Primary = primaryMember(group.Members)
+	}
+
+	slices.SortFunc(groups, func(a, b *FileGroup) int {
+		return cmp.Compare(a.FileNumber, b.FileNumber)
+	})
+
+	return groups
+}
+
+// primaryMember returns the member that stands for the whole group. Section
+// 6.2.1.3 of the specification gives a DCF basic file priority over a DCF extended
+// image file. It says nothing about a DCF optional file, the other kind of JPG, so
+// ranking every JPG first is a choice. Section 4.6 makes a THM only the thumbnail
+// of another file, so it comes last. The specification leaves the rest open, so an
+// image comes before a video, and a video before any other file. Among equals, the
+// first in name order wins.
+func primaryMember(members []*Object) *Object {
+	choices := []func(string) bool{isJPG, isImageFile, isVideoFile, isNotTHM}
+
+	for _, choice := range choices {
+		for _, member := range members {
+			if choice(member.Path) {
+				return member
+			}
+		}
+	}
+
+	return members[0]
+}
+
+func isNotTHM(name string) bool {
+	return !strings.EqualFold(path.Ext(name), ".THM")
+}

pkg/dcf/filegroup_internal_test.go

@@ -0,0 +1,232 @@
+package dcf
+
+import (
+	"slices"
+	"testing"
+	"testing/fstest"
+)
+
+// mustFileGroups returns the file groups of directory, and stops the test unless
+// there are exactly count of them.
+func mustFileGroups(t *testing.T, directory *Directory, count int) []*FileGroup {
+	t.Helper()
+
+	if len(directory.FileGroups) != count {
+		t.Fatalf("%s has %d file groups, want %d",
+			directory.Name, len(directory.FileGroups), count)
+	}
+
+	return directory.FileGroups
+}
+
+// assertFileGroup checks the numbers, the primary member and the members of group.
+func assertFileGroup(
+	t *testing.T,
+	group *FileGroup,
+	directoryNumber, fileNumber int,
+	primary string,
+	members ...string,
+) {
+	t.Helper()
+
+	if group.DirectoryNumber != directoryNumber || group.FileNumber != fileNumber {
+		t.Errorf("group numbers = %d and %d, want %d and %d",
+			group.DirectoryNumber, group.FileNumber, directoryNumber, fileNumber)
+	}
+
+	if group.Primary.Path != primary {
+		t.Errorf("Primary = %q, want %q", group.Primary.Path, primary)
+	}
+
+	paths := make([]string, 0, len(group.Members))
+
+	for _, member := range group.Members {
+		paths = append(paths, member.Path)
+	}
+
+	assertPaths(t, "members", paths, members...)
+
+	if got := group.MembersCount(); got != len(members) {
+		t.Errorf("MembersCount() = %d, want %d", got, len(members))
+	}
+}
+
+// assertSizes checks the total size of group and the size of each member.
+func assertSizes(t *testing.T, group *FileGroup, total int64, sizes ...int64) {
+	t.Helper()
+
+	if got := group.TotalSize(); got != total {
+		t.Errorf("TotalSize() = %d, want %d", got, total)
+	}
+
+	got := make([]int64, 0, len(group.Members))
+
+	for _, member := range group.Members {
+		got = append(got, member.Size)
+	}
+
+	if !slices.Equal(got, sizes) {
+		t.Errorf("member sizes = %v, want %v", got, sizes)
+	}
+}
+
+// assertPrimaryMember checks which of the named members primaryMember picks.
+func assertPrimaryMember(t *testing.T, want string, names ...string) {
+	t.Helper()
+
+	members := make([]*Object, 0, len(names))
+
+	for _, name := range names {
+		members = append(members, &Object{Path: name})
+	}
+
+	if got := primaryMember(members).Path; got != want {
+		t.Errorf("primaryMember of %q = %q, want %q", names, got, want)
+	}
+}
+
+func TestParseStoreGroupsFilesSharingAFileNumber(t *testing.T) {
+	t.Parallel()
+
+	store := mustParseStore(t, fstest.MapFS{
+		"DCIM/100ABCDE/DSC00001.ARW": {Data: make([]byte, 2000)},
+		"DCIM/100ABCDE/DSC00001.JPG": {Data: make([]byte, 500)},
+		"DCIM/100ABCDE/MVI_0002.MP4": {Data: make([]byte, 9000)},
+		"DCIM/100ABCDE/MVI_0002.THM": {Data: make([]byte, 100)},
+		"DCIM/100ABCDE/DSC00003.JPG": {Data: make([]byte, 700)},
+		"DCIM/101ABCDE/DSC00001.JPG": {Data: make([]byte, 300)},
+	})
+
+	first := mustFileGroups(t, store.Directories[0], 3)
+	second := mustFileGroups(t, store.Directories[1], 1)
+
+	assertFileGroup(t, first[0], 100, 1, "DCIM/100ABCDE/DSC00001.JPG",
+		"DCIM/100ABCDE/DSC00001.ARW",
+		"DCIM/100ABCDE/DSC00001.JPG",
+	)
+	assertSizes(t, first[0], 2500, 2000, 500)
+
+	assertFileGroup(t, first[1], 100, 2, "DCIM/100ABCDE/MVI_0002.MP4",
+		"DCIM/100ABCDE/MVI_0002.MP4",
+		"DCIM/100ABCDE/MVI_0002.THM",
+	)
+	assertSizes(t, first[1], 9100, 9000, 100)
+
+	assertFileGroup(t, first[2], 100, 3, "DCIM/100ABCDE/DSC00003.JPG",
+		"DCIM/100ABCDE/DSC00003.JPG",
+	)
+	assertSizes(t, first[2], 700, 700)
+
+	assertFileGroup(t, second[0], 101, 1, "DCIM/101ABCDE/DSC00001.JPG",
+		"DCIM/101ABCDE/DSC00001.JPG",
+	)
+	assertSizes(t, second[0], 300, 300)
+
+	// The images, the videos and the totals still count each file on its own.
+	if got := store.ImagesCount(); got != 4 {
+		t.Errorf("ImagesCount() = %d, want 4", got)
+	}
+
+	if got := store.VideosCount(); got != 1 {
+		t.Errorf("VideosCount() = %d, want 1", got)
+	}
+
+	if got := store.TotalSize(); got != 12500 {
+		t.Errorf("TotalSize() = %d, want 12500", got)
+	}
+}
+
+func TestParseStoreGroupsFilesWhoseFirstFourCharactersDiffer(t *testing.T) {
+	t.Parallel()
+
+	store := mustParseStore(t, newCard(
+		"DCIM/100ABCDE/ABCD0004.TIF",
+		"DCIM/100ABCDE/THMB0004.THM",
+	))
+
+	groups := mustFileGroups(t, store.Directories[0], 1)
+
+	assertFileGroup(t, groups[0], 100, 4, "DCIM/100ABCDE/ABCD0004.TIF",
+		"DCIM/100ABCDE/ABCD0004.TIF",
+		"DCIM/100ABCDE/THMB0004.THM",
+	)
+}
+
+func TestParseStoreOrdersFileGroupsByFileNumber(t *testing.T) {
+	t.Parallel()
+
+	store := mustParseStore(t, newCard(
+		"DCIM/100ABCDE/ABCD0002.JPG",
+		"DCIM/100ABCDE/WXYZ0001.JPG",
+	))
+
+	groups := mustFileGroups(t, store.Directories[0], 2)
+
+	assertFileGroup(t, groups[0], 100, 1, "DCIM/100ABCDE/WXYZ0001.JPG",
+		"DCIM/100ABCDE/WXYZ0001.JPG",
+	)
+	assertFileGroup(t, groups[1], 100, 2, "DCIM/100ABCDE/ABCD0002.JPG",
+		"DCIM/100ABCDE/ABCD0002.JPG",
+	)
+}
+
+func TestParseStoreLeavesOtherFilesOutOfFileGroups(t *testing.T) {
+	t.Parallel()
+
+	store := mustParseStore(t, newCard(
+		"DCIM/100ABCDE/ABCD0004.ARW",
+		"DCIM/100ABCDE/ABCD0004.JPG",
+		"DCIM/100ABCDE/PQRS0004.JPG",
+		"DCIM/100ABCDE/README.TXT",
+		"DCIM/MISC/ABCD0005.JPG",
+		"PRIVATE/M4ROOT/CLIP/C0006.MP4",
+	))
+
+	if len(store.Directories) != 1 {
+		t.Fatalf("got %d directories, want 1", len(store.Directories))
+	}
+
+	groups := mustFileGroups(t, store.Directories[0], 1)
+
+	assertFileGroup(t, groups[0], 100, 4, "DCIM/100ABCDE/ABCD0004.ARW",
+		"DCIM/100ABCDE/ABCD0004.ARW",
+	)
+}
+
+func TestParseStoreGivesDirectoryWithoutDCFFilesNoFileGroups(t *testing.T) {
+	t.Parallel()
+
+	store := mustParseStore(t, newCard("DCIM/100ABCDE/README.TXT"))
+
+	mustFileGroups(t, store.Directories[0], 0)
+}
+
+func TestPrimaryMember(t *testing.T) {
+	t.Parallel()
+
+	// A JPG comes first, in either case.
+	assertPrimaryMember(t, "A0010001.JPG", "A0010001.ARW", "A0010001.JPG")
+	assertPrimaryMember(t, "A0010001.JPG", "A0010001.JPG", "A0010001.MOV")
+	assertPrimaryMember(t, "a0010001.jpg", "a0010001.arw", "a0010001.jpg")
+
+	// Then any other image.
+	assertPrimaryMember(t, "Z0010001.ARW", "A0010001.MP4", "Z0010001.ARW")
+	assertPrimaryMember(t, "Z0010001.ARW", "A0010001.WAV", "Z0010001.ARW")
+	assertPrimaryMember(t, "A0010001.TIF", "A0010001.MOV", "A0010001.TIF")
+	assertPrimaryMember(t, "Z0010001.TIF", "A0010001.WAV", "Z0010001.TIF")
+
+	// Then a video.
+	assertPrimaryMember(t, "A0010001.MP4", "A0010001.AAC", "A0010001.MP4")
+	assertPrimaryMember(t, "Z0010001.MP4", "A0010001.THM", "Z0010001.MP4")
+
+	// Then any other file except a THM, in either case.
+	assertPrimaryMember(t, "Z0010001.WAV", "A0010001.THM", "Z0010001.WAV")
+	assertPrimaryMember(t, "a0010001.wav", "a0010001.thm", "a0010001.wav")
+
+	// A THM is primary only when the group holds nothing else.
+	assertPrimaryMember(t, "A0010001.THM", "A0010001.THM")
+
+	// Among equals, the first in name order wins.
+	assertPrimaryMember(t, "A0010001.THM", "A0010001.THM", "Z0010001.THM")
+	assertPrimaryMember(t, "A0010001.WAV", "A0010001.WAV", "Z0010001.XML")
+}

pkg/dcf/helpers.go

@@ -1,21 +1,28 @@
 package dcf
 
 import (
+	"context"
 	"fmt"
 	"log/slog"
 	"os"
-	"path/filepath"
 	"slices"
 
 	"github.com/shirou/gopsutil/disk"
 )
 
-func findAllMountPoints() ([]string, error) {
+func findAllMountPoints(ctx context.Context) ([]string, error) {
 	mountpoints := []string{}
 
-	partitions, err := disk.Partitions(false) // physical devices only, so false
+	// disk.PartitionsWithContext does not check ctx on Linux.
+	err := ctx.Err()
 	if err != nil {
-		return nil, err
+		return nil, fmt.Errorf("listing the mounted filesystems: %w", err)
+	}
+
+	// physical devices only, so false
+	partitions, err := disk.PartitionsWithContext(ctx, false)
+	if err != nil {
+		return nil, fmt.Errorf("listing the mounted filesystems: %w", err)
 	}
 
 	for _, partition := range partitions {
@@ -27,46 +34,46 @@ func findAllMountPoints() ([]string, error) {
 	return mountpoints, nil
 }
 
-// findDCFMountPoints returns a list of strings of the paths of mountpoints
-// that contain a DCIM or M4ROOT directory, which is how we detect DCF stores.
-// Its only argument is an integer of how many mountpoints to return. If the
-// argument is 0, all mountpoints are returned.
-func findDCFMountPoints(requestedCount int) ([]string, error) {
-	filteredMountpoints := []string{}
-
-	mountpoints, err := findAllMountPoints()
+// findDCFMountPoints returns the paths of the mountpoints that hold a DCF store.
+func findDCFMountPoints(ctx context.Context) ([]string, error) {
+	mountpoints, err := findAllMountPoints(ctx)
 	if err != nil {
 		return nil, err
 	}
 
+	return filterDCFMountPoints(mountpoints), nil
+}
+
+// filterDCFMountPoints returns the mountpoints that hold a DCF store.
+func filterDCFMountPoints(mountpoints []string) []string {
+	filteredMountpoints := []string{}
+
 	for _, mountpoint := range mountpoints {
-		dcimPath := filepath.Join(mountpoint, "DCIM")
-		m4rootPath := filepath.Join(mountpoint, "M4ROOT")
-		shouldKeep := false
-
-		_, err = os.Stat(dcimPath)
-		if err == nil {
-			shouldKeep = true
-
-			slog.Debug(fmt.Sprintf("Found DCIM directory at %s\n", dcimPath))
+		if !isDCFMountPoint(mountpoint) {
+			continue
 		}
 
-		_, err = os.Stat(m4rootPath)
-		if err == nil {
-			shouldKeep = true
+		slog.Debug(fmt.Sprintf("Keeping mountpoint %s\n", mountpoint))
 
-			slog.Debug(fmt.Sprintf("Found M4ROOT directory at %s\n", m4rootPath))
-		}
+		filteredMountpoints = append(filteredMountpoints, mountpoint)
+	}
 
-		if shouldKeep {
-			slog.Debug(fmt.Sprintf("Keeping mountpoint %s\n", mountpoint))
+	return filteredMountpoints
+}
 
-			filteredMountpoints = append(filteredMountpoints, mountpoint)
-			if (requestedCount > 0) && (len(filteredMountpoints)+1 >= requestedCount) {
-				break
-			}
+// isDCFMountPoint reports whether the root of mountpoint holds a DCIM or
+// PRIVATE/M4ROOT directory, which is how we detect DCF stores.
+func isDCFMountPoint(mountpoint string) bool {
+	fsys := os.DirFS(mountpoint)
+
+	for _, directory := range []string{dcimDirectory, m4RootDirectory} {
+		found, err := hasDirectory(fsys, directory)
+		if err == nil && found {
+			slog.Debug(fmt.Sprintf("Found %s directory at %s\n", directory, mountpoint))
+
+			return true
 		}
 	}
 
-	return filteredMountpoints, nil
+	return false
 }

pkg/dcf/helpers_internal_test.go

@@ -0,0 +1,87 @@
+package dcf
+
+import (
+	"os"
+	"path/filepath"
+	"slices"
+	"testing"
+)
+
+// newMountPoint returns a temporary directory holding the given directories.
+func newMountPoint(t *testing.T, directories ...string) string {
+	t.Helper()
+
+	root := t.TempDir()
+
+	for _, directory := range directories {
+		err := os.MkdirAll(filepath.Join(root, directory), 0o750)
+		if err != nil {
+			t.Fatalf("MkdirAll: %v", err)
+		}
+	}
+
+	return root
+}
+
+func TestIsDCFMountPoint(t *testing.T) {
+	t.Parallel()
+
+	cases := map[string]bool{
+		"DCIM":           true,
+		"PRIVATE/M4ROOT": true,
+		"M4ROOT":         false,
+		"PRIVATE":        false,
+	}
+
+	for directory, want := range cases {
+		got := isDCFMountPoint(newMountPoint(t, directory))
+		if got != want {
+			t.Errorf("isDCFMountPoint with only %q = %v, want %v",
+				directory, got, want)
+		}
+	}
+}
+
+func TestIsDCFMountPointIgnoresPlainFile(t *testing.T) {
+	t.Parallel()
+
+	root := t.TempDir()
+
+	err := os.WriteFile(filepath.Join(root, "DCIM"), nil, 0o600)
+	if err != nil {
+		t.Fatalf("WriteFile: %v", err)
+	}
+
+	if isDCFMountPoint(root) {
+		t.Fatal("isDCFMountPoint counted a plain file named DCIM as a directory")
+	}
+}
+
+func TestIsDCFMountPointIgnoresSymlink(t *testing.T) {
+	t.Parallel()
+
+	outside := newMountPoint(t, "M4ROOT")
+
+	for _, link := range []string{dcimDirectory, privateDirectory, m4RootDirectory} {
+		root := newMountPoint(t, filepath.Dir(link))
+		addSymlink(t, root, link, outside)
+
+		if isDCFMountPoint(root) {
+			t.Errorf("isDCFMountPoint counted a symlink at %s as a directory", link)
+		}
+	}
+}
+
+func TestFilterDCFMountPointsKeepsThoseHoldingAStore(t *testing.T) {
+	t.Parallel()
+
+	first := newMountPoint(t, "DCIM")
+	second := newMountPoint(t, "PRIVATE/M4ROOT")
+
+	got := filterDCFMountPoints([]string{newMountPoint(t), first, second})
+
+	want := []string{first, second}
+	if !slices.Equal(got, want) {
+		t.Errorf("filterDCFMountPoints = %v, want %v", got, want)
+	}
+}

pkg/dcf/parse.go

@@ -0,0 +1,310 @@
+package dcf
+
+import (
+	"context"
+	"errors"
+	"fmt"
+	"io/fs"
+	"path"
+	"regexp"
+	"strconv"
+	"strings"
+)
+
+// The two directories at the root of a card that hold its files.
+const (
+	dcimDirectory   = "DCIM"
+	m4RootDirectory = "PRIVATE/M4ROOT"
+)
+
+// The naming rules of the DCF specification, CIPA DC-009-2010, sections 2.3,
+// 4.2.2 and 4.3.1. Names use A-Z, 0-9 and "_", and lower-case letters count as
+// upper-case. Each pattern captures the number in the name.
+var (
+	dcfDirectoryName = regexp.MustCompile(`^([1-9][0-9]{2})[0-9A-Za-z_]{5}$`)
+	dcfFileName      = regexp.MustCompile(`^[0-9A-Za-z_]{4}([0-9]{4})\.[0-9A-Za-z_]{3}$`)
+)
+
+// parseStore reads one card from fsys, which is rooted at the card's mount
+// point. The root argument is the path of that mount point. Only DCIM and
+// PRIVATE/M4ROOT are read. A card can lack either one. A symlink is never
+// followed. An error names the mount point, because fsys reports only paths
+// inside it. When ctx is cancelled, the read stops before the next directory.
+func parseStore(ctx context.Context, fsys fs.FS, root string) (*Store, error) {
+	store := &Store{RootDirectory: root}
+
+	err := parseDCIM(ctx, fsys, store)
+	if err == nil {
+		err = parseM4Root(ctx, fsys, store)
+	}
+
+	if err != nil {
+		return nil, fmt.Errorf("reading the card at %s: %w", root, err)
+	}
+
+	return store, nil
+}
+
+// hasDirectory reports whether the card holds the named directory. A plain file
+// or a symlink at any step of the path counts as missing. Any other failure to
+// look is an error.
+func hasDirectory(fsys fs.FS, name string) (bool, error) {
+	parent := path.Dir(name)
+	if parent != "." {
+		found, err := hasDirectory(fsys, parent)
+		if err != nil || !found {
+			return false, err
+		}
+	}
+
+	info, err := fs.Lstat(fsys, name)
+	if errors.Is(err, fs.ErrNotExist) {
+		return false, nil
+	}
+
+	if err != nil {
+		return false, err
+	}
+
+	return info.IsDir(), nil
+}
+
+// readDir returns the entries of the named directory, unless ctx is cancelled.
+func readDir(ctx context.Context, fsys fs.FS, name string) ([]fs.DirEntry, error) {
+	err := ctx.Err()
+	if err != nil {
+		return nil, err
+	}
+
+	return fs.ReadDir(fsys, name)
+}
+
+// parseDCIM adds the DCF directories under DCIM to store. Everything else there
+// is not conforming. That includes directories that share a directory number,
+// which section 7.1.2 of the specification treats as non-DCF directories.
+func parseDCIM(ctx context.Context, fsys fs.FS, store *Store) error {
+	found, err := hasDirectory(fsys, dcimDirectory)
+	if err != nil || !found {
+		return err
+	}
+
+	entries, err := readDir(ctx, fsys, dcimDirectory)
+	if err != nil {
+		return err
+	}
+
+	directoriesWithNumber := map[int]int{}
+
+	for _, entry := range entries {
+		if entry.IsDir() {
+			directoriesWithNumber[parseNumber(dcfDirectoryName, entry.Name())]++
+		}
+	}
+
+	for _, entry := range entries {
+		number := parseNumber(dcfDirectoryName, entry.Name())
+
+		if entry.IsDir() && number != 0 && directoriesWithNumber[number] == 1 {
+			err = parseDirectory(ctx, fsys, store, entry.Name(), number)
+		} else {
+			err = addNonConforming(ctx, fsys, store, dcimDirectory, entry)
+		}
+
+		if err != nil {
+			return err
+		}
+	}
+
+	return nil
+}
+
+// parseDirectory adds one DCF directory and the DCF files directly inside it to
+// store. A DCF file is a regular file, so a symlink with a DCF name is not one.
+// Everything else there is not conforming. That includes JPG files that share a
+// file number, which section 7.2.2 of the specification says must not show DCF
+// numbers. Other files that share a file number do conform.
+func parseDirectory(
+	ctx context.Context,
+	fsys fs.FS,
+	store *Store,
+	name string,
+	number int,
+) error {
+	directoryPath := path.Join(dcimDirectory, name)
+
+	entries, err := readDir(ctx, fsys, directoryPath)
+	if err != nil {
+		return err
+	}
+
+	jpgsWithNumber := map[int]int{}
+
+	for _, entry := range entries {
+		if entry.Type().IsRegular() && isJPG(entry.Name()) {
+			jpgsWithNumber[parseNumber(dcfFileName, entry.Name())]++
+		}
+	}
+
+	directory := &Directory{Number: number, Name: name}
+
+	for _, entry := range entries {
+		fileNumber := parseNumber(dcfFileName, entry.Name())
+		sharedJPG := isJPG(entry.Name()) && jpgsWithNumber[fileNumber] > 1
+
+		if !entry.Type().IsRegular() || fileNumber == 0 || sharedJPG {
+			err = addNonConforming(ctx, fsys, store, directoryPath, entry)
+			if err != nil {
+				return err
+			}
+
+			continue
+		}
+
+		entryPath := path.Join(directoryPath, entry.Name())
+
+		object, err := newObject(fsys, store, entryPath, entry)
+		if err != nil {
+			return err
+		}
+
+		object.DirectoryNumber = number
+		object.FileNumber = fileNumber
+		directory.Objects = append(directory.Objects, &object)
+
+		addMedia(store, object)
+	}
+
+	directory.FileGroups = newFileGroups(directory.Objects)
+	store.Directories = append(store.Directories, directory)
+
+	return nil
+}
+
+// parseM4Root adds the images and videos under PRIVATE/M4ROOT to store. The DCF
+// naming rules do not apply there, so these files have no DCF numbers. It skips
+// a symlink. It also skips a name that is not valid UTF-8, because fs.FS refuses
+// to open one.
+func parseM4Root(ctx context.Context, fsys fs.FS, store *Store) error {
+	found, err := hasDirectory(fsys, m4RootDirectory)
+	if err != nil || !found {
+		return err
+	}
+
+	return fs.WalkDir(fsys, m4RootDirectory,
+		func(filePath string, entry fs.DirEntry, err error) error {
+			if err != nil {
+				return err
+			}
+
+			validName := fs.ValidPath(filePath)
+
+			if entry.IsDir() && !validName {
+				return fs.SkipDir
+			}
+
+			// fs.WalkDir reads a directory right after this call for it.
+			if entry.IsDir() {
+				return ctx.Err()
+			}
+
+			if !entry.Type().IsRegular() || !validName {
+				return nil
+			}
+
+			object, err := newObject(fsys, store, filePath, entry)
+			if err != nil {
+				return err
+			}
+
+			addMedia(store, object)
+
+			return nil
+		})
+}
+
+// addNonConforming lists one entry of the directory dir on store as not
+// conforming. If the entry is a directory, it lists everything inside it too. It
+// does not follow a symlink. It does not read a directory whose name is not
+// valid UTF-8, because fs.FS refuses to open one.
+func addNonConforming(
+	ctx context.Context,
+	fsys fs.FS,
+	store *Store,
+	dir string,
+	entry fs.DirEntry,
+) error {
+	entryPath := path.Join(dir, entry.Name())
+	store.NonConforming = append(store.NonConforming, entryPath)
+
+	if !entry.IsDir() || !fs.ValidPath(entryPath) {
+		return nil
+	}
+
+	entries, err := readDir(ctx, fsys, entryPath)
+	if err != nil {
+		return err
+	}
+
+	for _, inner := range entries {
+		err = addNonConforming(ctx, fsys, store, entryPath, inner)
+		if err != nil {
+			return err
+		}
+	}
+
+	return nil
+}
+
+// newObject returns the Object for the file at filePath, with no DCF numbers.
+func newObject(
+	fsys fs.FS,
+	store *Store,
+	filePath string,
+	entry fs.DirEntry,
+) (Object, error) {
+	info, err := entry.Info()
+	if err != nil {
+		return Object{}, err
+	}
+
+	return Object{
+		DCFStoreRoot: store.RootDirectory,
+		Path:         filePath,
+		Size:         info.Size(),
+		Extension:    path.Ext(filePath),
+		fsys:         fsys,
+	}, nil
+}
+
+// addMedia adds object to the images or videos of store. A file with any other
+// extension is neither, and is not added.
+func addMedia(store *Store, object Object) {
+	if isImageFile(object.Path) {
+		store.Images = append(store.Images, &Image{Object: object})
+	}
+
+	if isVideoFile(object.Path) {
+		store.Videos = append(store.Videos, &Video{Object: object})
+	}
+}
+
+// parseNumber returns the number that pattern captures in name. It returns 0
+// when name does not match. No DCF number is 0, so this also rejects the file
+// number 0000, which the specification forbids.
+func parseNumber(pattern *regexp.Regexp, name string) int {
+	match := pattern.FindStringSubmatch(name)
+	if match == nil {
+		return 0
+	}
+
+	number, err := strconv.Atoi(match[1])
+	if err != nil {
+		return 0
+	}
+
+	return number
+}
+
+func isJPG(name string) bool {
+	return strings.EqualFold(path.Ext(name), ".JPG")
+}

pkg/dcf/parse_internal_test.go

@@ -0,0 +1,906 @@
+package dcf
+
+import (
+	"context"
+	"errors"
+	"io/fs"
+	"os"
+	"path"
+	"path/filepath"
+	"reflect"
+	"slices"
+	"testing"
+	"testing/fstest"
+)
+
+const (
+	testRoot         = "/mnt/card"
+	privateDirectory = "PRIVATE"
+)
+
+// newCard returns a card holding an empty file at each of the given paths.
+func newCard(paths ...string) fstest.MapFS {
+	card := fstest.MapFS{}
+
+	for _, filePath := range paths {
+		card[filePath] = &fstest.MapFile{}
+	}
+
+	return card
+}
+
+// newCardOnDisk returns a temporary directory holding the three bytes "abc" in a
+// file at each of the given paths. It is for tests that fstest.MapFS cannot do.
+func newCardOnDisk(t *testing.T, paths ...string) string {
+	t.Helper()
+
+	root := t.TempDir()
+
+	for _, filePath := range paths {
+		fullPath := filepath.Join(root, filePath)
+
+		err := os.MkdirAll(filepath.Dir(fullPath), 0o750)
+		if err != nil {
+			t.Fatalf("MkdirAll: %v", err)
+		}
+
+		err = os.WriteFile(fullPath, []byte("abc"), 0o600)
+		if err != nil {
+			t.Fatalf("WriteFile: %v", err)
+		}
+	}
+
+	return root
+}
+
+// addSymlink creates a symlink to target at the named path under root.
+func addSymlink(t *testing.T, root, name, target string) {
+	t.Helper()
+
+	err := os.Symlink(target, filepath.Join(root, name))
+	if err != nil {
+		t.Fatalf("Symlink: %v", err)
+	}
+}
+
+func mustParseStore(t *testing.T, fsys fs.FS) *Store {
+	t.Helper()
+
+	store, err := parseStore(t.Context(), fsys, testRoot)
+	if err != nil {
+		t.Fatalf("parseStore: %v", err)
+	}
+
+	return store
+}
+
+func imagePaths(store *Store) []string {
+	paths := make([]string, 0, len(store.Images))
+
+	for _, image := range store.Images {
+		paths = append(paths, image.Path)
+	}
+
+	return paths
+}
+
+func videoPaths(store *Store) []string {
+	paths := make([]string, 0, len(store.Videos))
+
+	for _, video := range store.Videos {
+		paths = append(paths, video.Path)
+	}
+
+	return paths
+}
+
+func objectPaths(directory *Directory) []string {
+	paths := make([]string, 0, len(directory.Objects))
+
+	for _, object := range directory.Objects {
+		paths = append(paths, object.Path)
+	}
+
+	return paths
+}
+
+func directoryNumbers(store *Store) []int {
+	numbers := make([]int, 0, len(store.Directories))
+
+	for _, directory := range store.Directories {
+		numbers = append(numbers, directory.Number)
+	}
+
+	return numbers
+}
+
+// wantDirectory returns the Directory that parseStore should build for objects.
+func wantDirectory(number int, name string, objects ...*Object) *Directory {
+	return &Directory{
+		Number:     number,
+		Name:       name,
+		Objects:    objects,
+		FileGroups: newFileGroups(objects),
+	}
+}
+
+func assertPaths(t *testing.T, what string, got []string, want ...string) {
+	t.Helper()
+
+	if !slices.Equal(got, want) {
+		t.Errorf("%s = %q, want %q", what, got, want)
+	}
+}
+
+func TestParseStorePutsEachFileWhereItBelongs(t *testing.T) {
+	t.Parallel()
+
+	card := fstest.MapFS{
+		"DCIM/100MSDCF/ABCD0001.ARW":       {Data: make([]byte, 2000)},
+		"DCIM/100MSDCF/readme.txt":         {Data: make([]byte, 7)},
+		"DCIM/101CANON/MVI_0002.MP4":       {Data: make([]byte, 9000)},
+		"DCIM/101CANON/MVI_0002.THM":       {Data: make([]byte, 100)},
+		"DCIM/MISC/DSC00003.JPG":           {Data: make([]byte, 7)},
+		"DCIM/DSC00004.JPG":                {Data: make([]byte, 7)},
+		"PRIVATE/M4ROOT/CLIP/C0002.MP4":    {Data: make([]byte, 400)},
+		"PRIVATE/M4ROOT/CLIP/C0002M01.XML": {Data: make([]byte, 7)},
+		"PRIVATE/OTHER/C0003.MP4":          {Data: make([]byte, 7)},
+		"BACKUP/DSC00005.JPG":              {Data: make([]byte, 7)},
+		"DSC00006.JPG":                     {Data: make([]byte, 7)},
+	}
+
+	store := mustParseStore(t, card)
+
+	arw := Object{
+		DCFStoreRoot:    testRoot,
+		Path:            "DCIM/100MSDCF/ABCD0001.ARW",
+		Size:            2000,
+		Extension:       ".ARW",
+		DirectoryNumber: 100,
+		FileNumber:      1,
+		fsys:            card,
+	}
+	mp4 := Object{
+		DCFStoreRoot:    testRoot,
+		Path:            "DCIM/101CANON/MVI_0002.MP4",
+		Size:            9000,
+		Extension:       ".MP4",
+		DirectoryNumber: 101,
+		FileNumber:      2,
+		fsys:            card,
+	}
+	thm := Object{
+		DCFStoreRoot:    testRoot,
+		Path:            "DCIM/101CANON/MVI_0002.THM",
+		Size:            100,
+		Extension:       ".THM",
+		DirectoryNumber: 101,
+		FileNumber:      2,
+		fsys:            card,
+	}
+	clip := Object{
+		DCFStoreRoot: testRoot,
+		Path:         "PRIVATE/M4ROOT/CLIP/C0002.MP4",
+		Size:         400,
+		Extension:    ".MP4",
+		fsys:         card,
+	}
+
+	wantDirectories := []*Directory{
+		wantDirectory(100, "100MSDCF", &arw),
+		wantDirectory(101, "101CANON", &mp4, &thm),
+	}
+	if !reflect.DeepEqual(store.Directories, wantDirectories) {
+		t.Errorf("Directories = %+v, want %+v", store.Directories, wantDirectories)
+	}
+
+	wantImages := []*Image{{Object: arw}}
+	if !reflect.DeepEqual(store.Images, wantImages) {
+		t.Errorf("Images = %+v, want %+v", store.Images, wantImages)
+	}
+
+	wantVideos := []*Video{{Object: mp4}, {Object: clip}}
+	if !reflect.DeepEqual(store.Videos, wantVideos) {
+		t.Errorf("Videos = %+v, want %+v", store.Videos, wantVideos)
+	}
+
+	assertPaths(t, "NonConforming", store.NonConforming,
+		"DCIM/100MSDCF/readme.txt",
+		"DCIM/DSC00004.JPG",
+		"DCIM/MISC",
+		"DCIM/MISC/DSC00003.JPG",
+	)
+}
+
+func TestParseStoreCountsOnlyImagesAndVideos(t *testing.T) {
+	t.Parallel()
+
+	card := fstest.MapFS{
+		"DCIM/100ABCDE/ABCD0001.ARW":    {Data: make([]byte, 2000)},
+		"DCIM/100ABCDE/ABCD0002.THM":    {Data: make([]byte, 100)},
+		"DCIM/100ABCDE/ABCD0003.MP4":    {Data: make([]byte, 9000)},
+		"DCIM/MISC/ABCD0004.JPG":        {Data: make([]byte, 50)},
+		"PRIVATE/M4ROOT/CLIP/C0004.MP4": {Data: make([]byte, 400)},
+		"ABCD0005.JPG":                  {Data: make([]byte, 30)},
+	}
+
+	store := mustParseStore(t, card)
+
+	if got := store.ImagesCount(); got != 1 {
+		t.Errorf("ImagesCount() = %d, want 1", got)
+	}
+
+	if got := store.VideosCount(); got != 2 {
+		t.Errorf("VideosCount() = %d, want 2", got)
+	}
+
+	if got := store.TotalSize(); got != 11400 {
+		t.Errorf("TotalSize() = %d, want 11400", got)
+	}
+}
+
+func TestParseStoreSetsFullFilePath(t *testing.T) {
+	t.Parallel()
+
+	store := mustParseStore(t, newCard("DCIM/100MSDCF/DSC00001.JPG"))
+
+	want := testRoot + "/DCIM/100MSDCF/DSC00001.JPG"
+	if got := store.Images[0].FullFilePath(); got != want {
+		t.Fatalf("FullFilePath() = %q, want %q", got, want)
+	}
+}
+
+func TestParseNumberOfDirectoryName(t *testing.T) {
+	t.Parallel()
+
+	cases := map[string]int{
+		"100MSDCF":    100,
+		"999VWXYZ":    999,
+		"100D3500":    100,
+		"101abcde":    101,
+		"102A_B_C":    102,
+		"099ABCDE":    0,
+		"000ABCDE":    0,
+		"103A":        0,
+		"IM104ABC":    0,
+		"105_abcdefg": 0,
+		"107@-%+]":    0,
+		"IM08ABCD":    0,
+		"100ABC-E":    0,
+		"100ABCD":     0,
+		"100ABCDEF":   0,
+		"1000ABCDE":   0,
+		"X100ABCDE":   0,
+		"TEMP":        0,
+	}
+
+	for name, want := range cases {
+		if got := parseNumber(dcfDirectoryName, name); got != want {
+			t.Errorf("directory %q parsed to %d, want %d", name, got, want)
+		}
+	}
+}
+
+func TestParseNumberOfFileName(t *testing.T) {
+	t.Parallel()
+
+	cases := map[string]int{
+		"DSC00001.ARW":    1,
+		"ABCD9999.JPG":    9999,
+		"abcd0002.jpg":    2,
+		"_ABC0001.JPG":    1,
+		"IMG_0001.JPG":    1,
+		"12340005.JPG":    5,
+		"ABCD0000.JPG":    0,
+		"A0004.JPG":       0,
+		"ABCDE0005.JPG":   0,
+		"ABCD10000.JPG":   0,
+		"abcdefg0006.JPG": 0,
+		"+-@]0007.JPG":    0,
+		"ABCD00~1.JPG":    0,
+		"ABCDEFGH.JPG":    0,
+		"ABCD0001":        0,
+		"ABCD0001.":       0,
+		"ABCD0001.JP":     0,
+		"ABCD0001.JPEG":   0,
+		"ABCD0001.J-G":    0,
+		"._DSC0001.JPG":   0,
+		".DS_Store":       0,
+	}
+
+	for name, want := range cases {
+		if got := parseNumber(dcfFileName, name); got != want {
+			t.Errorf("file %q parsed to %d, want %d", name, got, want)
+		}
+	}
+}
+
+func TestParseStoreTreatsLowerCaseNamesAsDCFNames(t *testing.T) {
+	t.Parallel()
+
+	card := newCard("DCIM/101abcde/abcd0002.jpg")
+
+	store := mustParseStore(t, card)
+
+	want := []*Directory{wantDirectory(101, "101abcde", &Object{
+		DCFStoreRoot:    testRoot,
+		Path:            "DCIM/101abcde/abcd0002.jpg",
+		Extension:       ".jpg",
+		DirectoryNumber: 101,
+		FileNumber:      2,
+		fsys:            card,
+	})}
+	if !reflect.DeepEqual(store.Directories, want) {
+		t.Errorf("Directories = %+v, want %+v", store.Directories, want)
+	}
+
+	assertPaths(t, "Images", imagePaths(store), "DCIM/101abcde/abcd0002.jpg")
+	assertPaths(t, "NonConforming", store.NonConforming)
+}
+
+func TestParseStoreAcceptsUnderscoreInNames(t *testing.T) {
+	t.Parallel()
+
+	store := mustParseStore(t, newCard("DCIM/100A_B_C/_ABC0001.JPG"))
+
+	assertPaths(t, "Images", imagePaths(store), "DCIM/100A_B_C/_ABC0001.JPG")
+	assertPaths(t, "NonConforming", store.NonConforming)
+}
+
+func TestParseStoreRejectsDirectoriesSharingANumber(t *testing.T) {
+	t.Parallel()
+
+	store := mustParseStore(t, newCard(
+		"DCIM/102ABCDE/ABCD0001.JPG",
+		"DCIM/102PQRST/ABCD0002.JPG",
+		"DCIM/103ABCDE/ABCD0003.JPG",
+	))
+
+	if got, want := directoryNumbers(store), []int{103}; !slices.Equal(got, want) {
+		t.Errorf("directory numbers = %v, want %v", got, want)
+	}
+
+	assertPaths(t, "Images", imagePaths(store), "DCIM/103ABCDE/ABCD0003.JPG")
+	assertPaths(t, "NonConforming", store.NonConforming,
+		"DCIM/102ABCDE",
+		"DCIM/102ABCDE/ABCD0001.JPG",
+		"DCIM/102PQRST",
+		"DCIM/102PQRST/ABCD0002.JPG",
+	)
+}
+
+func TestParseStoreDoesNotCountPlainFileAsSharingADirectoryNumber(t *testing.T) {
+	t.Parallel()
+
+	store := mustParseStore(t, newCard(
+		"DCIM/100ABCDE",
+		"DCIM/100PQRST/ABCD0001.JPG",
+	))
+
+	if got, want := directoryNumbers(store), []int{100}; !slices.Equal(got, want) {
+		t.Errorf("directory numbers = %v, want %v", got, want)
+	}
+
+	assertPaths(t, "NonConforming", store.NonConforming, "DCIM/100ABCDE")
+}
+
+func TestParseStoreRejectsJPGsSharingAFileNumber(t *testing.T) {
+	t.Parallel()
+
+	store := mustParseStore(t, newCard(
+		"DCIM/100ABCDE/ABCD0003.JPG",
+		"DCIM/100ABCDE/PQRS0003.jpg",
+		"DCIM/101ABCDE/ABCD0003.JPG",
+	))
+
+	assertPaths(t, "objects", objectPaths(store.Directories[0]))
+	assertPaths(t, "objects", objectPaths(store.Directories[1]),
+		"DCIM/101ABCDE/ABCD0003.JPG",
+	)
+	assertPaths(t, "Images", imagePaths(store), "DCIM/101ABCDE/ABCD0003.JPG")
+	assertPaths(t, "NonConforming", store.NonConforming,
+		"DCIM/100ABCDE/ABCD0003.JPG",
+		"DCIM/100ABCDE/PQRS0003.jpg",
+	)
+}
+
+func TestParseStoreKeepsOtherFilesSharingAFileNumber(t *testing.T) {
+	t.Parallel()
+
+	store := mustParseStore(t, newCard(
+		"DCIM/100ABCDE/DSC00004.ARW",
+		"DCIM/100ABCDE/DSC00004.JPG",
+		"DCIM/100ABCDE/MVI_0005.MP4",
+		"DCIM/100ABCDE/MVI_0005.THM",
+	))
+
+	assertPaths(t, "objects", objectPaths(store.Directories[0]),
+		"DCIM/100ABCDE/DSC00004.ARW",
+		"DCIM/100ABCDE/DSC00004.JPG",
+		"DCIM/100ABCDE/MVI_0005.MP4",
+		"DCIM/100ABCDE/MVI_0005.THM",
+	)
+	assertPaths(t, "Images", imagePaths(store),
+		"DCIM/100ABCDE/DSC00004.ARW",
+		"DCIM/100ABCDE/DSC00004.JPG",
+	)
+	assertPaths(t, "Videos", videoPaths(store), "DCIM/100ABCDE/MVI_0005.MP4")
+	assertPaths(t, "NonConforming", store.NonConforming)
+}
+
+func TestParseStoreKeepsARWSharingANumberWithTwoJPGs(t *testing.T) {
+	t.Parallel()
+
+	store := mustParseStore(t, newCard(
+		"DCIM/100ABCDE/ABCD0004.ARW",
+		"DCIM/100ABCDE/ABCD0004.JPG",
+		"DCIM/100ABCDE/PQRS0004.JPG",
+	))
+
+	assertPaths(t, "objects", objectPaths(store.Directories[0]),
+		"DCIM/100ABCDE/ABCD0004.ARW",
+	)
+	assertPaths(t, "NonConforming", store.NonConforming,
+		"DCIM/100ABCDE/ABCD0004.JPG",
+		"DCIM/100ABCDE/PQRS0004.JPG",
+	)
+}
+
+func TestParseStoreDoesNotCountDirectoryAsSharingAFileNumber(t *testing.T) {
+	t.Parallel()
+
+	store := mustParseStore(t, newCard(
+		"DCIM/100ABCDE/ABCD0003.JPG",
+		"DCIM/100ABCDE/PQRS0003.JPG/README.TXT",
+	))
+
+	assertPaths(t, "objects", objectPaths(store.Directories[0]),
+		"DCIM/100ABCDE/ABCD0003.JPG",
+	)
+	assertPaths(t, "NonConforming", store.NonConforming,
+		"DCIM/100ABCDE/PQRS0003.JPG",
+		"DCIM/100ABCDE/PQRS0003.JPG/README.TXT",
+	)
+}
+
+func TestParseStoreDoesNotCountDirectoryNamedLikeAVideo(t *testing.T) {
+	t.Parallel()
+
+	store := mustParseStore(t, newCard("PRIVATE/M4ROOT/CLIP.MP4/C0001.XML"))
+
+	assertPaths(t, "Videos", videoPaths(store))
+}
+
+func TestParseStoreRejectsSubdirectoryOfDCFDirectory(t *testing.T) {
+	t.Parallel()
+
+	store := mustParseStore(t, newCard(
+		"DCIM/100ABCDE/SUB/ABCD0001.JPG",
+		"DCIM/100ABCDE/ABCD0002.JPG/ABCD0003.JPG",
+	))
+
+	assertPaths(t, "objects", objectPaths(store.Directories[0]))
+	assertPaths(t, "Images", imagePaths(store))
+	assertPaths(t, "NonConforming", store.NonConforming,
+		"DCIM/100ABCDE/ABCD0002.JPG",
+		"DCIM/100ABCDE/ABCD0002.JPG/ABCD0003.JPG",
+		"DCIM/100ABCDE/SUB",
+		"DCIM/100ABCDE/SUB/ABCD0001.JPG",
+	)
+}
+
+func TestParseStoreListsEmptyDirectories(t *testing.T) {
+	t.Parallel()
+
+	store := mustParseStore(t, fstest.MapFS{
+		"DCIM/100EMPTY": {Mode: fs.ModeDir},
+		"DCIM/MISC":     {Mode: fs.ModeDir},
+	})
+
+	if got, want := directoryNumbers(store), []int{100}; !slices.Equal(got, want) {
+		t.Errorf("directory numbers = %v, want %v", got, want)
+	}
+
+	assertPaths(t, "NonConforming", store.NonConforming, "DCIM/MISC")
+}
+
+func TestParseStoreWithoutDCIM(t *testing.T) {
+	t.Parallel()
+
+	store := mustParseStore(t, newCard("PRIVATE/M4ROOT/CLIP/C0001.MP4"))
+
+	if len(store.Directories) != 0 {
+		t.Errorf("Directories = %+v, want none", store.Directories)
+	}
+
+	assertPaths(t, "Videos", videoPaths(store), "PRIVATE/M4ROOT/CLIP/C0001.MP4")
+}
+
+func TestParseStoreWithoutM4Root(t *testing.T) {
+	t.Parallel()
+
+	store := mustParseStore(t, newCard("DCIM/100ABCDE/ABCD0001.JPG"))
+
+	assertPaths(t, "Images", imagePaths(store), "DCIM/100ABCDE/ABCD0001.JPG")
+	assertPaths(t, "Videos", videoPaths(store))
+}
+
+func TestParseStoreTreatsPlainFilesAsMissingDirectories(t *testing.T) {
+	t.Parallel()
+
+	store := mustParseStore(t, newCard("DCIM", "PRIVATE/M4ROOT"))
+
+	assertPaths(t, "Images", imagePaths(store))
+	assertPaths(t, "Videos", videoPaths(store))
+	assertPaths(t, "NonConforming", store.NonConforming)
+}
+
+// A real filesystem reports a lookup under a plain file differently from a
+// missing path, which fstest.MapFS does not reproduce.
+func TestParseStoreReadsRealFilesystemWithPlainFileNamedPrivate(t *testing.T) {
+	t.Parallel()
+
+	root := newCardOnDisk(t, "DCIM/100ABCDE/ABCD0001.JPG", "PRIVATE")
+
+	store := mustParseStore(t, os.DirFS(root))
+
+	assertPaths(t, "Images", imagePaths(store), "DCIM/100ABCDE/ABCD0001.JPG")
+
+	if got := store.TotalImageSize(); got != 3 {
+		t.Errorf("TotalImageSize() = %d, want 3", got)
+	}
+}
+
+// A real filesystem can hold a name with the byte 0xff, which is not valid UTF-8.
+// fs.FS refuses to open such a name.
+func TestParseStoreListsNamesThatFSRefuses(t *testing.T) {
+	t.Parallel()
+
+	root := newCardOnDisk(t,
+		"DCIM/100ABCDE/ABCD0001.JPG",
+		"DCIM/100ABCDE/A\xffDIR/INNER",
+		"DCIM/100ABCDE/A\xffFILE",
+		"DCIM/A\xffDIR/INNER",
+		"DCIM/A\xffFILE",
+		"DCIM/MISC/A\xffDIR/INNER",
+		"DCIM/MISC/A\xffFILE",
+	)
+
+	store := mustParseStore(t, os.DirFS(root))
+
+	assertPaths(t, "Images", imagePaths(store), "DCIM/100ABCDE/ABCD0001.JPG")
+	assertPaths(t, "NonConforming", store.NonConforming,
+		"DCIM/100ABCDE/A\xffDIR",
+		"DCIM/100ABCDE/A\xffFILE",
+		"DCIM/A\xffDIR",
+		"DCIM/A\xffFILE",
+		"DCIM/MISC",
+		"DCIM/MISC/A\xffDIR",
+		"DCIM/MISC/A\xffFILE",
+	)
+}
+
+func TestParseStoreListsDanglingSymlinks(t *testing.T) {
+	t.Parallel()
+
+	root := newCardOnDisk(t, "DCIM/100ABCDE/ABCD0001.JPG", "DCIM/MISC/README.TXT")
+
+	addSymlink(t, root, "DCIM/100ABCDE/ABCD0002.JPG", "absent")
+	addSymlink(t, root, "DCIM/100ABCDE/LINK", "absent")
+	addSymlink(t, root, "DCIM/101ABCDE", "absent")
+	addSymlink(t, root, "DCIM/LINK", "absent")
+	addSymlink(t, root, "DCIM/MISC/LINK", "absent")
+
+	store := mustParseStore(t, os.DirFS(root))
+
+	if got, want := directoryNumbers(store), []int{100}; !slices.Equal(got, want) {
+		t.Errorf("directory numbers = %v, want %v", got, want)
+	}
+
+	assertPaths(t, "Images", imagePaths(store), "DCIM/100ABCDE/ABCD0001.JPG")
+	assertPaths(t, "NonConforming", store.NonConforming,
+		"DCIM/100ABCDE/ABCD0002.JPG",
+		"DCIM/100ABCDE/LINK",
+		"DCIM/101ABCDE",
+		"DCIM/LINK",
+		"DCIM/MISC",
+		"DCIM/MISC/LINK",
+		"DCIM/MISC/README.TXT",
+	)
+}
+
+func TestParseStoreDoesNotFollowSymlinksUnderDCIM(t *testing.T) {
+	t.Parallel()
+
+	outside := newCardOnDisk(t, "OUTSIDE.JPG", "DIRECTORY/ABCD0009.JPG")
+	outsideFile := filepath.Join(outside, "OUTSIDE.JPG")
+	outsideDirectory := filepath.Join(outside, "DIRECTORY")
+
+	root := newCardOnDisk(t,
+		"DCIM/100ABCDE/ABCD0001.JPG",
+		"DCIM/102ABCDE/ABCD0002.JPG",
+		"DCIM/MISC/README.TXT",
+	)
+
+	// These two have DCF file names. The second has the number of a real JPG,
+	// which must not make that JPG shared.
+	addSymlink(t, root, "DCIM/100ABCDE/ABCD0003.JPG", outsideFile)
+	addSymlink(t, root, "DCIM/100ABCDE/PQRS0001.JPG", outsideFile)
+
+	// These two have DCF directory names. The second has the number of a real
+	// directory, which must not make that directory shared.
+	addSymlink(t, root, "DCIM/101ABCDE", outsideDirectory)
+	addSymlink(t, root, "DCIM/102PQRST", outsideDirectory)
+
+	addSymlink(t, root, "DCIM/LINK", outsideDirectory)
+	addSymlink(t, root, "DCIM/MISC/LINK", outsideDirectory)
+
+	store := mustParseStore(t, os.DirFS(root))
+
+	if got, want := directoryNumbers(store), []int{100, 102}; !slices.Equal(got, want) {
+		t.Errorf("directory numbers = %v, want %v", got, want)
+	}
+
+	assertPaths(t, "Images", imagePaths(store),
+		"DCIM/100ABCDE/ABCD0001.JPG",
+		"DCIM/102ABCDE/ABCD0002.JPG",
+	)
+
+	if got := store.TotalImageSize(); got != 6 {
+		t.Errorf("TotalImageSize() = %d, want 6", got)
+	}
+
+	assertPaths(t, "NonConforming", store.NonConforming,
+		"DCIM/100ABCDE/ABCD0003.JPG",
+		"DCIM/100ABCDE/PQRS0001.JPG",
+		"DCIM/101ABCDE",
+		"DCIM/102PQRST",
+		"DCIM/LINK",
+		"DCIM/MISC",
+		"DCIM/MISC/LINK",
+		"DCIM/MISC/README.TXT",
+	)
+}
+
+func TestParseStoreSkipsSymlinksAndNamesThatFSRefusesUnderM4Root(t *testing.T) {
+	t.Parallel()
+
+	outside := newCardOnDisk(t, "OUTSIDE.MP4")
+
+	// The names with the byte 0xff sort before CLIP, so a walk that stops at one
+	// of them does not reach it.
+	root := newCardOnDisk(t,
+		"PRIVATE/M4ROOT/A\xffDIR/C0002.MP4",
+		"PRIVATE/M4ROOT/A\xffFILE.MP4",
+		"PRIVATE/M4ROOT/CLIP/C0001.MP4",
+	)
+
+	addSymlink(t, root, "PRIVATE/M4ROOT/CLIP/C0003.MP4",
+		filepath.Join(outside, "OUTSIDE.MP4"))
+	addSymlink(t, root, "PRIVATE/M4ROOT/CLIP/C0004.MP4", "absent")
+	addSymlink(t, root, "PRIVATE/M4ROOT/LINK", outside)
+
+	store := mustParseStore(t, os.DirFS(root))
+
+	assertPaths(t, "Videos", videoPaths(store), "PRIVATE/M4ROOT/CLIP/C0001.MP4")
+
+	if got := store.TotalVideoSize(); got != 3 {
+		t.Errorf("TotalVideoSize() = %d, want 3", got)
+	}
+}
+
+func TestParseStoreTreatsSymlinksAsMissingDirectories(t *testing.T) {
+	t.Parallel()
+
+	outside := newCardOnDisk(t,
+		"100ABCDE/ABCD0001.JPG",
+		"M4ROOT/C0001.MP4",
+		"C0002.MP4",
+	)
+
+	cases := []struct{ link, target string }{
+		{dcimDirectory, outside},
+		{dcimDirectory, dcimDirectory},
+		{privateDirectory, outside},
+		{privateDirectory, privateDirectory},
+		{m4RootDirectory, outside},
+	}
+
+	for _, testCase := range cases {
+		root := newMountPoint(t, filepath.Dir(testCase.link))
+		addSymlink(t, root, testCase.link, testCase.target)
+
+		store := mustParseStore(t, os.DirFS(root))
+
+		if !reflect.DeepEqual(store, &Store{RootDirectory: testRoot}) {
+			t.Errorf("symlink at %s to %s read as %+v, want an empty store",
+				testCase.link, testCase.target, store)
+		}
+	}
+}
+
+// failingFS is a card on which one call fails for one path, the way a failing
+// card does. The call is Lstat, ReadDir or Info.
+type failingFS struct {
+	fstest.MapFS
+
+	call string
+	name string
+}
+
+func (f failingFS) Lstat(name string) (fs.FileInfo, error) {
+	err := f.failure("Lstat", name)
+	if err != nil {
+		return nil, err
+	}
+
+	return f.MapFS.Lstat(name)
+}
+
+func (f failingFS) ReadDir(name string) ([]fs.DirEntry, error) {
+	err := f.failure("ReadDir", name)
+	if err != nil {
+		return nil, err
+	}
+
+	entries, err := f.MapFS.ReadDir(name)
+	if err != nil {
+		return nil, err
+	}
+
+	for i, entry := range entries {
+		err = f.failure("Info", path.Join(name, entry.Name()))
+		if err != nil {
+			entries[i] = failingEntry{DirEntry: entry, err: err}
+		}
+	}
+
+	return entries, nil
+}
+
+func (f failingFS) failure(call, name string) error {
+	if call == f.call && name == f.name {
+		return &fs.PathError{Op: call, Path: name, Err: fs.ErrPermission}
+	}
+
+	return nil
+}
+
+// failingEntry is a directory entry whose Info always returns err.
+type failingEntry struct {
+	fs.DirEntry
+
+	err error
+}
+
+func (e failingEntry) Info() (fs.FileInfo, error) {
+	return nil, e.err
+}
+
+func TestParseStoreReturnsReadErrorsNamingTheCard(t *testing.T) {
+	t.Parallel()
+
+	card := newCard(
+		"DCIM/100ABCDE/ABCD0001.JPG",
+		"DCIM/100ABCDE/SUB/README.TXT",
+		"DCIM/MISC/SUB/README.TXT",
+		"PRIVATE/M4ROOT/CLIP/C0001.MP4",
+	)
+
+	cases := map[string][]string{
+		"Lstat": {dcimDirectory, privateDirectory, m4RootDirectory},
+		"ReadDir": {
+			dcimDirectory,
+			"DCIM/100ABCDE",
+			"DCIM/100ABCDE/SUB",
+			"DCIM/MISC",
+			"DCIM/MISC/SUB",
+			m4RootDirectory,
+			"PRIVATE/M4ROOT/CLIP",
+		},
+		"Info": {"DCIM/100ABCDE/ABCD0001.JPG", "PRIVATE/M4ROOT/CLIP/C0001.MP4"},
+	}
+
+	for call, names := range cases {
+		for _, name := range names {
+			fsys := failingFS{MapFS: card, call: call, name: name}
+
+			_, err := parseStore(t.Context(), fsys, testRoot)
+			if !errors.Is(err, fs.ErrPermission) {
+				t.Errorf("%s of %s: error = %v, want %v",
+					call, name, err, fs.ErrPermission)
+
+				continue
+			}
+
+			want := "reading the card at " + testRoot + ": " +
+				call + " " + name + ": permission denied"
+			if err.Error() != want {
+				t.Errorf("error = %q, want %q", err.Error(), want)
+			}
+		}
+	}
+}
+
+// cancelingFS is a card that cancels the read when the read reaches the named
+// directory. It records every directory read in read.
+type cancelingFS struct {
+	fstest.MapFS
+
+	name   string
+	cancel context.CancelFunc
+	read   *[]string
+}
+
+func (f cancelingFS) ReadDir(name string) ([]fs.DirEntry, error) {
+	*f.read = append(*f.read, name)
+
+	if name == f.name {
+		f.cancel()
+	}
+
+	return f.MapFS.ReadDir(name)
+}
+
+func TestParseStoreStopsBeforeTheNextDirectoryWhenCancelled(t *testing.T) {
+	t.Parallel()
+
+	card := newCard(
+		"DCIM/100ABCDE/SUB/README.TXT",
+		"DCIM/OTHER/SUB/README.TXT",
+		"PRIVATE/M4ROOT/CLIP/SUB/C0001.MP4",
+	)
+
+	// Each of these directories holds another directory, which must not be read.
+	names := []string{
+		dcimDirectory,
+		"DCIM/100ABCDE",
+		"DCIM/OTHER",
+		m4RootDirectory,
+		"PRIVATE/M4ROOT/CLIP",
+	}
+
+	for _, name := range names {
+		ctx, cancel := context.WithCancel(t.Context())
+
+		var read []string
+
+		fsys := cancelingFS{MapFS: card, name: name, cancel: cancel, read: &read}
+
+		_, err := parseStore(ctx, fsys, testRoot)
+		if !errors.Is(err, context.Canceled) {
+			t.Errorf("cancelled at %s: error = %v, want %v",
+				name, err, context.Canceled)
+		}
+
+		if read[len(read)-1] != name {
+			t.Errorf("cancelled at %s: read %q", name, read)
+		}
+	}
+}
+
+func TestParseStoreReadsNoDirectoryWhenAlreadyCancelled(t *testing.T) {
+	t.Parallel()
+
+	ctx, cancel := context.WithCancel(t.Context())
+	cancel()
+
+	cards := []fstest.MapFS{
+		newCard("DCIM/README.TXT"),
+		newCard("PRIVATE/M4ROOT/C0001.MP4"),
+	}
+
+	for _, card := range cards {
+		var read []string
+
+		// The context is already cancelled, so this needs no name to cancel at.
+		fsys := cancelingFS{MapFS: card, read: &read}
+
+		_, err := parseStore(ctx, fsys, testRoot)
+		if !errors.Is(err, context.Canceled) {
+			t.Errorf("error = %v, want %v", err, context.Canceled)
+		}
+
+		assertPaths(t, "directories read", read)
+	}
+}