Extract thumbnail path helpers into internal/thumb

The scanner (thumbnailForVideo, thumbnailForImage), the service importer
(generateThumbnail), and writeService.RegenerateThumbnail all reinvented
the same ".thumbnails/<base>.jpg" layout inline. Centralise the
convention in internal/thumb/path.go (ThumbnailDir, ThumbnailNameFor,
ThumbnailPathFor + DirName) so changing the on-disk layout is a one-line
change and the three callers cannot drift apart. Pure path math, no I/O —
callers retain their existing MkdirAll mechanism (scanner FS, os.MkdirAll
in service) for testability.

Covered by table-driven unit tests for trailing slashes, missing
extensions, dotfiles, and multi-dot filenames.

2 files changed, 117 insertions, 0 deletions

diff --git a/player-server/internal/thumb/path.go b/player-server/internal/thumb/path.go
new file mode 100644
index 0000000..66f3145
--- /dev/null
+++ b/player-server/internal/thumb/path.go
@@ -0,0 +1,38 @@
+package thumb
+
+import (
+	"path/filepath"
+	"strings"
+)
+
+// DirName is the on-disk directory name used to store generated thumbnails.
+// It is a hidden subdirectory placed alongside the source media files.
+const DirName = ".thumbnails"
+
+// thumbExt is the canonical extension used for all generated thumbnails.
+// Thumbnails are always JPEGs regardless of the source media type.
+const thumbExt = ".jpg"
+
+// ThumbnailDir returns the conventional thumbnail directory inside parent.
+// The directory is a hidden ".thumbnails" subfolder sitting next to the
+// source media. filepath.Join handles any trailing slashes on parent.
+func ThumbnailDir(parent string) string {
+	return filepath.Join(parent, DirName)
+}
+
+// ThumbnailNameFor returns the canonical thumbnail filename for the given
+// source file (basename without extension + ".jpg"). For dotfiles (e.g.
+// ".bashrc") filepath.Ext returns the leading-dot name itself; TrimSuffix
+// then yields an empty base, producing just ".jpg" — matching the historical
+// behaviour of the inlined helpers being replaced.
+func ThumbnailNameFor(srcPath string) string {
+	base := filepath.Base(srcPath)
+	return strings.TrimSuffix(base, filepath.Ext(base)) + thumbExt
+}
+
+// ThumbnailPathFor returns the full path to the thumbnail for srcPath stored
+// under parent/.thumbnails/. parent is the directory that should contain the
+// thumbnail folder (typically the set directory or the source's parent dir).
+func ThumbnailPathFor(srcPath, parent string) string {
+	return filepath.Join(ThumbnailDir(parent), ThumbnailNameFor(srcPath))
+}
diff --git a/player-server/internal/thumb/path_test.go b/player-server/internal/thumb/path_test.go
new file mode 100644
index 0000000..c7f45f0
--- /dev/null
+++ b/player-server/internal/thumb/path_test.go
@@ -0,0 +1,79 @@
+package thumb
+
+import "testing"
+
+func TestThumbnailDir(t *testing.T) {
+	tests := []struct {
+		name   string
+		parent string
+		want   string
+	}{
+		{"simple", "/media/set1", "/media/set1/.thumbnails"},
+		{"trailing slash", "/media/set1/", "/media/set1/.thumbnails"},
+		{"double trailing slash", "/media/set1//", "/media/set1/.thumbnails"},
+		{"relative", "set1", "set1/.thumbnails"},
+		{"empty parent", "", ".thumbnails"},
+		{"root", "/", "/.thumbnails"},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			if got := ThumbnailDir(tt.parent); got != tt.want {
+				t.Errorf("ThumbnailDir(%q) = %q, want %q", tt.parent, got, tt.want)
+			}
+		})
+	}
+}
+
+func TestThumbnailNameFor(t *testing.T) {
+	tests := []struct {
+		name    string
+		srcPath string
+		want    string
+	}{
+		{"video mp4", "/media/set1/clip.mp4", "clip.jpg"},
+		{"image jpg", "/media/set1/photo.JPG", "photo.jpg"},
+		{"audio with cover", "/media/set1/song.mp3", "song.jpg"},
+		{"no extension", "/media/set1/README", "README.jpg"},
+		{"multi-dot filename", "/media/set1/my.movie.final.mkv", "my.movie.final.jpg"},
+		{"dotfile", "/media/set1/.bashrc", ".jpg"},
+		{"just basename", "clip.mp4", "clip.jpg"},
+		{"path with trailing slash", "/media/set1/clip.mp4/", "clip.jpg"},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			if got := ThumbnailNameFor(tt.srcPath); got != tt.want {
+				t.Errorf("ThumbnailNameFor(%q) = %q, want %q", tt.srcPath, got, tt.want)
+			}
+		})
+	}
+}
+
+func TestThumbnailPathFor(t *testing.T) {
+	tests := []struct {
+		name    string
+		srcPath string
+		parent  string
+		want    string
+	}{
+		{"video", "/media/set1/clip.mp4", "/media/set1", "/media/set1/.thumbnails/clip.jpg"},
+		{"image trailing slash parent", "/media/set1/photo.jpg", "/media/set1/", "/media/set1/.thumbnails/photo.jpg"},
+		{"src elsewhere", "/uploads/raw/photo.png", "/media/set1", "/media/set1/.thumbnails/photo.jpg"},
+		{"relative", "clip.mp4", "set1", "set1/.thumbnails/clip.jpg"},
+		{"no extension", "/media/set1/RAW", "/media/set1", "/media/set1/.thumbnails/RAW.jpg"},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			if got := ThumbnailPathFor(tt.srcPath, tt.parent); got != tt.want {
+				t.Errorf("ThumbnailPathFor(%q, %q) = %q, want %q", tt.srcPath, tt.parent, got, tt.want)
+			}
+		})
+	}
+}
+
+// TestDirNameConstant guards the on-disk convention; many tests in
+// internal/service compare against the literal ".thumbnails" name.
+func TestDirNameConstant(t *testing.T) {
+	if DirName != ".thumbnails" {
+		t.Errorf("DirName = %q, want %q (changing this breaks on-disk layout)", DirName, ".thumbnails")
+	}
+}