Flag slave-only songs whose album or artist the master has

Scans now mark slave-only songs as missing from the master when the master
has their album folder or at least their artist folder. A new Missing from
master filter in Review lists them, split by album and artist.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/engine/missing.go

@@ -0,0 +1,91 @@
+package engine
+
+import (
+	"fmt"
+	"path/filepath"
+	"strings"
+)
+
+// A slave-only song is "missing from the master" when the master has its
+// album folder (the song was probably never added there) or at least its
+// artist folder (the album is missing). Folders are compared with disc
+// folders ignored and release labels such as "Deluxe Edition" stripped, so
+// "Robot Face (Deluxe)" in the slave finds "Robot Face" in the master.
+
+// Missing levels.
+const (
+	MissingAlbum  = "album"
+	MissingArtist = "artist"
+)
+
+// folderNorm is a folder name reduced to letters and digits, release labels removed.
+func folderNorm(name string) string {
+	if w := words(stripRelease(name)); len(w) > 0 {
+		return strings.Join(w, "")
+	}
+	return strings.ToLower(name)
+}
+
+// folderKeys returns the normalized artist key and album key of a song's
+// folders; the album key is empty for a song directly in the artist folder.
+func folderKeys(rel string) (artist, album string) {
+	var parts []string
+	for _, p := range strings.Split(filepath.Dir(rel), string(filepath.Separator)) {
+		if p != "." && !discFolder.MatchString(p) {
+			parts = append(parts, folderNorm(p))
+		}
+	}
+	if len(parts) == 0 {
+		return "", ""
+	}
+	if len(parts) > 1 {
+		album = strings.Join(parts, "/")
+	}
+	return parts[0], album
+}
+
+// masterFolders indexes the master's artist and album folders by key, to
+// their real spelling.
+type masterFolders struct {
+	artists map[string]string
+	albums  map[string]string
+}
+
+func buildMasterFolders(rels []string) masterFolders {
+	mf := masterFolders{artists: map[string]string{}, albums: map[string]string{}}
+	for _, rel := range rels {
+		artist, album := folderKeys(rel)
+		if artist == "" {
+			continue
+		}
+		if _, ok := mf.artists[artist]; !ok {
+			mf.artists[artist] = firstComponent(rel)
+		}
+		if album != "" {
+			if _, ok := mf.albums[album]; !ok {
+				d := filepath.Dir(rel)
+				for d != "." && discFolder.MatchString(filepath.Base(d)) {
+					d = filepath.Dir(d)
+				}
+				mf.albums[album] = d
+			}
+		}
+	}
+	return mf
+}
+
+// missing reports whether the master has a slave-only song's album or
+// artist, with a reason naming the master's folder.
+func (mf masterFolders) missing(relSlave string) (level, reason string) {
+	artist, album := folderKeys(relSlave)
+	if artist == "" {
+		return "", ""
+	}
+	if d, ok := mf.albums[album]; ok && album != "" {
+		return MissingAlbum, fmt.Sprintf("the master has the album %q but not this song", d)
+	}
+	if d, ok := mf.artists[artist]; ok {
+		return MissingArtist, fmt.Sprintf("the master has the artist %q but not this album", d)
+	}
+	return "", ""
+}