command/Database: add "sort" parameter to "find" and "search"

Implement the second part of https://bugs.musicpd.org/view.php?id=3990

src/tag/Tag.cxx

@@ -96,3 +96,68 @@ Tag::HasType(TagType type) const
 {
 	return GetValue(type) != nullptr;
 }
+
+static TagType
+DecaySort(TagType type)
+{
+	switch (type) {
+	case TAG_ARTIST_SORT:
+		return TAG_ARTIST;
+
+	case TAG_ALBUM_SORT:
+		return TAG_ALBUM;
+
+	case TAG_ALBUM_ARTIST_SORT:
+		return TAG_ALBUM_ARTIST;
+
+	default:
+		return TAG_NUM_OF_ITEM_TYPES;
+	}
+}
+
+static TagType
+Fallback(TagType type)
+{
+	switch (type) {
+	case TAG_ALBUM_ARTIST:
+		return TAG_ARTIST;
+
+	case TAG_MUSICBRAINZ_ALBUMARTISTID:
+		return TAG_MUSICBRAINZ_ARTISTID;
+
+	default:
+		return TAG_NUM_OF_ITEM_TYPES;
+	}
+}
+
+const char *
+Tag::GetSortValue(TagType type) const
+{
+	const char *value = GetValue(type);
+	if (value != nullptr)
+		return value;
+
+	/* try without *_SORT */
+	const auto no_sort_type = DecaySort(type);
+	if (no_sort_type != TAG_NUM_OF_ITEM_TYPES) {
+		value = GetValue(no_sort_type);
+		if (value != nullptr)
+			return value;
+	}
+
+	/* fall back from TAG_ALBUM_ARTIST to TAG_ALBUM */
+
+	type = Fallback(type);
+	if (type != TAG_NUM_OF_ITEM_TYPES)
+		return GetSortValue(type);
+
+	if (no_sort_type != TAG_NUM_OF_ITEM_TYPES) {
+		type = Fallback(no_sort_type);
+		if (type != TAG_NUM_OF_ITEM_TYPES)
+			return GetSortValue(type);
+	}
+
+	/* finally fall back to empty string */
+
+	return "";
+}

src/tag/Tag.hxx

@@ -142,6 +142,15 @@ struct Tag {
 	gcc_pure
 	bool HasType(TagType type) const;
 
+	/**
+	 * Returns a value for sorting on the specified type, with
+	 * automatic fallbacks to the next best tag type
+	 * (e.g. #TAG_ALBUM_ARTIST falls back to #TAG_ARTIST).  If
+	 * there is no such value, returns an empty string.
+	 */
+	gcc_pure
+	const char *GetSortValue(TagType type) const;
+
 	class const_iterator {
 		friend struct Tag;
 		const TagItem *const*cursor;