stats: added num_artists, num_albums

Don't recalculate the number of artists and albums each time a client
requests statistics.  Calculate that once in stats_update().

src/stats.c

@@ -33,12 +33,6 @@ void stats_global_init(void)
 	stats.start_time = time(NULL);
 }
 
-void stats_update(void)
-{
-	stats.song_count = countSongsIn(NULL);
-	stats.song_duration = sumSongTimesIn(NULL);
-}
-
 struct visit_data {
 	enum tag_type type;
 	struct strset *set;
@@ -78,6 +72,14 @@ getNumberOfTagItems(enum tag_type type)
 	return ret;
 }
 
+void stats_update(void)
+{
+	stats.song_count = countSongsIn(NULL);
+	stats.song_duration = sumSongTimesIn(NULL);
+	stats.artist_count = getNumberOfTagItems(TAG_ITEM_ARTIST);
+	stats.album_count = getNumberOfTagItems(TAG_ITEM_ALBUM);
+}
+
 int stats_print(struct client *client)
 {
 	client_printf(client,
@@ -88,8 +90,8 @@ int stats_print(struct client *client)
 		      "playtime: %li\n"
 		      "db_playtime: %li\n"
 		      "db_update: %li\n",
-		      getNumberOfTagItems(TAG_ITEM_ARTIST),
+		      stats.artist_count,
-		      getNumberOfTagItems(TAG_ITEM_ALBUM),
+		      stats.album_count,
 		      stats.song_count,
 		      time(NULL) - stats.start_time,
 		      (long)(getPlayerTotalPlayTime() + 0.5),

src/stats.h

@@ -30,6 +30,12 @@ struct stats {
 	/** sum of all song durations in the music directory (in
 	    seconds) */
 	unsigned long song_duration;
+
+	/** number of distinct artist names in the music directory */
+	unsigned artist_count;
+
+	/** number of distinct album names in the music directory */
+	unsigned album_count;
 };
 
 extern struct stats stats;