state_file: save only if something has changed

NEWS

@@ -36,6 +36,7 @@ ver 0.16 (20??/??/??)
 * renamed option "--stdout" to "--stderr"
 * removed options --create-db and --no-create-db
 * automatically update the database with Linux inotify
+* state_file: save only if something has changed
 * obey $(sysconfdir) for default mpd.conf location
 
 

src/output_command.c

@@ -31,6 +31,8 @@
 #include "mixer_control.h"
 #include "idle.h"
 
+extern unsigned audio_output_state_version;
+
 bool
 audio_output_enable_index(unsigned idx)
 {
@@ -44,6 +46,8 @@ audio_output_enable_index(unsigned idx)
 	ao->enabled = true;
 	idle_add(IDLE_OUTPUT);
 
+	++audio_output_state_version;
+
 	return true;
 }
 
@@ -67,5 +71,7 @@ audio_output_disable_index(unsigned idx)
 		idle_add(IDLE_MIXER);
 	}
 
+	++audio_output_state_version;
+
 	return true;
 }

src/output_state.c

@@ -34,6 +34,8 @@
 
 #define AUDIO_DEVICE_STATE "audio_device_state:"
 
+unsigned audio_output_state_version;
+
 void
 audio_output_state_save(FILE *fp)
 {
@@ -80,3 +82,9 @@ audio_output_state_read(const char *line)
 	ao->enabled = false;
 	return true;
 }
+
+unsigned
+audio_output_state_get_version(void)
+{
+	return audio_output_state_version;
+}

src/output_state.h

@@ -34,4 +34,12 @@ audio_output_state_read(const char *line);
 void
 audio_output_state_save(FILE *fp);
 
+/**
+ * Generates a version number for the current state of the audio
+ * outputs.  This is used by timer_save_state_file() to determine
+ * whether the state has changed and the state file should be saved.
+ */
+unsigned
+audio_output_state_get_version(void);
+
 #endif

src/playlist_state.c

@@ -200,3 +200,21 @@ playlist_state_restore(const char *line, FILE *fp, struct playlist *playlist)
 
 	return true;
 }
+
+unsigned
+playlist_state_get_hash(const struct playlist *playlist)
+{
+	return playlist->queue.version ^
+		(getPlayerElapsedTime() << 8) ^
+		(playlist->current >= 0
+		 ? (queue_order_to_position(&playlist->queue,
+					    playlist->current) << 16)
+		 : 0) ^
+		((int)getPlayerCrossFade() << 20) ^
+		(getPlayerState() << 24) ^
+		(playlist->queue.random << 27) ^
+		(playlist->queue.repeat << 28) ^
+		(playlist->queue.single << 29) ^
+		(playlist->queue.consume << 30) ^
+		(playlist->queue.random << 31);
+}

src/playlist_state.h

@@ -36,4 +36,13 @@ playlist_state_save(FILE *fp, const struct playlist *playlist);
 bool
 playlist_state_restore(const char *line, FILE *fp, struct playlist *playlist);
 
+/**
+ * Generates a hash number for the current state of the playlist and
+ * the playback options.  This is used by timer_save_state_file() to
+ * determine whether the state has changed and the state file should
+ * be saved.
+ */
+unsigned
+playlist_state_get_hash(const struct playlist *playlist);
+
 #endif

src/state_file.c

@@ -36,6 +36,13 @@ static char *state_file_path;
 /** the GLib source id for the save timer */
 static guint save_state_source_id;
 
+/**
+ * These version numbers determine whether we need to save the state
+ * file.  If nothing has changed, we won't let the hard drive spin up.
+ */
+static unsigned prev_volume_version, prev_output_version,
+	prev_playlist_version;
+
 static void
 state_file_write(void)
 {
@@ -57,6 +64,10 @@ state_file_write(void)
 	playlist_state_save(fp, &g_playlist);
 
 	while(fclose(fp) && errno == EINTR) /* nothing */;
+
+	prev_volume_version = sw_volume_state_get_hash();
+	prev_output_version = audio_output_state_get_version();
+	prev_playlist_version = playlist_state_get_hash(&g_playlist);
 }
 
 static void
@@ -88,6 +99,10 @@ state_file_read(void)
 	}
 
 	while(fclose(fp) && errno == EINTR) /* nothing */;
+
+	prev_volume_version = sw_volume_state_get_hash();
+	prev_output_version = audio_output_state_get_version();
+	prev_playlist_version = playlist_state_get_hash(&g_playlist);
 }
 
 /**
@@ -97,6 +112,13 @@ state_file_read(void)
 static gboolean
 timer_save_state_file(G_GNUC_UNUSED gpointer data)
 {
+	if (prev_volume_version == sw_volume_state_get_hash() &&
+	    prev_output_version == audio_output_state_get_version() &&
+	    prev_playlist_version == playlist_state_get_hash(&g_playlist))
+		/* nothing has changed - don't save the state file,
+		   don't spin up the hard disk */
+		return true;
+
 	state_file_write();
 	return true;
 }

src/volume.c

@@ -122,3 +122,9 @@ void save_sw_volume_state(FILE *fp)
 {
 	fprintf(fp, SW_VOLUME_STATE "%u\n", volume_software_set);
 }
+
+unsigned
+sw_volume_state_get_hash(void)
+{
+	return volume_software_set;
+}

src/volume.h

@@ -36,4 +36,13 @@ read_sw_volume_state(const char *line);
 
 void save_sw_volume_state(FILE *fp);
 
+/**
+ * Generates a hash number for the current state of the software
+ * volume control.  This is used by timer_save_state_file() to
+ * determine whether the state has changed and the state file should
+ * be saved.
+ */
+unsigned
+sw_volume_state_get_hash(void);
+
 #endif