command: allow changing replay gain mode on-the-fly

The new command "replay_gain_mode" allows the user to switch the
replay gain mode on-the-fly.  No more mpd.conf editing.

src/command.c

@@ -44,6 +44,7 @@
 #include "client.h"
 #include "tag_print.h"
 #include "path.h"
+#include "replay_gain.h"
 #include "idle.h"
 #include "config.h"
 
@@ -1521,6 +1522,28 @@ handle_listplaylists(struct client *client,
 	return COMMAND_RETURN_OK;
 }
 
+static enum command_return
+handle_replay_gain_mode(struct client *client,
+			G_GNUC_UNUSED int argc, char *argv[])
+{
+	if (!replay_gain_set_mode_string(argv[1])) {
+		command_error(client, ACK_ERROR_ARG,
+			      "Unrecognized replay gain mode");
+		return COMMAND_RETURN_ERROR;
+	}
+
+	return COMMAND_RETURN_OK;
+}
+
+static enum command_return
+handle_replay_gain_status(struct client *client,
+			  G_GNUC_UNUSED int argc, G_GNUC_UNUSED char *argv[])
+{
+	client_printf(client, "replay_gain_mode: %s\n",
+		      replay_gain_get_mode_string());
+	return COMMAND_RETURN_OK;
+}
+
 static enum command_return
 handle_idle(struct client *client,
 	    G_GNUC_UNUSED int argc, G_GNUC_UNUSED char *argv[])
@@ -1765,6 +1788,10 @@ static const struct command commands[] = {
 	{ "random", PERMISSION_CONTROL, 1, 1, handle_random },
 	{ "rename", PERMISSION_CONTROL, 2, 2, handle_rename },
 	{ "repeat", PERMISSION_CONTROL, 1, 1, handle_repeat },
+	{ "replay_gain_mode", PERMISSION_CONTROL, 1, 1,
+	  handle_replay_gain_mode },
+	{ "replay_gain_status", PERMISSION_READ, 0, 0,
+	  handle_replay_gain_status },
 	{ "rescan", PERMISSION_ADMIN, 0, 1, handle_rescan },
 	{ "rm", PERMISSION_CONTROL, 1, 1, handle_rm },
 	{ "save", PERMISSION_CONTROL, 1, 1, handle_save },

src/replay_gain.c

@@ -42,7 +42,26 @@ enum replay_gain_mode replay_gain_mode = REPLAY_GAIN_OFF;
 static float replay_gain_preamp = 1.0;
 static float replay_gain_missing_preamp = 1.0;
 
-static bool
+const char *
+replay_gain_get_mode_string(void)
+{
+	switch (replay_gain_mode) {
+	case REPLAY_GAIN_OFF:
+		return "off";
+
+	case REPLAY_GAIN_TRACK:
+		return "track";
+
+	case REPLAY_GAIN_ALBUM:
+		return "album";
+	}
+
+	/* unreachable */
+	assert(false);
+	return "off";
+}
+
+bool
 replay_gain_set_mode_string(const char *p)
 {
 	assert(p != NULL);

src/replay_gain.h

@@ -23,6 +23,8 @@
 #ifndef MPD_REPLAY_GAIN_H
 #define MPD_REPLAY_GAIN_H
 
+#include <stdbool.h>
+
 enum replay_gain_mode {
 	REPLAY_GAIN_OFF = -1,
 	REPLAY_GAIN_ALBUM,
@@ -52,6 +54,20 @@ void replay_gain_info_free(struct replay_gain_info *info);
 
 void replay_gain_global_init(void);
 
+/**
+ * Returns the current replay gain mode as a machine-readable string.
+ */
+const char *
+replay_gain_get_mode_string(void);
+
+/**
+ * Sets the replay gain mode, parsed from a string.
+ *
+ * @return true on success, false if the string could not be parsed
+ */
+bool
+replay_gain_set_mode_string(const char *p);
+
 void
 replay_gain_apply(struct replay_gain_info *info, char *buffer, int bufferSize,
 		  const struct audio_format *format);