Replaygain workaround for internet radios

[vpatkov]

Different internet radios have different volumes, that requires manual volume adjustment when stations are switched. I suggest use a URL fragment like #gain=-5 to normalize volumes for internet radios, applied if replaygain setting is not off.

[vpatkov]

Or maybe allow to set arbitrary tags in URL fragment, separated by &. This not only provide a way to set ReplayGain tags, but also the stream name (for those streams that doesn't provide it), genre, and other tags.

[vpatkov]

Here's quick and dirty patch, for gain only.

--- a/src/decoder/Thread.cxx
+++ b/src/decoder/Thread.cxx
@@ -21,8 +21,10 @@
 #include "util/Domain.hxx"
 #include "util/ScopeExit.hxx"
 #include "util/StringCompare.hxx"
+#include "util/UriQueryParser.hxx"
 #include "thread/Name.hxx"
 #include "tag/ApeReplayGain.hxx"
+#include "tag/ReplayGainParser.hxx"
 #include "Log.hxx"
 
 #include <stdexcept>
@@ -277,6 +279,19 @@
 	ReplayGainInfo info;
 	if (replay_gain_ape_read(is, info))
 		client.SubmitReplayGain(&info);
+
+	const char *fragment = uri_get_fragment(is.GetURI());
+	if (fragment != nullptr) {
+		const auto gain = UriFindRawQueryParameter(fragment, "gain");
+		if (gain.data() != nullptr) {
+			if (ParseReplayGainTag(info, "replaygain_track_gain",
+					std::string(gain).c_str())) {
+				info.album.gain = info.track.gain;
+				info.album.peak = info.track.peak = 0.0;
+				client.SubmitReplayGain(&info);
+			}
+		}
+	}
 }
 
 /**