opus: move EbuR128 -> ReplayGain to OpusTags

Logically, the offset belongs to the conversion of Opus Ebu R128 metadata
to the internal ReplayGain structures.

Movin the code does not change change the logic at all (as addition is
commutative), but by moving the conversion closer to the source of the
data and avoiding EBU R128-scaled data as far as possible, I hope to
reduce the chance of future logic errors.

Author: horazont

src/decoder/plugins/OpusDecoderPlugin.cxx

@@ -47,17 +47,6 @@ IsOpusTags(const ogg_packet &packet) noexcept
 	return packet.bytes >= 8 && memcmp(packet.packet, "OpusTags", 8) == 0;
 }
 
-/**
- * Convert an EBU R128 value to ReplayGain.
- */
-static constexpr float
-EbuR128ToReplayGain(float ebu_r128) noexcept
-{
-	/* add 5dB to compensate for the different reference levels
-	   between ReplayGain (89dB) and EBU R128 (-23 LUFS) */
-	return ebu_r128 + 5;
-}
-
 static bool
 mpd_opus_init([[maybe_unused]] const ConfigBlock &block)
 {
@@ -282,11 +271,12 @@ MPDOpusDecoder::HandleTags(const ogg_packet &packet)
 
 	if (rgi.IsDefined()) {
 		/* submit all valid EBU R128 values with output_gain
-		   applied */
+		   applied. conversion from EBU R128 -> ReplayGain
+		   happened in ScanOpusTags already. */
 		if (rgi.track.IsDefined())
-			rgi.track.gain += EbuR128ToReplayGain(output_gain);
+			rgi.track.gain += output_gain;
 		if (rgi.album.IsDefined())
-			rgi.album.gain += EbuR128ToReplayGain(output_gain);
+			rgi.album.gain += output_gain;
 		client.SubmitReplayGain(&rgi);
 		submitted_replay_gain = true;
 	}

src/decoder/plugins/OpusTags.cxx

@@ -17,6 +17,17 @@
 
 using std::string_view_literals::operator""sv;
 
+/**
+ * Convert an EBU R128 value to ReplayGain.
+ */
+static constexpr float
+EbuR128ToReplayGain(float ebu_r128) noexcept
+{
+	/* add 5dB to compensate for the different reference levels
+	   between ReplayGain (89dB) and EBU R128 (-23 LUFS) */
+	return ebu_r128 + 5;
+}
+
 [[gnu::pure]]
 static TagType
 ParseOpusTagName(std::string_view name) noexcept
@@ -47,14 +58,14 @@ ScanOneOpusTag(std::string_view name, std::string_view value,
 		   dB */
 
 		if (const auto i = ParseInteger<int_least32_t>(value))
-			rgi->track.gain = float(*i) / 256.0f;
+			rgi->track.gain = EbuR128ToReplayGain(float(*i) / 256.0f);
 	} else if (rgi != nullptr &&
 		   StringIsEqualIgnoreCase(name, "R128_ALBUM_GAIN"sv)) {
 		/* R128_ALBUM_GAIN is a Q7.8 fixed point number in
 		   dB */
 
 		if (const auto i = ParseInteger<int_least32_t>(value))
-			rgi->album.gain = float(*i) / 256.0f;
+			rgi->album.gain = EbuR128ToReplayGain(float(*i) / 256.0f);
 	}
 
 	handler.OnPair(name, value);