fix(DmSynth): implement stop-gap solution to fix audio flicker when s…

…witching bands

This only really works for Gothic 1 and doesn't actually fix the root cause which is, that newly allocated TSFs will produce PCM that is drastically different from what was produced by the previous one, causing audio flicker.

In the future, a solution might be to fade out the audio to zero before the transition happens.

src/Band.c

@@ -125,6 +125,28 @@ DmResult DmBand_download(DmBand* slf, DmLoader* loader) {
 	return rv;
 }
 
+bool DmBand_isSortOfSameAs(DmBand* slf, DmBand* oth) {
+	if (slf == oth) {
+		return true;
+	}
+
+	if (slf == NULL || oth == NULL) {
+		return false;
+	}
+
+	if (slf->instrument_count != oth->instrument_count) {
+		return false;
+	}
+
+	for (size_t i = 0; i < oth->instrument_count; ++i) {
+		if (slf->instruments[i].dls != oth->instruments[i].dls) {
+			return false;
+		}
+	}
+
+	return true;
+}
+
 void DmInstrument_free(DmInstrument* slf) {
 	if (slf == NULL || slf->dls == NULL) {
 		return;

src/Synth.c

@@ -63,22 +63,40 @@ void DmSynth_reset(DmSynth* slf) {
 //                   instead of reloading the entire instrument list and re-creating all TSFs.
 // See also: https://documentation.help/DirectMusic/usingbands.htm
 void DmSynth_sendBandUpdate(DmSynth* slf, DmBand* band) {
-	if (slf == NULL) {
+	if (slf == NULL || band == NULL) {
 		return;
 	}
 
 	// Optimization: Don't re-alloc the entire thing if we're getting the same band
-	if (band == slf->band) {
+	// TODO(lmichaelis): This is a stop-gap solution to prevent audio flicker when replacing TSFs. It
+	//                   only works if all band instruments are in the same order and use the same DLS.
+	//                   When switching between actual bands, this will break!
+	if (DmBand_isSortOfSameAs(slf->band, band)) {
+		for (size_t i = 0; i < band->instrument_count; ++i) {
+			DmInstrument* ins = &band->instruments[i];
+			if (slf->channels[ins->channel].synth == NULL) {
+				continue;
+			}
+
+			float pan = (ins->flags & DmInstrument_PAN) ? (float) ins->pan / DmInt_MIDI_MAX : DmInt_PAN_CENTER;
+			float vol = (ins->flags & DmInstrument_VOLUME) ? (float) ins->volume / DmInt_MIDI_MAX : DmInt_VOLUME_MAX;
+
+			bool res = tsf_channel_set_pan(slf->channels[ins->channel].synth, 0, pan);
+			if (!res) {
+				Dm_report(DmLogLevel_ERROR, "DmSynth: tsf_channel_set_pan encountered an error.");
+			}
+
+			slf->channels[ins->channel].pitch_bend_reset = DmInt_PITCH_BEND_NEUTRAL;
+			slf->channels[ins->channel].volume = vol;
+			slf->channels[ins->channel].volume_reset = vol;
+			slf->channels[ins->channel].pan_reset = pan;
+		}
 		return;
 	}
 
 	slf->band = band;
 	DmSynth_freeChannels(slf);
 
-	if (band == NULL) {
-		return;
-	}
-
 	// Calculate the number of required performance channels
 	for (size_t i = 0; i < band->instrument_count; ++i) {
 		slf->channel_count = max_usize(band->instruments[i].channel, slf->channel_count);

src/_Internal.h

@@ -488,6 +488,7 @@ DMINT DmBand* DmBand_retain(DmBand* slf);
 DMINT void DmBand_release(DmBand* slf);
 DMINT DmResult DmBand_parse(DmBand* slf, DmRiff* rif);
 DMINT DmResult DmBand_download(DmBand* slf, DmLoader* loader);
+DMINT bool DmBand_isSortOfSameAs(DmBand* slf, DmBand* oth);
 DMINT void DmInstrument_free(DmInstrument* slf);
 
 DMINT DmResult DmStyle_create(DmStyle** slf);