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opl: Internally swap MUS/MID percussion channel #s.

MIDI uses channel 9 for percussion but MUS uses channel 15. As the
channel numbers matter when we run out of free voices (#468),
internally swap channels 9 and 15 so that channel precedence is
decided correctly.

--- a/src/i_oplmusic.c

+++ b/src/i_oplmusic.c

@@ -610,6 +610,26 @@

     OPL_WriteRegister(OPL_REGS_FREQ_2 + voice->index, voice->freq >> 8);

 }

+static opl_channel_data_t *TrackChannelForEvent(opl_track_data_t *track,

+                                                midi_event_t *event)

+{

+    unsigned int channel_num = event->data.channel.channel;

+

+    // MIDI uses track #9 for percussion, but for MUS it's track #15

+    // instead. Because DMX works on MUS data internally, we need to

+    // swap back to the MUS version of the channel number.

+    if (channel_num == 9)

+    {

+        channel_num = 15;

+    }

+    else if (channel_num == 15)

+    {

+        channel_num = 9;

+    }

+

+    return &track->channels[channel_num];

+}

+

 // Get the frequency that we should be using for a voice.

 static void KeyOffEvent(opl_track_data_t *track, midi_event_t *event)

@@ -625,7 +645,7 @@

            event->data.channel.param2);

 */

-    channel = &track->channels[event->data.channel.channel];

+    channel = TrackChannelForEvent(track, event);

     key = event->data.channel.param1;

     // Turn off voices being used to play this key.

@@ -826,9 +846,7 @@

 {

     genmidi_instr_t *instrument;

     opl_channel_data_t *channel;

-    unsigned int note;

-    unsigned int key;

-    unsigned int volume;

+    unsigned int note, key, volume;

 /*

     printf("note on: channel %i, %i, %i\n",

@@ -851,10 +869,9 @@

     }

     // The channel.

-    channel = &track->channels[event->data.channel.channel];

+    channel = TrackChannelForEvent(track, event);

-    // Percussion channel (10) is treated differently.

-

+    // Percussion channel is treated differently.

     if (event->data.channel.channel == 9)

     {

         if (key < 35 || key > 81)

@@ -891,14 +908,14 @@

 static void ProgramChangeEvent(opl_track_data_t *track, midi_event_t *event)

 {

-    int channel;

+    opl_channel_data_t *channel;

     int instrument;

     // Set the instrument used on this channel.

-    channel = event->data.channel.channel;

+    channel = TrackChannelForEvent(track, event);

     instrument = event->data.channel.param1;

-    track->channels[channel].instrument = &main_instrs[instrument];

+    channel->instrument = &main_instrs[instrument];

     // TODO: Look through existing voices that are turned on on this

     // channel, and change the instrument.

@@ -938,9 +955,9 @@

 static void ControllerEvent(opl_track_data_t *track, midi_event_t *event)

 {

+    opl_channel_data_t *channel;

     unsigned int controller;

     unsigned int param;

-    opl_channel_data_t *channel;

 /*

     printf("change controller: channel %i, %i, %i\n",

@@ -949,7 +966,7 @@

            event->data.channel.param2);

 */

-    channel = &track->channels[event->data.channel.channel];

+    channel = TrackChannelForEvent(track, event);

     controller = event->data.channel.param1;

     param = event->data.channel.param2;

@@ -981,7 +998,7 @@

     // Update the channel bend value.  Only the MSB of the pitch bend

     // value is considered: this is what Doom does.

-    channel = &track->channels[event->data.channel.channel];

+    channel = TrackChannelForEvent(track, event);

     channel->bend = event->data.channel.param2 - 64;

     // Update all voices for this channel.