better channel allocation strategy

issue #376

src/engine/engine.cpp

@@ -2138,10 +2138,12 @@ void DivEngine::noteOff(int chan) {
 }
 
 void DivEngine::autoNoteOn(int ch, int ins, int note, int vol) {
-  //if (ch<0 || ch>=chans) return;
+  bool isViable[DIV_MAX_CHANS];
+  bool canPlayAnyway=false;
   if (midiBaseChan<0) midiBaseChan=0;
   if (midiBaseChan>=chans) midiBaseChan=chans-1;
   int finalChan=midiBaseChan;
+  int finalChanType=getChannelType(finalChan);
 
   if (!playing) {
     reset();
@@ -2149,16 +2151,55 @@ void DivEngine::autoNoteOn(int ch, int ins, int note, int vol) {
     playing=true;
   }
 
+  // 1. check which channels are viable for this instrument
+  DivInstrument* insInst=getIns(ins);
+  for (int i=0; i<chans; i++) {
+    if (ins==-1 || ins>=song.insLen || getPreferInsType(i)==insInst->type || getPreferInsSecondType(i)==insInst->type) {
+      if (insInst->type==DIV_INS_OPL) {
+        if (insInst->fm.ops==2 || getChannelType(i)==DIV_CH_OP) {
+          isViable[i]=true;
+          canPlayAnyway=true;
+        } else {
+          isViable[i]=false;
+        }
+      } else {
+        isViable[i]=true;
+        canPlayAnyway=true;
+      }
+    } else {
+      isViable[i]=false;
+    }
+  }
+
+  if (!canPlayAnyway) return;
+
+  // 2. find a free channel
   do {
-    if ((ins==-1 || ins>=song.insLen || getPreferInsType(finalChan)==getIns(ins)->type || getPreferInsSecondType(finalChan)==getIns(ins)->type) && chan[finalChan].midiNote==-1) {
+    if (isViable[finalChan] && chan[finalChan].midiNote==-1 && getChannelType(finalChan)==finalChanType) {
       chan[finalChan].midiNote=note;
+      chan[finalChan].midiAge=midiAgeCounter++;
       pendingNotes.push(DivNoteEvent(finalChan,ins,note,vol,true));
-      break;
+      return;
     }
     if (++finalChan>=chans) {
       finalChan=0;
     }
   } while (finalChan!=midiBaseChan);
+
+  // 3. find the oldest channel
+  int candidate=finalChan;
+  do {
+    if (isViable[finalChan] && getChannelType(finalChan)==finalChanType && chan[finalChan].midiAge<chan[candidate].midiAge) {
+      candidate=finalChan;
+    }
+    if (++finalChan>=chans) {
+      finalChan=0;
+    }
+  } while (finalChan!=midiBaseChan);
+
+  chan[candidate].midiNote=note;
+  chan[candidate].midiAge=midiAgeCounter++;
+  pendingNotes.push(DivNoteEvent(candidate,ins,note,vol,true));
 }
 
 void DivEngine::autoNoteOff(int ch, int note, int vol) {

src/engine/engine.h

@@ -89,6 +89,7 @@ struct DivChannelState {
   bool arpYield, delayLocked, inPorta, scheduledSlideReset, shorthandPorta, noteOnInhibit, resetArp;
 
   int midiNote, curMidiNote, midiPitch;
+  size_t midiAge;
   bool midiAftertouch;
 
   DivChannelState():
@@ -135,6 +136,7 @@ struct DivChannelState {
     midiNote(-1),
     curMidiNote(-1),
     midiPitch(-1),
+    midiAge(0),
     midiAftertouch(false) {}
 };
 
@@ -339,6 +341,7 @@ class DivEngine {
   int reversePitchTable[4096];
   int pitchTable[4096];
   int midiBaseChan;
+  size_t midiAgeCounter;
 
   blip_buffer_t* samp_bb;
   size_t samp_bbInLen;
@@ -894,6 +897,7 @@ class DivEngine {
       audioEngine(DIV_AUDIO_NULL),
       exportMode(DIV_EXPORT_MODE_ONE),
       midiBaseChan(0),
+      midiAgeCounter(0),
       samp_bb(NULL),
       samp_bbInLen(0),
       samp_temp(0),