better loop tracking

src/engine/engine.cpp

@@ -1419,14 +1419,20 @@ void DivEngine::setLoops(int loops) {
   remainingLoops=loops;
 }
 
-void DivEngine::playSub() {
+void DivEngine::playSub(bool preserveDrift) {
   reset();
+  if (preserveDrift && curOrder==0) return;
   int goal=curOrder;
   curOrder=0;
   curRow=0;
+  int prevDrift=clockDrift;
   clockDrift=0;
   cycles=0;
+  if (preserveDrift) {
+    endOfSong=false;
+  } else {
     ticks=1;
+  }
   speedAB=false;
   playing=true;
   dispatch->setSkipRegisterWrites(true);
@@ -1435,14 +1441,20 @@ void DivEngine::playSub() {
   }
   dispatch->setSkipRegisterWrites(false);
   dispatch->forceIns();
+  if (preserveDrift) {
+    clockDrift=prevDrift;
+  } else {
     clockDrift=0;
+  }
+  if (!preserveDrift) {
     cycles=0;
     ticks=1;
   }
+}
 
 void DivEngine::play() {
   isBusy.lock();
-  playSub();
+  playSub(false);
   isBusy.unlock();
 }
 
@@ -1682,7 +1694,7 @@ void DivEngine::setOrder(unsigned char order) {
   curOrder=order;
   if (order>=song.ordersLen) curOrder=0;
   if (playing) {
-    playSub();
+    playSub(false);
   }
   isBusy.unlock();
 }

src/engine/engine.h

@@ -113,7 +113,7 @@ class DivEngine {
   bool perSystemPostEffect(int ch, unsigned char effect, unsigned char effectVal);
   void renderSamples();
   void reset();
-  void playSub();
+  void playSub(bool preserveDrift);
 
   public:
     DivSong song;

src/engine/playback.cpp

@@ -697,7 +697,7 @@ bool DivEngine::nextTick() {
   if (--ticks<=0) {
     ret=endOfSong;
     if (endOfSong) {
-      reset();
+      playSub(true);
     }
     endOfSong=false;
     nextRow();