add ability to play by one row

src/engine/engine.cpp

@@ -4756,6 +4756,7 @@ void DivEngine::playSub(bool preserveDrift, int goalRow) {
   int goal=curOrder;
   curOrder=0;
   curRow=0;
+  stepPlay=0;
   int prevDrift;
   prevDrift=clockDrift;
   clockDrift=0;
@@ -4815,8 +4816,12 @@ int DivEngine::calcFreq(int base, int pitch, bool period, int octave) {
 
 void DivEngine::play() {
   isBusy.lock();
-  freelance=false;
-  playSub(false);
+  if (stepPlay==0) {
+    freelance=false;
+    playSub(false);
+  } else {
+    stepPlay=0;
+  }
   isBusy.unlock();
 }
 
@@ -4827,11 +4832,23 @@ void DivEngine::playToRow(int row) {
   isBusy.unlock();
 }
 
+void DivEngine::stepOne(int row) {
+  isBusy.lock();
+  if (!isPlaying()) {
+    freelance=false;
+    playSub(false,row);
+  }
+  stepPlay=2;
+  ticks=1;
+  isBusy.unlock();
+}
+
 void DivEngine::stop() {
   isBusy.lock();
   freelance=false;
   playing=false;
   extValuePresent=false;
+  stepPlay=0;
   remainingLoops=-1;
   isBusy.unlock();
 }
@@ -5086,6 +5103,10 @@ bool DivEngine::isPlaying() {
   return (playing && !freelance);
 }
 
+bool DivEngine::isStepping() {
+  return !(stepPlay==0);
+}
+
 bool DivEngine::isChannelMuted(int chan) {
   return isMuted[chan];
 }

src/engine/engine.h

@@ -150,7 +150,7 @@ class DivEngine {
   bool halted;
   bool forceMono;
   int ticks, curRow, curOrder, remainingLoops, nextSpeed, divider;
-  int cycles, clockDrift;
+  int cycles, clockDrift, stepPlay;
   int changeOrd, changePos, totalSeconds, totalTicks, totalTicksR, totalCmds, lastCmds, cmdsPerSecond, globalPitch;
   unsigned char extValue;
   unsigned char speed1, speed2;
@@ -288,6 +288,9 @@ class DivEngine {
     // play to row
     void playToRow(int row);
 
+    // play by one row
+    void stepOne(int row);
+
     // stop
     void stop();
 
@@ -406,6 +409,9 @@ class DivEngine {
     // is playing
     bool isPlaying();
 
+    // is stepping
+    bool isStepping();
+
     // is exporting
     bool isExporting();
 
@@ -587,6 +593,7 @@ class DivEngine {
       divider(60),
       cycles(0),
       clockDrift(0),
+      stepPlay(0),
       changeOrd(-1),
       changePos(0),
       totalSeconds(0),

src/engine/playback.cpp

@@ -844,7 +844,7 @@ bool DivEngine::nextTick(bool noAccum) {
   }
 
   if (!freelance) {
-    if (--ticks<=0) {
+    if (stepPlay!=1) if (--ticks<=0) {
       ret=endOfSong;
       if (endOfSong) {
         if (song.loopModality!=2) {
@@ -852,6 +852,7 @@ bool DivEngine::nextTick(bool noAccum) {
         }
       }
       endOfSong=false;
+      if (stepPlay==2) stepPlay=1;
       nextRow();
     }
     // process stuff