add broken speed alternation flag - dev70

papers/format.md

@@ -29,6 +29,7 @@ furthermore, an `or reserved` indicates this field is always present, but is res
 
 the format versions are:
 
+- 70: Furnace dev70
 - 69: Furnace dev69
 - 68: Furnace dev68
 - 67: Furnace dev67
@@ -235,6 +236,9 @@ size | description
  STR | song comment
   4f | master volume, 1.0f=100% (>=59)
      | this is 2.0f for modules before 59
+ --- | **extended compatibility flags** (>=70)
+  1  | broken speed selection
+ 31  | reserved
 ```
 
 # instrument

src/engine/engine.h

@@ -38,8 +38,8 @@
     warnings+=(String("\n")+x); \
   }
 
-#define DIV_VERSION "dev69"
-#define DIV_ENGINE_VERSION 69
+#define DIV_VERSION "dev70"
+#define DIV_ENGINE_VERSION 70
 
 // for imports
 #define DIV_VERSION_MOD 0xff01

src/engine/fileOps.cpp

@@ -144,6 +144,7 @@ bool DivEngine::loadDMF(unsigned char* file, size_t len) {
     ds.oneTickCut=false;
     ds.newInsTriggersInPorta=true;
     ds.arp0Reset=true;
+    ds.brokenSpeedSel=true;
 
     // 1.1 compat flags
     if (ds.version>24) {
@@ -1067,6 +1068,14 @@ bool DivEngine::loadFur(unsigned char* file, size_t len) {
       ds.masterVol=2.0f;
     }
 
+    if (ds.version>=70) {
+      // extended compat flags
+      ds.brokenSpeedSel=reader.readC();
+      for (int i=0; i<31; i++) {
+        reader.readC();
+      }
+    }
+
     // read instruments
     for (int i=0; i<ds.insLen; i++) {
       DivInstrument* ins=new DivInstrument;
@@ -1905,6 +1914,12 @@ SafeWriter* DivEngine::saveFur(bool notPrimary) {
 
   w->writeF(song.masterVol);
 
+  // extended compat flags
+  w->writeC(song.brokenSpeedSel);
+  for (int i=0; i<31; i++) {
+    w->writeC(0);
+  }
+
   /// INSTRUMENT
   for (int i=0; i<song.insLen; i++) {
     DivInstrument* ins=song.ins[i];

src/engine/playback.cpp

@@ -1179,6 +1179,15 @@ void DivEngine::nextRow() {
     if (haltOn==DIV_HALT_PATTERN) halted=true;
   }
 
+  if (song.brokenSpeedSel) {
+    if ((song.patLen&1) && curOrder&1) {
+      ticks=((curRow&1)?speed2:speed1)*(song.timeBase+1);
+      nextSpeed=(curRow&1)?speed1:speed2;
+    } else {
+      ticks=((curRow&1)?speed1:speed2)*(song.timeBase+1);
+      nextSpeed=(curRow&1)?speed2:speed1;
+    }
+  } else {
     if (speedAB) {
       ticks=speed2*(song.timeBase+1);
       nextSpeed=speed1;
@@ -1187,6 +1196,7 @@ void DivEngine::nextRow() {
       nextSpeed=speed2;
     }
     speedAB=!speedAB;
+  }
 
   // post row details
   for (int i=0; i<chans; i++) {

src/engine/song.h

@@ -301,6 +301,7 @@ struct DivSong {
   bool oneTickCut;
   bool newInsTriggersInPorta;
   bool arp0Reset;
+  bool brokenSpeedSel;
 
   DivOrders orders;
   std::vector<DivInstrument*> ins;
@@ -373,7 +374,8 @@ struct DivSong {
     brokenDACMode(false),
     oneTickCut(false),
     newInsTriggersInPorta(true),
-    arp0Reset(true) {
+    arp0Reset(true),
+    brokenSpeedSel(false) {
     for (int i=0; i<32; i++) {
       system[i]=DIV_SYSTEM_NULL;
       systemVol[i]=64;

src/gui/compatFlags.cpp

@@ -81,6 +81,10 @@ void FurnaceGUI::drawCompatFlags() {
     if (ImGui::IsItemHovered()) {
       ImGui::SetTooltip("when enabled, a one-tick note cut will be inserted between non-legato/non-portamento notes.\nthis simulates the behavior of some Amiga/SNES music engines.");
     }
+    ImGui::Checkbox("Broken speed alternation",&e->song.brokenSpeedSel);
+    if (ImGui::IsItemHovered()) {
+      ImGui::SetTooltip("determines next speed based on whether the row is odd/even instead of alternating between speeds.");
+    }
 
     ImGui::Text("Loop modality:");
     if (ImGui::RadioButton("Reset channels",e->song.loopModality==0)) {