From 154ef3f9a3aaaf9fbee3e90f713882c88fc4e543 Mon Sep 17 00:00:00 2001
From: tildearrow <tildearrow@protonmail.com>
Date: Sat, 26 Mar 2022 23:39:20 -0500
Subject: [PATCH] Amiga: filter emulation

---
 papers/doc/7-systems/amiga.md |  2 +-
 src/engine/dispatch.h         |  2 ++
 src/engine/fileOps.cpp        |  5 +++-
 src/engine/platform/amiga.cpp | 43 ++++++++++++++++++++++++++++++-----
 src/engine/platform/amiga.h   |  6 +++++
 src/engine/playback.cpp       | 11 +++++++++
 6 files changed, 61 insertions(+), 8 deletions(-)

diff --git a/papers/doc/7-systems/amiga.md b/papers/doc/7-systems/amiga.md
index fba62c430..713300c29 100644
--- a/papers/doc/7-systems/amiga.md
+++ b/papers/doc/7-systems/amiga.md
@@ -6,4 +6,4 @@ in this very computer music trackers were born...
 
 # effects
 
-none. as of this moment the Amiga doesn't need any effects in particular, but some may be added in a future.
+- `10xx`: toggle low-pass filter. `0` turns it off and `1` turns it on.
\ No newline at end of file
diff --git a/src/engine/dispatch.h b/src/engine/dispatch.h
index a3cd1e810..b535df52d 100644
--- a/src/engine/dispatch.h
+++ b/src/engine/dispatch.h
@@ -106,6 +106,8 @@ enum DivDispatchCmds {
 
   DIV_CMD_SAA_ENVELOPE,
 
+  DIV_CMD_AMIGA_FILTER,
+
   DIV_CMD_LYNX_LFSR_LOAD,
   
   DIV_CMD_QSOUND_ECHO_FEEDBACK,
diff --git a/src/engine/fileOps.cpp b/src/engine/fileOps.cpp
index d9806db57..0e82b6b79 100644
--- a/src/engine/fileOps.cpp
+++ b/src/engine/fileOps.cpp
@@ -1573,6 +1573,9 @@ bool DivEngine::loadMod(unsigned char* file, size_t len) {
               fxTyp=fxVal>>4;
               fxVal&=0x0f;
               switch (fxTyp) {
+                case 0:
+                  writeFxCol(0x10,!fxVal);
+                  break;
                 case 1: // single note slide up
                 case 2: // single note slide down
                   writeFxCol(fxTyp-1+0xf1,fxVal);
@@ -1613,7 +1616,7 @@ bool DivEngine::loadMod(unsigned char* file, size_t len) {
     ds.systemLen=(chCount+3)/4;
     for(int i=0; i<ds.systemLen; i++) {
       ds.system[i]=DIV_SYSTEM_AMIGA;
-      ds.systemFlags[i]=1|(80<<8)|(bypassLimits?4:0); // PAL
+      ds.systemFlags[i]=1|(80<<8)|(bypassLimits?4:0)|((ds.systemLen>1 || bypassLimits)?2:0); // PAL
     }
     for(int i=0; i<chCount; i++) {
       ds.chanShow[i]=true;
diff --git a/src/engine/platform/amiga.cpp b/src/engine/platform/amiga.cpp
index 13cb9604c..a58de52f5 100644
--- a/src/engine/platform/amiga.cpp
+++ b/src/engine/platform/amiga.cpp
@@ -63,10 +63,20 @@ const char** DivPlatformAmiga::getRegisterSheet() {
   return regCheatSheetAmiga;
 }
 
+const char* DivPlatformAmiga::getEffectName(unsigned char effect) {
+  switch (effect) {
+    case 0x10:
+      return "10xx: Toggle filter (0 disables; 1 enables)";
+      break;
+  }
+  return NULL;
+}
+
 void DivPlatformAmiga::acquire(short* bufL, short* bufR, size_t start, size_t len) {
+  static int outL, outR;
   for (size_t h=start; h<start+len; h++) {
-    bufL[h]=0;
-    bufR[h]=0;
+    outL=0;
+    outR=0;
     for (int i=0; i<4; i++) {
       if (chan[i].sample>=0 && chan[i].sample<parent->song.sampleLen) {
         chan[i].audSub-=AMIGA_DIVIDER;
@@ -102,14 +112,20 @@ void DivPlatformAmiga::acquire(short* bufL, short* bufR, size_t start, size_t le
       }
       if (!isMuted[i]) {
         if (i==0 || i==3) {
-          bufL[h]+=((chan[i].audDat*chan[i].outVol)*sep1)>>7;
-          bufR[h]+=((chan[i].audDat*chan[i].outVol)*sep2)>>7;
+          outL+=((chan[i].audDat*chan[i].outVol)*sep1)>>7;
+          outR+=((chan[i].audDat*chan[i].outVol)*sep2)>>7;
         } else {
-          bufL[h]+=((chan[i].audDat*chan[i].outVol)*sep2)>>7;
-          bufR[h]+=((chan[i].audDat*chan[i].outVol)*sep1)>>7;
+          outL+=((chan[i].audDat*chan[i].outVol)*sep2)>>7;
+          outR+=((chan[i].audDat*chan[i].outVol)*sep1)>>7;
         }
       }
     }
+    filter[0][0]+=(filtConst*(outL-filter[0][0]))>>12;
+    filter[0][1]+=(filtConst*(filter[0][0]-filter[0][1]))>>12;
+    filter[1][0]+=(filtConst*(outR-filter[1][0]))>>12;
+    filter[1][1]+=(filtConst*(filter[1][0]-filter[1][1]))>>12;
+    bufL[h]=filter[0][1];
+    bufR[h]=filter[1][1];
   }
 }
 
@@ -299,6 +315,10 @@ int DivPlatformAmiga::dispatch(DivCommand c) {
       chan[c.chan].audPos=c.value;
       chan[c.chan].setPos=true;
       break;
+    case DIV_CMD_AMIGA_FILTER:
+      filterOn=c.value;
+      filtConst=filterOn?filtConstOn:filtConstOff;
+      break;
     case DIV_CMD_GET_VOLMAX:
       return 64;
       break;
@@ -332,7 +352,11 @@ void* DivPlatformAmiga::getChanState(int ch) {
 void DivPlatformAmiga::reset() {
   for (int i=0; i<4; i++) {
     chan[i]=DivPlatformAmiga::Channel();
+    filter[0][i]=0;
+    filter[1][i]=0;
   }
+  filterOn=false;
+  filtConst=filterOn?filtConstOn:filtConstOff;
 }
 
 bool DivPlatformAmiga::isStereo() {
@@ -372,6 +396,13 @@ void DivPlatformAmiga::setFlags(unsigned int flags) {
   sep2=127-((flags>>8)&127);
   amigaModel=flags&2;
   bypassLimits=flags&4;
+  if (amigaModel) {
+    filtConstOff=4000;
+    filtConstOn=sin(M_PI*8000.0/(double)rate)*4096.0;
+  } else {
+    filtConstOff=sin(M_PI*16000.0/(double)rate)*4096.0;
+    filtConstOn=sin(M_PI*5500.0/(double)rate)*4096.0;
+  }
 }
 
 int DivPlatformAmiga::init(DivEngine* p, int channels, int sugRate, unsigned int flags) {
diff --git a/src/engine/platform/amiga.h b/src/engine/platform/amiga.h
index 02dcb3de9..d73918e96 100644
--- a/src/engine/platform/amiga.h
+++ b/src/engine/platform/amiga.h
@@ -68,6 +68,11 @@ class DivPlatformAmiga: public DivDispatch {
   bool isMuted[4];
   bool bypassLimits;
   bool amigaModel;
+  bool filterOn;
+
+  int filter[2][4];
+  int filtConst;
+  int filtConstOff, filtConstOn;
 
   int sep1, sep2;
 
@@ -88,6 +93,7 @@ class DivPlatformAmiga: public DivDispatch {
     void notifyWaveChange(int wave);
     void notifyInsDeletion(void* ins);
     const char** getRegisterSheet();
+    const char* getEffectName(unsigned char effect);
     int init(DivEngine* parent, int channels, int sugRate, unsigned int flags);
     void quit();
 };
diff --git a/src/engine/playback.cpp b/src/engine/playback.cpp
index 71fd58ddd..7d089e609 100644
--- a/src/engine/playback.cpp
+++ b/src/engine/playback.cpp
@@ -113,6 +113,8 @@ const char* cmdName[DIV_CMD_MAX]={
   "AY_AUTO_PWM",
 
   "SAA_ENVELOPE",
+
+  "AMIGA_FILTER",
   
   "LYNX_LFSR_LOAD",
 
@@ -649,6 +651,15 @@ bool DivEngine::perSystemPostEffect(int ch, unsigned char effect, unsigned char
           return false;
       }
       break;
+    case DIV_SYSTEM_AMIGA:
+      switch (effect) {
+        case 0x10: // toggle filter
+          dispatchCmd(DivCommand(DIV_CMD_AMIGA_FILTER,ch,effectVal));
+          break;
+        default:
+          return false;
+      }
+      break;
     case DIV_SYSTEM_SEGAPCM:
     case DIV_SYSTEM_SEGAPCM_COMPAT:
       switch (effect) {