class MyMass:CMass
{
  F64 radius;
};

class MySpring:CSpring
{
};

CMathODE *ode=NULL;

U0 DrawIt(CTask *,CDC *dc)
{
  MyMass   *tmpm;
  MySpring *tmps;
  dc->color=RED;
  tmps=ode->next_spring;
  while (tmps!=&ode->next_spring)
  {
    GrLine(dc,tmps->end1->x,tmps->end1->y,tmps->end2->x,tmps->end2->y);
    tmps=tmps->next;
  }
  dc->color=BLACK;
  tmpm=ode->next_mass;
  while (tmpm!=&ode->next_mass)
  {
    GrCircle(dc,tmpm->x,tmpm->y,tmpm->radius);
    tmpm=tmpm->next;
  }
}

U0 MyDerivative(CMathODE *ode,F64,COrder2D3 *,COrder2D3 *)
{//The forces due to springs and drag are
//automatically handled by the ode code.
  //We can add new forces here.
  F64 d,dd;
  CD3 p;
  MyMass *tmpm1,*tmpm2;
  tmpm1=ode->next_mass;
  while (tmpm1!=&ode->next_mass)
  {
    tmpm2=tmpm1->next;
    while (tmpm2!=&ode->next_mass)
    {
      D3Sub(&p,&tmpm2->state->x,&tmpm1->state->x);
      dd=D3NormSqr(&p);
      if (dd<=Sqr(tmpm1->radius+tmpm2->radius))
      {
        d=Sqrt(dd)+0.0001;
        dd=10.0*Sqr(Sqr(Sqr(tmpm1->radius+tmpm2->radius)-dd));
        D3MulEqu(&p,dd/d);
        D3AddEqu(&tmpm2->DstateDt->DxDt,&p);
        D3SubEqu(&tmpm1->DstateDt->DxDt,&p);
      }
      tmpm2=tmpm2->next;
    }
    tmpm1=tmpm1->next;
  }
}

U0 PlaceMass(I64 x, I64 y)
{
  MyMass *tmpm=CAlloc(sizeof(MyMass));
  tmpm->mass=1.0;
  tmpm->drag_profile_factor=100.0;
  tmpm->x=x;
  tmpm->y=y;
  tmpm->radius=10*(Rand+0.25);
  QueIns(tmpm,ode->last_mass);
}

U0 PlaceSpring(MyMass *tmpm1,MyMass *tmpm2)
{
  MySpring *tmps=CAlloc(sizeof(MySpring));
  tmps->end1=tmpm1;
  tmps->end2=tmpm2;
  tmps->const=10000;
  tmps->rest_len=100;
  QueIns(tmps,ode->last_spring);
}

U0 Init()
{
  ode=ODENew(0,1e-4,ODEF_HAS_MASSES);
  ode->derive=&MyDerivative;
  ode->drag_v2=0.002;
  ode->drag_v3=0.00001;
  ode->acceleration_limit=5e3;
  QueIns(ode,Fs->last_ode);
}

U0 CleanUp()
{
  QueRem(ode);
  QueDel(&ode->next_mass,TRUE);
  QueDel(&ode->next_spring,TRUE);
  ODEDel(ode);
}

U0 MassSpringDemo()
{
  I64 msg_code,arg1,arg2;
  MyMass *tmpm1=NULL,*tmpm2=NULL;
  PopUpOk("Left-Click to place mas\n"
          "Right-Click and drag to\n"
          "connect with spring.\n\n"
          "Springs are 100 pixs long.\n");
  SettingsPush; //See SettingsPush
  AutoComplete;
  WinBorder;
  WinMax;
  DocCursor;
  DocClear;
  Fs->win_inhibit|=WIG_DBL_CLICK;
  MenuPush(
  "File {"
  "  Abort(,CH_SHIFT_ESC);"
  "  Exit(,CH_ESC);"
  "}"
  "Play {"
  "  Restart(,'\n');"
  "}"
  );
  Init;
  Fs->draw_it=&DrawIt;
  try
  {
    while (TRUE)
    {
      msg_code=GetMsg(&arg1,&arg2,
                      1<<MSG_MS_L_DOWN|1<<MSG_MS_R_DOWN|1<<MSG_MS_R_UP|1<<MSG_KEY_DOWN);
      switch (msg_code)
      {
      case MSG_MS_L_DOWN:
        PlaceMass(arg1,arg2);
        break;
      case MSG_MS_R_DOWN:
        tmpm1=MassFind(ode,arg1,arg2);
        tmpm2=NULL;
        break;
      case MSG_MS_R_UP:
        if (tmpm1 && (tmpm2=MassFind(ode,arg1,arg2)) && tmpm1!=tmpm2)
          PlaceSpring(tmpm1,tmpm2);
        tmpm1=tmpm2=NULL;
        break;
      case MSG_KEY_DOWN:
        switch (arg1)
        {
        case '\n':
          CleanUp;
          Init;
          break;
        case CH_SHIFT_ESC:
        case CH_ESC:
          goto ms_done;
        }
        break;
      }
    }
ms_done: //Don't goto out of try
    GetMsg(,,1<<MSG_KEY_UP);
  }
  catch
    PutExcept;
  SettingsPop;
  CleanUp;
  MenuPop;
}

MassSpringDemo;