Shooting cannon source code

program cannon;
uses Graph,Crt;
const dt=0.035;
      g=98.1;
var a,v0,x,y,t:Real;
    gd,gm,maxX,maxY,midX,midY,i:Integer;
    PathToDriver,stat,xs,ys,ts:String;
    clr1:Boolean;
    ch:Char;

procedure TUI1;
begin
     WriteLn;
     Write('Angle:');
     ReadLn(a);
     a:=a*Pi/180;
     Write('Initial speed:');
     ReadLn(v0);
     v0:=v0*10;
end;

procedure startgraph;
begin
     gd:=0;
     PathToDriver:='P:\BGI\';
     DetectGraph(gd,gm);
     InitGraph(gd,gm,PathToDriver);
     maxX:=GetMaxX;
     maxY:=GetMaxY;
     midX:=maxX div 2;
     midY:=maxY div 2;
end;

procedure DrawGrid;
begin
     MoveTo(midX,midY);
     SetColor(15);
     Rectangle(midX-5,midY-3,midX+5,midY+3);
     MoveTo(0,0);
     for i:=1 to 64 do
     begin
          SetColor(i);
          SetFillStyle(SolidFill,i);
          Bar(8*i,8,8*i+8,16);
     end;
end;

procedure DrawTrack;
begin
     x:=midX;
     y:=midY;
     t:=0;
     clr1:=False;
     MoveTo(Round(x),Round(y));
     while (GetX>0) and (GetX<maxX) {and (GetY>0)} and (GetY<maxY) do
     begin
          if clr1 then
          begin
               SetColor(64);
               clr1:=False;
          end else
          begin
               SetColor(7);
               clr1:=True;
          end;
          t:=t+dt;
          x:=midX + v0*Cos(a)*t;
          y:=midY - v0*Sin(a)*t + (g*t*t)/2;
          LineTo(Round(x),Round(y));

          MoveTo(32,32);
          SetFillStyle(SolidFill,16);
          Bar(32,32,300,48);
          SetColor(15);
          Str(x:0:4,xs);
          Str(y:0:4,ys);
          Str(t:0:4,ts);
          stat:='x: '+xs+' y: '+ys+' t: '+ts;
          OutText(stat);
          MoveTo(Round(x),Round(y));
          Delay(Round(dt*1000));

          if KeyPressed then ch:=ReadKey;
          case ch of
               #75: t:=Abs(t-10*dt);
               #77: t:=t+dt;
               #32: Readkey;
          end;
     end;

end;

{main}
begin
     TUI1;
     startgraph;
     drawgrid;
     drawtrack;
     ReadLn;
     CloseGraph;
end.