{************************************************}
{                                                }
{        FunctionGraph Demo Routine              }
{        FUNGR.INC                               }
{                                                }
{************************************************}

Function Zfun (X,Y : Real) : Real;
Begin
  Zfun:=Exp (- Sqr (X)-Sqr (Y)) ;
End;

Procedure FnGraph (Xmin,Xmax,Ymin,Ymax,Zmin,
                   Zmax:Real;Adr:Integer);

Const
  RetCode = $C9;

Var
  OldL,NewL : PolyPoint;
  J,K,MaxX,LevZ : Integer;
  StepH,StepV,DeltH : Integer;
  X,Y,StepX,StepY,MultZ,Fun : Real;
  Return : Byte Absolute $F000;

Procedure PushReal (X : Real); External $F000;

Function CallFunc (Adr : Integer) : Real;
Begin
  Inline (
    $2A / Adr /    {  ld   hl,(adr)   }
    $E9            {  jp   (hl)        });
End;

Procedure HorizLines;
Begin
  For K:=0 To 200 Do OldL.Co[K].Y:=0;
  Y:=Ymin;
  For J:=0 To 40 Do
    Begin
      X:=Xmin;
      For K:=0 To 200 Do
        Begin
          Return:=RetCode;
          PushReal (X); PushReal (Y);
          Fun:=CallFunc (Adr);
          NewL.Co[k].X:=K*StepH+J*DeltH;
          NewL.Co[k].Y:=Round (MultZ*Fun)+J
                               *StepV+LevZ;
          X:=X+StepX;
        End;
      NewL.N:=K;
      For K:=1 To 200 Do
        If OldL.Co[K].Y > NewL.Co[K-1].Y Then
          NewL.Co[K-1].Y:=OldL.Co[K].Y;
      OldL:=NewL;
      If J Mod 2 = 0 Then Line (NewL);
      Y:=Y+StepY;
    End;
End;

Procedure VertLines;
Begin
  For K:=0 To 200 Do OldL.Co[K].Y:=0;
  X:=Xmax;
  For J:=0 To 200 Do
    Begin
      Y:=Ymin;
      For K:=0 To 40 Do
        Begin
          Return:=RetCode;
          PushReal (X); PushReal (Y);
          Fun:=CallFunc (Adr);
          NewL.Co[k].X:=MaxX-J*DeltH+K*StepH;
          NewL.Co[k].Y:=Round (MultZ*Fun)+K*StepV
                               +LevZ;
          Y:=Y+StepY;
        End;
      NewL.N:=K;
      For K:=0 To 39 Do
        If OldL.Co[K].Y > NewL.Co[K+1].Y Then
          NewL.Co[K+1].Y:=OldL.Co[K].Y;
      OldL:=NewL;
      If J Mod 5 = 0 Then Line (NewL);
      X:=X-StepX;
    End;
End;

Begin
  StepX:=(Xmax-Xmin) / 200;
  StepY:=(Ymax-Ymin) / 40;
  Case Device Of
    Monitor :
      Begin
        MultZ:=136 / (Zmax-Zmin);
        MaxX:=600; DeltH:=3;
        StepH:=3; StepV:=3;
      End;
    Plotter,Printer :
      Begin
        MultZ:=400 / (Zmax-Zmin);
        MaxX:=800; DeltH:=4;
        StepH:=4; StepV:=8;
      End;
  End;
  LevZ:=- Round (Zmin*MultZ);
  HorizLines;
  VertLines;
End;