procedure FFT( var XREAL,XIMAG: ZISU; var N,NU: integer );

     type   ZISU= array[1..100] of real;
     var    N2,NU1,K,L,I,P,K1,K1N2: integer;
            ARG,C,S,TREAL,TIMAG   : real;

     function IBITR( J,NU: integer ): integer;
        var J1,I,J2,B: integer;
             begin
                 J1:=J;
                 B:=0;
                 for I:=1 to NU do
                   begin
                     J2:=J1 div 2;
                     B:=B*2+(J1-2*J2);
                     J1:=J2
                   end ;
                 IBITR:=B
             end;

         function BEKI( NU1: integer ): integer;
            var Z,C: integer ;
            begin
               C:=1;
               for Z:=1 to NU1 do
                    C:=C*2;
               BEKI:=C
            end;

            begin
               N2:=N div 2;
               NU1:=NU-1;
               K:=0;
               for L:=1 to NU do
                  begin
                     repeat
                     for I:=1 to N2 do
                        begin
                           P:=IBITR( K div BEKI(NU1),NU );
                           ARG:=2*3.14*P/N;
                           C:=cos(ARG);
                           S:=sin(ARG);
                           K1:=K+1;
                           K1N2:=K1+N2;
                           TREAL:=XREAL[K1N2]*C+XIMAG[K1N2]*S;
                           TIMAG:=XIMAG[K1N2]*C-XREAL[K1N2]*S;
                           XREAL[K1N2]:=XREAL[K1]-TREAL;
                           XIMAG[K1N2]:=XIMAG[K1]-TIMAG;
                           XREAL[K1]:=XREAL[K1]+TREAL;
                           XIMAG[K1]:=XIMAG[K1]+TIMAG;
                           K:=K+1
                        end;
                     K:=K+N2;
                     until(K>=N) ;
                     K:=0;
                     NU1:=NU1-1;
                     N2:=N2 div 2
                  end;
               for K:=1 to N do
                  begin
                     I:=IBITR(K-1,NU)+1;
                     if I>K then
                        begin
                           TREAL:=XREAL[K];
                           TIMAG:=XIMAG[K];
                           XREAL[K]:=XREAL[I];
                           XIMAG[K]:=XIMAG[I];
                           XREAL[I]:=TREAL;
                           XIMAG[I]:=TIMAG
                        end
                  end
            end;