Procedure REALFT(var DATA:array of real; N, ISIGN:integer);
var
    I,I1,I2,I3,I4,N2P3:integer;
    WR,WI,C1,C2,THETA,WPR,WPI,WIS,WRS,WRI,H1R,H1I,H2R,H2I,WTEMP:real;
begin
    THETA:=6.28318530717959 / 2 / N;
    C1:=0.5;
    If ISIGN = 1 Then
    begin
        C2:=-0.5;
        FOUR1(DATA, N, 1);
    end
    Else
    begin
        C2:=0.5;
        THETA:=-THETA;
    end; 
    WPR:=-2* Sqr(Sin(0.5 * THETA));
    WPI:=Sin(THETA);
    WR:=1 + WPR;
    WI:=WPI;
    N2P3:= 2 * N + 3;
    For I:=2 To (N div 2) + 1 do
    begin
        I1:=2 * I - 1;
        I2:=I1 + 1;
        I3:=N2P3 - I2;
        I4:=I3 + 1;
        WRS:=WR;
        WIS:=WI;
        H1R:=C1 * (DATA[I1] + DATA[I3]);
        H1I:=C1 * (DATA[I2] - DATA[I4]);
        H2R:=-C2 * (DATA[I2] + DATA[I4]);
        H2I:=C2 * (DATA[I1] - DATA[I3]);
        DATA[I1]:=H1R + WRS * H2R - WIS * H2I;
        DATA[I2]:=H1I + WRS * H2I + WIS * H2R;
        DATA[I3]:=H1R - WRS * H2R + WIS * H2I;
        DATA[I4]:=-H1I + WRS * H2I + WIS * H2R;
        WTEMP:=WR;
        WR:=WR * WPR - WI * WRI + WR;
        WI:=WI * WPR + WTEMP * WPI + WI;
    end;
    If ISIGN = 1 Then
    begin
        H1R:=DATA[1];
        DATA[1]:=H1R + DATA[2];
        DATA[2]:=H1R - DATA[2];
    end
    Else
    begin
        H1R:=DATA[1];
        DATA[1]:=C1 * (H1R + DATA[2]);
        DATA[2]:=C1 * (H1R - DATA[2]);
        FOUR1(DATA, N, -1);
    end; 
end;