Procedure FOUR1(var DATA:array of real; NN, ISIGN:integer);
var
    N,J,JJ,I,M,II,MM,ISTEP,MMAX:integer;
    THETA,WR,WI,WPR,SUM,WTEMP,Y1,Y2,TEMPR,TEMPI,WPI:real;
begin
    N:=2 * NN;
    J:=1;
    For II:=1 To NN do
    begin
        I:= II * 2 - 1;
        If J > I Then
        begin
            TEMPR:=DATA[J];
            TEMPI:=DATA[J + 1];
            DATA[J]:=DATA[I];
            DATA[J + 1]:=DATA[I + 1];
            DATA[I]:=TEMPR;
            DATA[I + 1]:=TEMPI;
        end; 
        M:=N div 2;
        While (M >= 2) And (J > M) do
        begin
            J:=J - M;
            M:=M div 2;
        end;
        J:=J + M;
    end; 
    MMAX:=2;
    While N > MMAX do
    begin
        ISTEP:=2 * MMAX;
        THETA:=6.28318530717959 / (ISIGN * MMAX);
        WPR:=-2 * Sqr(Sin(0.5 * THETA));
        WPI:=Sin(THETA);
        WR:=1;
        WI:=0;
        For II:=1 To (MMAX div 2) do
        begin
            M:= II * 2 - 1;
            For JJ:=0 To ((N-M) div ISTEP) do
            begin
                I:= M + JJ*ISTEP ;
                J:=I + MMAX;
                TEMPR:=WR * DATA[J] - WI * DATA[J + 1];
                TEMPI:=WR * DATA[J + 1] + WI * DATA[J];
                DATA[J]:=DATA[I] - TEMPR;
                DATA[J + 1]:=DATA[I + 1] - TEMPI;
                DATA[I]:=DATA[I] + TEMPR;
                DATA[I + 1]:=DATA[I + 1] + TEMPI;
            end; 
            WTEMP:=WR;
            WR:=WR * WPR - WI * WPI + WR;
            WI:=WI * WPR + WTEMP * WPI + WI;
        end; 
        MMAX:=ISTEP;
    end;
end;