Sub REALFT(DATA(), N, ISIGN)
    THETA = 6.28318530717959 / 2# / N
    C1 = 0.5
    If ISIGN = 1 Then
        C2 = -0.5
        Call FOUR1(DATA(), N, 1)
    Else
        C2 = 0.5
        THETA = -THETA
    End If
    WPR = -2# * Sin(0.5 * THETA) ^ 2
    WPI = Sin(THETA)
    WR = 1# + WPR
    WI = WPI
    N2P3 = 2 * N + 3
    For I = 2 To N / 2 + 1
        I1 = 2 * I - 1
        I2 = I1 + 1
        I3 = N2P3 - I2
        I4 = I3 + 1
        WRS = CSng(WR)
        WIS = CSng(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
    Next I
    If ISIGN = 1 Then
        H1R = DATA(1)
        DATA(1) = H1R + DATA(2)
        DATA(2) = H1R - DATA(2)
    Else
        H1R = DATA(1)
        DATA(1) = C1 * (H1R + DATA(2))
        DATA(2) = C1 * (H1R - DATA(2))
        Call FOUR1(DATA(), N, -1)
    End If
End Sub