/*****************************************************************************
*                                                                            *
*   FCIRCONV.SPL   Copyright (C) 2003, 2010 DSP Development Corporation      *
*                               All Rights Reserved                          *
*                                                                            *
*   Author:      Randy Race                                                  *
*                                                                            *
*   Synopsis:    Circular Convolution using FFT's                            *
*                                                                            *
*   Revisions:    4 Apr 2003  RRR  Creation - from FCONV.SPL                 *
*                18 Jun 2010  RRR  real series check                         *
*                                                                            *
*****************************************************************************/

#if @HELP_FCIRCONV

    FCIRCONV

    Purpose: Circular convolution using the FFT method

    Syntax:  FCIRCONV(s1, s2)

                 s1 - input series

                 s2 - input series

                  N - Optional. An integer, the convolution length, defaults
                      to the maximum length of the input series.

    Returns: A series

    Example:
             W1: {1, 2, 3}
             W2: {4, 5, 6}
             W3: circonv(w1, w2)

             W3 contains the series {31, 31, 28}, the result of circular
             convolution of W1 and W2.

    Remarks:
             FCIRCONV performs circular convolution by computing the FFT's
             of the input series. This method is faster than CIRCONV for
             large series.

             See CIRCONV for a time domain implementation.

    See Also:
             Circonv
             Conv
             Fconv
#endif


/* frequency domain circular convolution */
fcirconv(s1, s2, N)
{
        local c;

        if (argc < 3)
        {
                if (argc < 2) error("fcirconv - two input series required");
                
                /* default to maximum length */
                N = max(length(s1), length(s2));
        }

        /* multiply DFTs and inverse */
        c = ifft(fft(s1, N) * fft(s2, N));

        /* make real if necessary */
        if (isreal(s1) && isreal(s2))
        {
                c = real(c);
        }

        /* set xoffset */
        setxoffset(c, xoffset(s1) + xoffset(s2));

        return(c);
}