/*Description
        Convolve two vectors using the FFT for computation. c = fftconv (X, Y) returns a vector of length equal to 'length(X) + length (Y) - 1'.  If X and Y are         the coefficient vectors of two polynomials, the returned value is the coefficient vector of the product polynomial.
        If the optional argument n is specified, an N-point FFT is used.
    Calling Sequence
        Y = fftconv(X, Y)
        Y = fftconv(X, Y, n)
    Parameters
        X, Y: Vectors
    Examples
        fftconv([1,2,3], [3,4,5])
        ans = 
            3.    10.    22.    22.    15. 
 */
 function y = fftconv(X, Y, n)
     funcprot(0);
     rhs = argn(2);
     if(rhs<2 | rhs>3)
         error("Wrong number of input arguments.");
     end
     shape_x = size(X);
     shape_y = size(Y);
     if (shape_x(1) ~= 1 || length(shape_x) ~= 2 || shape_y(1) ~= 1 || length(shape_y) ~= 2)
         error('The inputs must be a vector');
     end
     nx=length(X);
     ny=length(Y);
     select(rhs)
     case 2 then
         n=nx + ny;
         X=resize_matrix(X,1,n);
         Y=resize_matrix(Y,1,n);
         fftX=fft(X);
         fftY=fft(Y);
         y=fft(fftX.*fftY,1);
         y=y(1:nx+ny-1);
     case 3 then
         n = 2^(fix(log2(nx+ny))+1);
         X=resize_matrix(X,1,n);
         Y=resize_matrix(Y,1,n);
         fftX=fft(X);
         fftY=fft(Y);
         y=fft(fftX.*fftY,1);
         y=y(1:nx+ny-1);
     end
     y=clean(y);
 endfunction