/*
 * A specialzed vector de-mux for estimating power within defined sub-bands of a vector that
 *   is the output of an FFT+complex-to-MAG
 *
 * It takes in a fixed-size float vector, and a list of descriptors describing the sub-bands that are to
 *   be added together to form an output channel. The output is a number of channels (one per descriptor).
 * 
 * Consider the following example:
 * 
 * Input vector: XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
 *                    ^^^^^^         ^^^^                  ^^^^^^^
 *                     CH 0          CH 1                    CH 2    (etc)
 * 
 * For simple multi-channel power estimation this is significantly more efficient than many other techniques
 *   that might be used.  Channel-width resolution is based purely on the resolution of the FFT+complex-to-MAG
 *   that feeds this specialized demux. Resolutions of a few hundred Hz with 25MHz input bandwidths appear to
 *   be feasible with this technique.  For uniformly-spaced channels with relatively-coarse resolution, a PFB
 *   channel bank might be better.
 * 
 * This "rides" on the fact that the Gnu Radio FFT implementation is very efficient, due to its use of
 *   FFTW, which provides several overlapping techniques to improve performance including:
 * 
 *    o an optimizer
 *    o aggressive use of SSEx/3DNow!  instruction sets
 *    o multi-threaded FFT calculations (useful for very large FFTs, which are required for fine resolution)
 * 
 * In many situations, the total number of post-FFT ops you need to do is quite modest compared to the FFT,
 *   since you may be ignoring the vast majority of the input vector. In the application domain of interest
 *   (multi-channel power estimation for ionospheric measurements), probably 98% of the input vector isn't
 *   touched. But the channels of interest are:
 * 
 *     o not evenly spaced
 *     o not of uniform bandwidth
 *     o between 1 and perhaps 10 sub-band "collections" (output channels) might be of interest
 * 
 * invector - input vector of size 'vecsize'
 * vec_begins - list of sub-channel start points (offsets from beginning of input vector)
 * vec_ends   - list of sub-channel end points   (offsets from beginning of input vector)
 * nchan      - number of output channels (or "collections of sub-bands from input")
 * outchans   - the output channels
 * 
 */

int
vector_power_estimator_ff (float invector[], int vecsize, int vec_begins[], int vec_ends[], int nchan, float outchans[])
{
	int i, j;

	/*
	 * For each output channel descriptor
	 */
	for (i = 0; i < nchan; i++)
	{
		/*
		 * Accumulate an output channel from sub-bands in the input
		 */
		outchans[i] = 0.0;
		for (j = vec_begins[i]; j < vec_ends[i]; j++)
		{
			if (j < vecsize)
			{
				/*
				 * Opportunity for volk-ifying here
				 */
				outchans[i] += invector[j];
			}
		}
	}
	return nchan;
}