/*
 * This file was stolen from ffmpeg and modified for usability here.
 *  The original does not have a "copyright by" line.
 *
 * This version is a derivative work of that version.  It is not free
 *  software; it is licensed under the GNU Lesser General Public
 *  License version 2.1, which places nontrivial restrictions on what
 *  may be done with it.  (I'm not terribly happy about that, but for
 *  my purposes accepting LGPL-infected code is a lower price than
 *  reimplementing it all myself.)
 *
 * The LGPL v2.1 as distributed with ffmpeg is in the accompanying file
 *  `LGPL-v2.1'.
 */

#include <math.h>
#include <assert.h>

#include "kbdwin.h"
#include "avutil-attributes.h"

#define BESSEL_I0_ITER 50 // default: 50 iterations of Bessel I0 approximation

av_cold void ff_kbd_window_init(float *window, float alpha, int n)
{
   int i, j;
   double sum = 0.0, bessel, tmp;
   double local_window[FF_KBD_WINDOW_MAX];
   double alpha2 = (alpha * M_PI / n) * (alpha * M_PI / n);

   assert(n <= FF_KBD_WINDOW_MAX);

   for (i = 0; i < n; i++) {
       tmp = i * (n - i) * alpha2;
       bessel = 1.0;
       for (j = BESSEL_I0_ITER; j > 0; j--)
           bessel = bessel * tmp / (j * j) + 1;
       sum += bessel;
       local_window[i] = sum;
   }

   sum++;
   for (i = 0; i < n; i++)
       window[i] = sqrt(local_window[i] / sum);
}