/*
  * Opus encoder
  * Copyright (c) 2017 Rostislav Pehlivanov <atomnuker@gmail.com>
  *
  * This file is part of FFmpeg.
  *
  * FFmpeg is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Lesser General Public
  * License as published by the Free Software Foundation; either
  * version 2.1 of the License, or (at your option) any later version.
  *
  * FFmpeg is distributed in the hope that it will be useful,
  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  * Lesser General Public License for more details.
  *
  * You should have received a copy of the GNU Lesser General Public
  * License along with FFmpeg; if not, write to the Free Software
  * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
  */
 

 #ifndef AVCODEC_OPUSENC_UTILS_H
 #define AVCODEC_OPUSENC_UTILS_H
 

 #include "opus.h"
 
 typedef struct FFBesselFilter {
     float a[3];
     float b[2];
     float x[3];
     float y[3];
 } FFBesselFilter;
 
 /* Fills the coefficients, returns 1 if filter will be unstable */
 static inline int bessel_reinit(FFBesselFilter *s, float n, float f0, float fs,
                                 int highpass)
 {
     int unstable;
     float c, cfreq, w0, k1, k2;
 
     if (!highpass) {
         c = (1.0f/sqrtf(sqrtf(pow(2.0f, 1.0f/n) - 3.0f/4.0f) - 0.5f))/sqrtf(3.0f);
         cfreq = c*f0/fs;
         unstable = (cfreq <= 0.0f || cfreq >= 1.0f/4.0f);
     } else {
         c = sqrtf(3.0f)*sqrtf(sqrtf(pow(2.0f, 1.0f/n) - 3.0f/4.0f) - 0.5f);
         cfreq = 0.5f - c*f0/fs;
         unstable = (cfreq <= 3.0f/8.0f || cfreq >= 1.0f/2.0f);
     }
 
     w0 = tanf(M_PI*cfreq);
     k1 = 3.0f * w0;
     k2 = 3.0f * w0;
 
     s->a[0] = k2/(1.0f + k1 + k2);
     s->a[1] = 2.0f * s->a[0];
     s->a[2] = s->a[0];
     s->b[0] = 2.0f * s->a[0] * (1.0f/k2 - 1.0f);
     s->b[1] = 1.0f - (s->a[0] + s->a[1] + s->a[2] + s->b[0]);
 
     if (highpass) {
         s->a[1] *= -1;
         s->b[0] *= -1;
     }
 
     return unstable;
 }
 
 static inline int bessel_init(FFBesselFilter *s, float n, float f0, float fs,
                               int highpass)
 {
     memset(s, 0, sizeof(FFBesselFilter));
     return bessel_reinit(s, n, f0, fs, highpass);
 }
 
 static inline float bessel_filter(FFBesselFilter *s, float x)
 {
     s->x[2] = s->x[1];
     s->x[1] = s->x[0];
     s->x[0] = x;
     s->y[2] = s->y[1];
     s->y[1] = s->y[0];
     s->y[0] = s->a[0]*s->x[0] + s->a[1]*s->x[1] + s->a[2]*s->x[2] + s->b[0]*s->y[1] + s->b[1]*s->y[2];
     return s->y[0];
 }

 
 #endif /* AVCODEC_OPUSENC_UTILS_H */