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  #ifdef HAVE_CONFIG_H
  #include "config.h"
  #endif
  
  /*
      Elliptic/Cauer filters designed with 0.1 dB passband ripple,
      80 dB minimum stopband attenuation, and
      [0.95 : 0.15 : 0.35] normalized cut off frequencies.
  */
  
  #include "main.h"
  
  /* Helper function, interpolates the filter taps */
  static OPUS_INLINE void
  silk_LP_interpolate_filter_taps(opus_int32 B_Q28[TRANSITION_NB],
                                  opus_int32 A_Q28[TRANSITION_NA],
                                  const opus_int ind, const opus_int32 fac_Q16) {
    opus_int nb, na;
  
    if (ind < TRANSITION_INT_NUM - 1) {
      if (fac_Q16 > 0) {
        if (fac_Q16 < 32768) { /* fac_Q16 is in range of a 16-bit int */
          /* Piece-wise linear interpolation of B and A */
          for (nb = 0; nb < TRANSITION_NB; nb++) {
            B_Q28[nb] = silk_SMLAWB(silk_Transition_LP_B_Q28[ind][nb],
                                    silk_Transition_LP_B_Q28[ind + 1][nb] -
                                        silk_Transition_LP_B_Q28[ind][nb],
                                    fac_Q16);
          }
          for (na = 0; na < TRANSITION_NA; na++) {
            A_Q28[na] = silk_SMLAWB(silk_Transition_LP_A_Q28[ind][na],
                                    silk_Transition_LP_A_Q28[ind + 1][na] -
                                        silk_Transition_LP_A_Q28[ind][na],
                                    fac_Q16);
          }
        } else { /* ( fac_Q16 - ( 1 << 16 ) ) is in range of a 16-bit int */
          silk_assert(fac_Q16 - (1 << 16) == silk_SAT16(fac_Q16 - (1 << 16)));
          /* Piece-wise linear interpolation of B and A */
          for (nb = 0; nb < TRANSITION_NB; nb++) {
            B_Q28[nb] = silk_SMLAWB(silk_Transition_LP_B_Q28[ind + 1][nb],
                                    silk_Transition_LP_B_Q28[ind + 1][nb] -
                                        silk_Transition_LP_B_Q28[ind][nb],
                                    fac_Q16 - ((opus_int32)1 << 16));
          }
          for (na = 0; na < TRANSITION_NA; na++) {
            A_Q28[na] = silk_SMLAWB(silk_Transition_LP_A_Q28[ind + 1][na],
                                    silk_Transition_LP_A_Q28[ind + 1][na] -
                                        silk_Transition_LP_A_Q28[ind][na],
                                    fac_Q16 - ((opus_int32)1 << 16));
          }
        }
      } else {
        silk_memcpy(B_Q28, silk_Transition_LP_B_Q28[ind],
                    TRANSITION_NB * sizeof(opus_int32));
        silk_memcpy(A_Q28, silk_Transition_LP_A_Q28[ind],
                    TRANSITION_NA * sizeof(opus_int32));
      }
    } else {
      silk_memcpy(B_Q28, silk_Transition_LP_B_Q28[TRANSITION_INT_NUM - 1],
                  TRANSITION_NB * sizeof(opus_int32));
      silk_memcpy(A_Q28, silk_Transition_LP_A_Q28[TRANSITION_INT_NUM - 1],
                  TRANSITION_NA * sizeof(opus_int32));
    }
  }
  
  /* Low-pass filter with variable cutoff frequency based on  */
  /* piece-wise linear interpolation between elliptic filters */
  /* Start by setting psEncC->mode <> 0;                      */
  /* Deactivate by setting psEncC->mode = 0;                  */
  void silk_LP_variable_cutoff(
      silk_LP_state *psLP, /* I/O  LP filter state                             */
      opus_int16 *frame,   /* I/O  Low-pass filtered output signal             */
      const opus_int frame_length /* I    Frame length */
  ) {
    opus_int32 B_Q28[TRANSITION_NB], A_Q28[TRANSITION_NA], fac_Q16 = 0;
    opus_int ind = 0;
  
    silk_assert(psLP->transition_frame_no >= 0 &&
                psLP->transition_frame_no <= TRANSITION_FRAMES);
  
    /* Run filter if needed */
    if (psLP->mode != 0) {
      /* Calculate index and interpolation factor for interpolation */
  #if (TRANSITION_INT_STEPS == 64)
      fac_Q16 =
          silk_LSHIFT(TRANSITION_FRAMES - psLP->transition_frame_no, 16 - 6);
  #else
      fac_Q16 = silk_DIV32_16(
          silk_LSHIFT(TRANSITION_FRAMES - psLP->transition_frame_no, 16),
          TRANSITION_FRAMES);
  #endif
      ind = silk_RSHIFT(fac_Q16, 16);
      fac_Q16 -= silk_LSHIFT(ind, 16);
  
      silk_assert(ind >= 0);
      silk_assert(ind < TRANSITION_INT_NUM);
  
      /* Interpolate filter coefficients */
      silk_LP_interpolate_filter_taps(B_Q28, A_Q28, ind, fac_Q16);
  
      /* Update transition frame number for next frame */
      psLP->transition_frame_no = silk_LIMIT(
          psLP->transition_frame_no + psLP->mode, 0, TRANSITION_FRAMES);
  
      /* ARMA low-pass filtering */
      silk_assert(TRANSITION_NB == 3 && TRANSITION_NA == 2);
      silk_biquad_alt(frame, B_Q28, A_Q28, psLP->In_LP_State, frame, frame_length,
                      1);
    }
  }
  