#include "fftease.h"


/* S is a spectrum in rfft format, i.e., it contains N real values
   arranged as real followed by imaginary values, except for first
   two values, which are real parts of 0 and Nyquist frequencies;
   convert first changes these into N/2+1 PAIRS of magnitude and
   phase values to be stored in output array C; the phases are then
   unwrapped and successive phase differences are used to compute
   estimates of the instantaneous frequencies for each phase vocoder
   analysis channel; decimation rate D and sampling rate R are used
   to render these frequency values directly in Hz. */

void lpp_convert(float *S, float *C, int N2, float *lastphase, float fundamental, float factor )
{
  float   phase, phasediff;
  int     real, imag, amp,freq;
  float   a, b;
  int     i;

/*  float myTWOPI, myPI; */
/*  double sin(), cos(), atan(), hypot();*/

/*  myTWOPI = 8.*atan(1.);
    myPI = 4.*atan(1.); */


  for ( i = 0; i <= N2; i++ ) {
    imag = freq = ( real = amp = i<<1 ) + 1;
    a = ( i == N2 ? S[1] : S[real] );
    b = ( i == 0 || i == N2 ? 0. : S[imag] );

    C[amp] = hypot( a, b );
    if ( C[amp] == 0. )
      phasediff = 0.;
    else {
      phasediff = ( phase = -atan2( b, a ) ) - lastphase[i];
      lastphase[i] = phase;

      while ( phasediff > PI )
        phasediff -= TWOPI;
      while ( phasediff < -PI )
        phasediff += TWOPI;
    }
    C[freq] = phasediff*factor + i*fundamental;
  }
}
