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fidmkf.h

//
//      mkfilter-derived code
//      ---------------------
//
//        Copyright (c) 2002-2004 Jim Peters <http://uazu.net/>.  This
//        file is released under the GNU Lesser General Public License
//        (LGPL) version 2.1 as published by the Free Software
//        Foundation.  See the file COPYING_LIB for details, or visit
//        <http://www.fsf.org/licenses/licenses.html>.
//
//    This is all code derived from 'mkfilter' by Tony Fisher of the
//    University of York.  I've rewritten it all in C, and given it
//    a thorough overhaul, so there is actually none of his code
//    here any more, but it is all still very strongly based on the
//    algorithms and techniques that he used in 'mkfilter'.
//
//    For those who didn't hear, Tony Fisher died in February 2000
//    at the age of 43.  See his web-site for information and a
//    tribute:
//
//        http://www-users.cs.york.ac.uk/~fisher/
//        http://www-users.cs.york.ac.uk/~fisher/tribute.html
//
//      The original C++ sources and the rest of the mkfilter tool-set
//      are still available from his site:
//
//        http://www-users.cs.york.ac.uk/~fisher/mkfilter/
//
//    
//    I've made a number of improvements and changes whilst
//    rewriting the code in C.  For example, I halved the
//    calculations required in designing the filters by storing only
//    one complex pole/zero out of each conjugate pair.  This also
//    made it much easier to output the filter as a list of
//    sub-filters without lots of searching around to match up
//    conjugate pairs later on.  Outputting as a list of subfilters
//    permits greater accuracy in calculation of the response, and
//    also in the execution of the final filter.  Also, some FIR
//    coefficients can be marked as 'constant', allowing optimised
//    routines to be generated for whole classes of filters, with
//    just the variable coefficients filled in at run-time.
//
//    On the down-side, complex numbers are not portably available
//    in C before C99, so complex calculations here are done on
//    double[] arrays with inline functions, which ends up looking
//    more like assembly language than C.  Never mind.
//

//
//    LEGAL STUFF
//    -----------
//
//    Tony Fisher released his software on his University of York
//    pages for free use and free download.  The software itself has
//    no licence terms attached, nor copyright messages, just the
//    author's name, E-mail address and date.  Nor are there any
//    licence terms indicated on the website.  I understand that
//    under the Berne convention copyright does not have to be
//    claimed explicitly, so these are in fact copyright files by
//    legal default.  However, the intention was obviously that
//    these files should be used by others.
//
//    None of this really helps, though, if we're going to try to be
//    100% legally correct, so I wrote to Anthony Moulds who is the
//    contact name on Tony Fisher's pages now.  I explained what I
//    planned to do with the code, and he answered as follows:
//
//    (Note that I was planning to use it 'as-is' at that time,
//    rather than rewrite it as I have done now)
//
//      > To: "Jim Peters" <jim@uazu.net>
//      > From: "Anthony Moulds" <anthony@cs.york.ac.uk>
//      > Subject: RE: mkfilter source
//      > Date: Tue, 29 Oct 2002 15:30:19 -0000
//      > 
//      > Hi Jim,
//      > 
//      > Thanks for your email.
//      > 
//      > The University will be happy to let you use Dr Fisher's mkfilter
//      > code since your intention is not to profit financially from his work.
//      > 
//      > It would be nice if in some way you could acknowledge his contribution.
//      > 
//      > Best wishes and good luck with your work,
//      > 
//      > Anthony Moulds
//      > Senior Experimental Officer, 
//      > Computer Science Department,  University of York,
//      > York, England, UK. Tel: 44(0)1904 434758  Fax: 44(0)19042767
//      > ============================================================
//      > 
//      > 
//      > > -----Original Message-----
//      > > From: Jim Peters [mailto:jim@uazu.net]
//      > > Sent: Monday, October 28, 2002 12:36 PM
//      > > To: anthony@cs.york.ac.uk
//      > > Subject: mkfilter source
//      > > 
//      > > 
//      > > I'm very sorry to hear (rather late, I know) that Tony Fisher died --
//      > > I've always gone straight to the filter page, rather than through his
//      > > home page.  I hope his work remains available for the future.
//      > > 
//      > > Anyway, the reason I'm writing is to clarify the status of the
//      > > mkfilter source code.  Because copyright is not claimed on the web
//      > > page nor in the source distribution, I guess that Tony's intention was
//      > > that this code should be in the public domain.  However, I would like
//      > > to check this now to avoid complications later.
//      > > 
//      > > I am using his code, modified, to provide a library of filter-design
//      > > routines for a GPL'd filter design app, which is not yet released.
//      > > The library could also be used standalone, permitting apps to design
//      > > filters at run-time rather than using hard-coded compile-time filters.
//      > > My interest in filters is as a part of my work on the OpenEEG project
//
//    So this looks pretty clear to me.  I am not planning to profit
//    from the work, so everything is fine with the University.  I
//    guess others might profit from the work, indirectly, as with
//    any free software release, but so long as I don't, we're fine.
//
//    I hope this is watertight enough for Debian/etc.  Otherwise
//    I'll have to go back to Anthony Moulds for clarification.
//
//    Even though there is no code cut-and-pasted from 'mkfilter'
//    here, it is all very obviously based on that code, so it
//    probably counts as a derived work -- although as ever "I Am
//    Not A Lawyer".
//


#ifdef HUGE_VAL
 #define INF HUGE_VAL
#else
 #define INF (1.0/0.0)
#endif

#define TWOPI (2*M_PI)


//
//    Complex square root: aa= aa^0.5
//

STATIC_INLINE double
my_sqrt(double aa) {
   return aa <= 0.0 ? 0.0 : sqrt(aa);
}

// 'csqrt' clashes with builtin in GCC 4, so call it 'c_sqrt'
STATIC_INLINE void 
c_sqrt(double *aa) {
   double mag= hypot(aa[0], aa[1]);
   double rr= my_sqrt((mag + aa[0]) * 0.5);
   double ii= my_sqrt((mag - aa[0]) * 0.5);
   if (aa[1] < 0.0) ii= -ii;
   aa[0]= rr;
   aa[1]= ii;
}

//
//    Complex imaginary exponent: aa= e^i.theta
//

STATIC_INLINE void 
cexpj(double *aa, double theta) {
   aa[0]= cos(theta);
   aa[1]= sin(theta);
}

//
//    Complex exponent: aa= e^aa
//

// 'cexp' clashes with builtin in GCC 4, so call it 'c_exp'
STATIC_INLINE void 
c_exp(double *aa) {
   double mag= exp(aa[0]);
   aa[0]= mag * cos(aa[1]);
   aa[1]= mag * sin(aa[1]);
}

//
//    Global temp buffer for generating filters.  *NOT THREAD SAFE*
//
//    Note that the poles and zeros are stored in a strange way.
//    Rather than storing both a pole (or zero) and its complex
//    conjugate, I'm storing just one of the pair.  Also, for real
//    poles, I'm not storing the imaginary part (which is zero).
//    This results in a list of numbers exactly half the length you
//    might otherwise expect.  However, since some of these numbers
//    are in pairs, and some are single, we need a separate flag
//    array to indicate which is which.  poltyp[] serves this
//    purpose.  An entry is 1 if the corresponding offset is a real
//    pole, or 2 if it is the first of a pair of values making up a
//    complex pole.  The second value of the pair has an entry of 0
//    attached.  (Similarly for zeros in zertyp[])
//

#define MAXPZ 64 

static int n_pol;       // Number of poles
static double pol[MAXPZ];     // Pole values (see above)
static char poltyp[MAXPZ];    // Pole value types: 1 real, 2 first of complex pair, 0 second
static int n_zer;       // Same for zeros ...
static double zer[MAXPZ];
static char zertyp[MAXPZ];    


//
//    Pre-warp a frequency
//

STATIC_INLINE double 
prewarp(double val) {
   return tan(val * M_PI) / M_PI;
}


//
//    Bessel poles; final one is a real value for odd numbers of
//    poles
//

static double bessel_1[]= {
 -1.00000000000e+00
};

static double bessel_2[]= {
 -1.10160133059e+00, 6.36009824757e-01,
};

static double bessel_3[]= {
 -1.04740916101e+00, 9.99264436281e-01,
 -1.32267579991e+00,
};

static double bessel_4[]= {
 -9.95208764350e-01, 1.25710573945e+00,
 -1.37006783055e+00, 4.10249717494e-01,
};

static double bessel_5[]= {
 -9.57676548563e-01, 1.47112432073e+00,
 -1.38087732586e+00, 7.17909587627e-01,
 -1.50231627145e+00,
};

static double bessel_6[]= {
 -9.30656522947e-01, 1.66186326894e+00,
 -1.38185809760e+00, 9.71471890712e-01,
 -1.57149040362e+00, 3.20896374221e-01,
};

static double bessel_7[]= {
 -9.09867780623e-01, 1.83645135304e+00,
 -1.37890321680e+00, 1.19156677780e+00,
 -1.61203876622e+00, 5.89244506931e-01,
 -1.68436817927e+00,
};

static double bessel_8[]= {
 -8.92869718847e-01, 1.99832584364e+00,
 -1.37384121764e+00, 1.38835657588e+00,
 -1.63693941813e+00, 8.22795625139e-01,
 -1.75740840040e+00, 2.72867575103e-01,
};

static double bessel_9[]= {
 -8.78399276161e-01, 2.14980052431e+00,
 -1.36758830979e+00, 1.56773371224e+00,
 -1.65239648458e+00, 1.03138956698e+00,
 -1.80717053496e+00, 5.12383730575e-01,
 -1.85660050123e+00,
};

static double bessel_10[]= {
 -8.65756901707e-01, 2.29260483098e+00,
 -1.36069227838e+00, 1.73350574267e+00,
 -1.66181024140e+00, 1.22110021857e+00,
 -1.84219624443e+00, 7.27257597722e-01,
 -1.92761969145e+00, 2.41623471082e-01,
};

static double *bessel_poles[10]= {
   bessel_1, bessel_2, bessel_3, bessel_4, bessel_5,
   bessel_6, bessel_7, bessel_8, bessel_9, bessel_10
};

//
//    Generate Bessel poles for the given order.
//

static void 
bessel(int order) {
   int a;

   if (order > 10) error("Maximum Bessel order is 10");
   n_pol= order;
   memcpy(pol, bessel_poles[order-1], n_pol * sizeof(double));

   for (a= 0; a<order-1; ) {
      poltyp[a++]= 2;
      poltyp[a++]= 0;
   }
   if (a < order) 
      poltyp[a++]= 1;
}

//
//    Generate Butterworth poles for the given order.  These are
//    regularly-spaced points on the unit circle to the left of the
//    real==0 line.
//

static void 
butterworth(int order) {
   int a;
   if (order > MAXPZ) 
      error("Maximum butterworth/chebyshev order is %d", MAXPZ);
   n_pol= order;
   for (a= 0; a<order-1; a += 2) {
      poltyp[a]= 2;
      poltyp[a+1]= 0;
      cexpj(pol+a, M_PI - (order-a-1) * 0.5 * M_PI / order);
   }
   if (a < order) {
      poltyp[a]= 1;
      pol[a]= -1.0;
   }
}

//
//    Generate Chebyshev poles for the given order and ripple.
//

static void 
chebyshev(int order, double ripple) {
   double eps, y;
   double sh, ch;
   int a;

   butterworth(order);
   if (ripple >= 0.0) error("Chebyshev ripple in dB should be -ve");

   eps= sqrt(-1.0 + pow(10.0, -0.1 * ripple));
   y= asinh(1.0 / eps) / order;
   if (y <= 0.0) error("Internal error; chebyshev y-value <= 0.0: %g", y);
   sh= sinh(y);
   ch= cosh(y);

   for (a= 0; a<n_pol; ) {
      if (poltyp[a] == 1)
       pol[a++] *= sh;
      else {
       pol[a++] *= sh;
       pol[a++] *= ch;
      }
   }
}


//
//    Adjust raw poles to LP filter
//

static void 
lowpass(double freq) {
   int a;

   // Adjust poles
   freq *= TWOPI;
   for (a= 0; a<n_pol; a++)
      pol[a] *= freq;

   // Add zeros
   n_zer= n_pol;
   for (a= 0; a<n_zer; a++) {
      zer[a]= -INF;
      zertyp[a]= 1;
   }
}

//
//    Adjust raw poles to HP filter
//

static void 
highpass(double freq) {
   int a;

   // Adjust poles
   freq *= TWOPI;
   for (a= 0; a<n_pol; ) {
      if (poltyp[a] == 1) {
       pol[a]= freq / pol[a];
       a++;
      } else {
       crecip(pol + a);
       pol[a++] *= freq;
       pol[a++] *= freq;
      }
   }

   // Add zeros
   n_zer= n_pol;
   for (a= 0; a<n_zer; a++) {
      zer[a]= 0.0;
      zertyp[a]= 1;
   }
}

//
//    Adjust raw poles to BP filter.  The number of poles is
//    doubled.
//

static void 
bandpass(double freq1, double freq2) {
   double w0= TWOPI * sqrt(freq1*freq2);
   double bw= 0.5 * TWOPI * (freq2-freq1);
   int a, b;

   if (n_pol * 2 > MAXPZ) 
      error("Maximum order for bandpass filters is %d", MAXPZ/2);
   
   // Run through the list backwards, expanding as we go
   for (a= n_pol, b= n_pol*2; a>0; ) {
      // hba= pole * bw;
      // temp= c_sqrt(1.0 - square(w0 / hba));
      // pole1= hba * (1.0 + temp);
      // pole2= hba * (1.0 - temp);

      if (poltyp[a-1] == 1) {
       double hba;
       a--; b -= 2;
       poltyp[b]= 2; poltyp[b+1]= 0;
       hba= pol[a] * bw;
       cassz(pol+b, 1.0 - (w0 / hba) * (w0 / hba), 0.0);
       c_sqrt(pol+b);
       caddz(pol+b, 1.0, 0.0);
       cmulr(pol+b, hba);
      } else {          // Assume poltyp[] data is valid
       double hba[2];
       a -= 2; b -= 4;
       poltyp[b]= 2; poltyp[b+1]= 0;
       poltyp[b+2]= 2; poltyp[b+3]= 0;
       cass(hba, pol+a);
       cmulr(hba, bw);
       cass(pol+b, hba);
       crecip(pol+b);
       cmulr(pol+b, w0);
       csqu(pol+b);
       cneg(pol+b);
       caddz(pol+b, 1.0, 0.0);
       c_sqrt(pol+b);
       cmul(pol+b, hba);
       cass(pol+b+2, pol+b);
       cneg(pol+b+2);
       cadd(pol+b, hba);
       cadd(pol+b+2, hba);
      } 
   }
   n_pol *= 2;
   
   // Add zeros
   n_zer= n_pol; 
   for (a= 0; a<n_zer; a++) {
      zertyp[a]= 1;
      zer[a]= (a<n_zer/2) ? 0.0 : -INF;
   }
}

//
//    Adjust raw poles to BS filter.  The number of poles is
//    doubled.
//

static void 
bandstop(double freq1, double freq2) {
   double w0= TWOPI * sqrt(freq1*freq2);
   double bw= 0.5 * TWOPI * (freq2-freq1);
   int a, b;

   if (n_pol * 2 > MAXPZ) 
      error("Maximum order for bandstop filters is %d", MAXPZ/2);

   // Run through the list backwards, expanding as we go
   for (a= n_pol, b= n_pol*2; a>0; ) {
      // hba= bw / pole;
      // temp= c_sqrt(1.0 - square(w0 / hba));
      // pole1= hba * (1.0 + temp);
      // pole2= hba * (1.0 - temp);

      if (poltyp[a-1] == 1) {
       double hba;
       a--; b -= 2;
       poltyp[b]= 2; poltyp[b+1]= 0;
       hba= bw / pol[a];
       cassz(pol+b, 1.0 - (w0 / hba) * (w0 / hba), 0.0);
       c_sqrt(pol+b);
       caddz(pol+b, 1.0, 0.0);
       cmulr(pol+b, hba);
      } else {          // Assume poltyp[] data is valid
       double hba[2];
       a -= 2; b -= 4;
       poltyp[b]= 2; poltyp[b+1]= 0;
       poltyp[b+2]= 2; poltyp[b+3]= 0;
       cass(hba, pol+a);
       crecip(hba);
       cmulr(hba, bw);
       cass(pol+b, hba);
       crecip(pol+b);
       cmulr(pol+b, w0);
       csqu(pol+b);
       cneg(pol+b);
       caddz(pol+b, 1.0, 0.0);
       c_sqrt(pol+b);
       cmul(pol+b, hba);
       cass(pol+b+2, pol+b);
       cneg(pol+b+2);
       cadd(pol+b, hba);
       cadd(pol+b+2, hba);
      } 
   }
   n_pol *= 2;
   
   // Add zeros
   n_zer= n_pol; 
   for (a= 0; a<n_zer; a+=2) {
      zertyp[a]= 2; zertyp[a+1]= 0;
      zer[a]= 0.0; zer[a+1]= w0;
   }
}

//
//    Convert list of poles+zeros from S to Z using bilinear
//    transform
//

static void 
s2z_bilinear() {
   int a;
   for (a= 0; a<n_pol; ) {
      // Calculate (2 + val) / (2 - val)
      if (poltyp[a] == 1) {
       if (pol[a] == -INF) 
          pol[a]= -1.0;
       else 
          pol[a]= (2 + pol[a]) / (2 - pol[a]);
       a++;
      } else {
       double val[2];
       cass(val, pol+a);
       cneg(val);
       caddz(val, 2, 0);
       caddz(pol+a, 2, 0);
       cdiv(pol+a, val);
       a += 2;
      }
   }
   for (a= 0; a<n_zer; ) {
      // Calculate (2 + val) / (2 - val)
      if (zertyp[a] == 1) {
       if (zer[a] == -INF) 
          zer[a]= -1.0;
       else 
          zer[a]= (2 + zer[a]) / (2 - zer[a]);
       a++;
      } else {
       double val[2];
       cass(val, zer+a);
       cneg(val);
       caddz(val, 2, 0);
       caddz(zer+a, 2, 0);
       cdiv(zer+a, val);
       a += 2;
      }
   }
}

//
//    Convert S to Z using matched-Z transform
//
    
static void 
s2z_matchedZ() {
   int a;
   
   for (a= 0; a<n_pol; ) {
      // Calculate cexp(val)
      if (poltyp[a] == 1) {
       if (pol[a] == -INF) 
          pol[a]= 0.0;
       else 
          pol[a]= exp(pol[a]);
       a++;
      } else {
       c_exp(pol+a);
       a += 2;
      }
   }

   for (a= 0; a<n_zer; ) {
      // Calculate cexp(val)
      if (zertyp[a] == 1) {
       if (zer[a] == -INF) 
          zer[a]= 0.0;
       else 
          zer[a]= exp(zer[a]);
       a++;
      } else {
       c_exp(zer+a);
       a += 2;
      }
   }
}

//
//    Generate a FidFilter for the current set of poles and zeros.
//    The given gain is inserted at the start of the FidFilter as a
//    one-coefficient FIR filter.  This is positioned to be easily
//    adjusted later to correct the filter gain.
//
//    'cbm' should be a bitmap indicating which FIR coefficients are
//    constants for this filter type.  Normal values are ~0 for all
//    constant, or 0 for none constant, or some other bitmask for a
//    mixture.  Filters generated with lowpass(), highpass() and
//    bandpass() above should pass ~0, but bandstop() requires 0x5.
//
//    This routine requires that any lone real poles/zeros are at
//    the end of the list.  All other poles/zeros are handled in
//    pairs (whether pairs of real poles/zeros, or conjugate pairs).
//

static FidFilter*
z2fidfilter(double gain, int cbm) {
   int n_head, n_val;
   int a;
   FidFilter *rv;
   FidFilter *ff;

   n_head= 1 + n_pol + n_zer;  // Worst case: gain + 2-element IIR/FIR
   n_val= 1 + 2 * (n_pol+n_zer); //   for each pole/zero

   rv= ff= FFALLOC(n_head, n_val);

   ff->typ= 'F';
   ff->len= 1;
   ff->val[0]= gain;
   ff= FFNEXT(ff);

   // Output as much as possible as 2x2 IIR/FIR filters
   for (a= 0; a <= n_pol-2 && a <= n_zer-2; a += 2) {
      // Look for a pair of values for an IIR
      if (poltyp[a] == 1 && poltyp[a+1] == 1) {
       // Two real values
         ff->typ= 'I';
         ff->len= 3;
         ff->val[0]= 1;
         ff->val[1]= -(pol[a] + pol[a+1]);
         ff->val[2]= pol[a] * pol[a+1];
       ff= FFNEXT(ff); 
      } else if (poltyp[a] == 2) {
       // A complex value and its conjugate pair
         ff->typ= 'I';
         ff->len= 3;
         ff->val[0]= 1;
         ff->val[1]= -2 * pol[a];
         ff->val[2]= pol[a] * pol[a] + pol[a+1] * pol[a+1];
       ff= FFNEXT(ff); 
      } else error("Internal error -- bad poltyp[] values for z2fidfilter()");       

      // Look for a pair of values for an FIR
      if (zertyp[a] == 1 && zertyp[a+1] == 1) {
       // Two real values
       // Skip if constant and 0/0
       if (!cbm || zer[a] != 0.0 || zer[a+1] != 0.0) {
          ff->typ= 'F';
          ff->cbm= cbm;
          ff->len= 3;
          ff->val[0]= 1;
          ff->val[1]= -(zer[a] + zer[a+1]);
          ff->val[2]= zer[a] * zer[a+1];
          ff= FFNEXT(ff); 
       }
      } else if (zertyp[a] == 2) {
       // A complex value and its conjugate pair
       // Skip if constant and 0/0
       if (!cbm || zer[a] != 0.0 || zer[a+1] != 0.0) {
          ff->typ= 'F';
          ff->cbm= cbm;
          ff->len= 3;
          ff->val[0]= 1;
          ff->val[1]= -2 * zer[a];
          ff->val[2]= zer[a] * zer[a] + zer[a+1] * zer[a+1];
          ff= FFNEXT(ff); 
       }
      } else error("Internal error -- bad zertyp[] values");       
   }

   // Clear up any remaining bits and pieces.  Should only be a 1x1
   // IIR/FIR.
   if (n_pol-a == 0 && n_zer-a == 0) 
      ;
   else if (n_pol-a == 1 && n_zer-a == 1) {
      if (poltyp[a] != 1 || zertyp[a] != 1) 
       error("Internal error; bad poltyp or zertyp for final pole/zero");
      ff->typ= 'I';
      ff->len= 2;
      ff->val[0]= 1;
      ff->val[1]= -pol[a];
      ff= FFNEXT(ff); 

      // Skip FIR if it is constant and zero
      if (!cbm || zer[a] != 0.0) {
       ff->typ= 'F';
       ff->cbm= cbm;
       ff->len= 2;
       ff->val[0]= 1;
       ff->val[1]= -zer[a];
       ff= FFNEXT(ff); 
      }
   } else 
      error("Internal error: unexpected poles/zeros at end of list");

   // End of list
   ff->typ= 0;
   ff->len= 0;
   ff= FFNEXT(ff);
   
   rv= realloc(rv, ((char*)ff)-((char*)rv));
   if (!rv) error("Out of memory");
   return rv;
}

//
//    Setup poles/zeros for a band-pass resonator.  'qfact' gives
//    the Q-factor; 0 is a special value indicating +infinity,
//    giving an oscillator.
//

static void 
bandpass_res(double freq, double qfact) {
   double mag;
   double th0, th1, th2;
   double theta= freq * TWOPI;
   double val[2];
   double tmp1[2], tmp2[2], tmp3[2], tmp4[2];
   int cnt;

   n_pol= 2;
   poltyp[0]= 2; poltyp[1]= 0;
   n_zer= 2;
   zertyp[0]= 1; zertyp[1]= 1;
   zer[0]= 1; zer[1]= -1;

   if (qfact == 0.0) {
      cexpj(pol, theta);
      return;
   }

   // Do a full binary search, rather than seeding it as Tony Fisher does
   cexpj(val, theta);
   mag= exp(-theta / (2.0 * qfact));
   th0= 0; th2= M_PI;
   for (cnt= 60; cnt > 0; cnt--) {
      th1= 0.5 * (th0 + th2);
      cexpj(pol, th1);
      cmulr(pol, mag);
      
      // Evaluate response of filter for Z= val
      memcpy(tmp1, val, 2*sizeof(double));
      memcpy(tmp2, val, 2*sizeof(double));
      memcpy(tmp3, val, 2*sizeof(double));
      memcpy(tmp4, val, 2*sizeof(double));
      csubz(tmp1, 1, 0);
      csubz(tmp2, -1, 0);
      cmul(tmp1, tmp2);
      csub(tmp3, pol); cconj(pol);
      csub(tmp4, pol); cconj(pol);
      cmul(tmp3, tmp4);
      cdiv(tmp1, tmp3);
      
      if (fabs(tmp1[1] / tmp1[0]) < 1e-10) break;

      //printf("%-24.16g%-24.16g -> %-24.16g%-24.16g\n", th0, th2, tmp1[0], tmp1[1]);

      if (tmp1[1] > 0.0) th2= th1;
      else th0= th1;
   }

   if (cnt <= 0) fprintf(stderr, "Resonator binary search failed to converge");
}

//
//    Setup poles/zeros for a bandstop resonator
//

static void 
bandstop_res(double freq, double qfact) {
   bandpass_res(freq, qfact);
   zertyp[0]= 2; zertyp[1]= 0;
   cexpj(zer, TWOPI * freq);
}

//
//    Setup poles/zeros for an allpass resonator
//

static void 
allpass_res(double freq, double qfact) {
   bandpass_res(freq, qfact);
   zertyp[0]= 2; zertyp[1]= 0;
   memcpy(zer, pol, 2*sizeof(double));
   cmulr(zer, 1.0 / (zer[0]*zer[0] + zer[1]*zer[1]));
}

//
//    Setup poles/zeros for a proportional-integral filter
//

static void 
prop_integral(double freq) {
   n_pol= 1;
   poltyp[0]= 1;
   pol[0]= 0.0;
   n_zer= 1;
   zertyp[0]= 1;
   zer[0]= -TWOPI * freq;
}
   
// END //

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