text: #define PJ_LIB__
  
  #include <math.h>
  
  #include "proj.h"
  #include "proj_internal.h"
  
  PROJ_HEAD(nell_h, "Nell-Hammer") "\n\tPCyl, Sph";
  
  #define NITER 9
  #define EPS 1e-7
  
  
  static PJ_XY s_forward (PJ_LP lp, PJ *P) {           /* Spheroidal, forward */
      PJ_XY xy = {0.0,0.0};
      (void) P;
  
      xy.x = 0.5 * lp.lam * (1. + cos(lp.phi));
      xy.y = 2.0 * (lp.phi - tan(0.5 *lp.phi));
  
      return xy;
  }
  
  
  static PJ_LP s_inverse (PJ_XY xy, PJ *P) {           /* Spheroidal, inverse */
      PJ_LP lp = {0.0,0.0};
      double V, c, p;
      int i;
      (void) P;
  
      p = 0.5 * xy.y;
      for (i = NITER; i ; --i) {
          c = cos(0.5 * lp.phi);
          lp.phi -= V = (lp.phi - tan(lp.phi/2) - p)/(1. - 0.5/(c*c));
          if (fabs(V) < EPS)
              break;
      }
      if (!i) {
          lp.phi = p < 0. ? -M_HALFPI : M_HALFPI;
          lp.lam = 2. * xy.x;
      } else
          lp.lam = 2. * xy.x / (1. + cos(lp.phi));
  
      return lp;
  }
  
  
  PJ *PROJECTION(nell_h) {
      P->es = 0.;
      P->inv = s_inverse;
      P->fwd = s_forward;
  
      return P;
  }
  