text: #define PJ_LIB__
  #include    <projects.h>
  
  PROJ_HEAD(eck4, "Eckert IV") "\n\tPCyl, Sph.";
  
  #define C_x .42223820031577120149
  #define C_y 1.32650042817700232218
  #define RC_y    .75386330736002178205
  #define C_p 3.57079632679489661922
  #define RC_p    .28004957675577868795
  #define EPS 1e-7
  #define NITER   6
  
  
  static XY s_forward (LP lp, PJ *P) {           /* Spheroidal, forward */
      XY xy = {0.0,0.0};
      double p, V, s, c;
      int i;
      (void) P;
  
      p = C_p * sin(lp.phi);
      V = lp.phi * lp.phi;
      lp.phi *= 0.895168 + V * ( 0.0218849 + V * 0.00826809 );
      for (i = NITER; i ; --i) {
          c = cos(lp.phi);
          s = sin(lp.phi);
          lp.phi -= V = (lp.phi + s * (c + 2.) - p) /
              (1. + c * (c + 2.) - s * s);
          if (fabs(V) < EPS)
              break;
      }
      if (!i) {
          xy.x = C_x * lp.lam;
          xy.y = lp.phi < 0. ? -C_y : C_y;
      } else {
          xy.x = C_x * lp.lam * (1. + cos(lp.phi));
          xy.y = C_y * sin(lp.phi);
      }
      return xy;
  }
  
  
  static LP s_inverse (XY xy, PJ *P) {           /* Spheroidal, inverse */
      LP lp = {0.0,0.0};
      double c;
  
      lp.phi = aasin(P->ctx,xy.y / C_y);
      lp.lam = xy.x / (C_x * (1. + (c = cos(lp.phi))));
      lp.phi = aasin(P->ctx,(lp.phi + sin(lp.phi) * (c + 2.)) / C_p);
      return lp;
  }
  
  
  static void *freeup_new (PJ *P) {                       /* Destructor */
      if (0==P)
          return 0;
      return pj_dealloc(P);
  }
  
  static void freeup (PJ *P) {
      freeup_new (P);
      return;
  }
  
  
  PJ *PROJECTION(eck4) {
      P->es = 0.0;
      P->inv = s_inverse;
      P->fwd = s_forward;
  
      return P;
  }
  
  
  #ifndef PJ_SELFTEST
  int pj_eck4_selftest (void) {return 0;}
  #else
  
  int pj_eck4_selftest (void) {
      double tolerance_lp = 1e-10;
      double tolerance_xy = 1e-7;
  
      char s_args[] = {"+proj=eck4   +a=6400000    +lat_1=0.5 +lat_2=2"};
  
      LP fwd_in[] = {
          { 2, 1},
          { 2,-1},
          {-2, 1},
          {-2,-1}
      };
  
      XY s_fwd_expect[] = {
          { 188646.38935641639,  132268.54017406539},
          { 188646.38935641639, -132268.54017406539},
          {-188646.38935641639,  132268.54017406539},
          {-188646.38935641639, -132268.54017406539},
      };
  
      XY inv_in[] = {
          { 200, 100},
          { 200,-100},
          {-200, 100},
          {-200,-100}
      };
  
      LP s_inv_expect[] = {
          { 0.0021202405520236059, 0.00075601458836610643},
          { 0.0021202405520236059, -0.00075601458836610643},
          {-0.0021202405520236059, 0.00075601458836610643},
          {-0.0021202405520236059, -0.00075601458836610643},
      };
  
      return pj_generic_selftest (0, s_args, tolerance_xy, tolerance_lp, 4, 4, fwd_in, 0, s_fwd_expect, inv_in, 0, s_inv_expect);
  }
  
  
  #endif
  