#include "sofa.h"
#include "sofam.h"

int iauGc2gde ( double a, double f, double xyz[3],
                double *elong, double *phi, double *height )
/*
**  - - - - - - - - - -
**   i a u G c 2 g d e
**  - - - - - - - - - -
**
**  Transform geocentric coordinates to geodetic for a reference
**  ellipsoid of specified form.
**
**  This function is part of the International Astronomical Union's
**  SOFA (Standards of Fundamental Astronomy) software collection.
**
**  Status:  support function.
**
**  Given:
**     a       double     equatorial radius (Notes 2,4)
**     f       double     flattening (Note 3)
**     xyz     double[3]  geocentric vector (Note 4)
**
**  Returned:
**     elong   double     longitude (radians, east +ve)
**     phi     double     latitude (geodetic, radians)
**     height  double     height above ellipsoid (geodetic, Note 4)
**
**  Returned (function value):
**             int        status:  0 = OK
**                                -1 = illegal f
**                                -2 = illegal a
**
**  Notes:
**
**  1) This function is based on the GCONV2H Fortran subroutine by
**     Toshio Fukushima (see reference).
**
**  2) The equatorial radius, a, can be in any units, but meters is
**     the conventional choice.
**
**  3) The flattening, f, is (for the Earth) a value around 0.00335,
**     i.e. around 1/298.
**
**  4) The equatorial radius, a, and the geocentric vector, xyz,
**     must be given in the same units, and determine the units of
**     the returned height, height.
**
**  5) If an error occurs (status < 0), elong, phi and height are
**     unchanged.
**
**  6) The inverse transformation is performed in the function
**     iauGd2gce.
**
**  7) The transformation for a standard ellipsoid (such as WGS84) can
**     more conveniently be performed by calling iauGc2gd, which uses a
**     numerical code to identify the required A and F values.
**
**  Reference:
**
**     Fukushima, T., "Transformation from Cartesian to geodetic
**     coordinates accelerated by Halley's method", J.Geodesy (2006)
**     79: 689-693
**
**  This revision:  2021 May 11
**
**  SOFA release 2023-10-11
**
**  Copyright (C) 2023 IAU SOFA Board.  See notes at end.
*/
{
   double aeps2, e2, e4t, ec2, ec, b, x, y, z, p2, absz, p, s0, pn, zc,
                 c0, c02, c03, s02, s03, a02, a0, a03, d0, f0, b0, s1,
                 cc, s12, cc2;


/* ------------- */
/* Preliminaries */
/* ------------- */

/* Validate ellipsoid parameters. */
   if ( f < 0.0 || f >= 1.0 ) return -1;
   if ( a <= 0.0 ) return -2;

/* Functions of ellipsoid parameters (with further validation of f). */
   aeps2 = a*a * 1e-32;
   e2 = (2.0 - f) * f;
   e4t = e2*e2 * 1.5;
   ec2 = 1.0 - e2;
   if ( ec2 <= 0.0 ) return -1;
   ec = sqrt(ec2);
   b = a * ec;

/* Cartesian components. */
   x = xyz[0];
   y = xyz[1];
   z = xyz[2];

/* Distance from polar axis squared. */
   p2 = x*x + y*y;

/* Longitude. */
   *elong = p2 > 0.0 ? atan2(y, x) : 0.0;

/* Unsigned z-coordinate. */
   absz = fabs(z);

/* Proceed unless polar case. */
   if ( p2 > aeps2 ) {

   /* Distance from polar axis. */
      p = sqrt(p2);

   /* Normalization. */
      s0 = absz / a;
      pn = p / a;
      zc = ec * s0;

   /* Prepare Newton correction factors. */
      c0 = ec * pn;
      c02 = c0 * c0;
      c03 = c02 * c0;
      s02 = s0 * s0;
      s03 = s02 * s0;
      a02 = c02 + s02;
      a0 = sqrt(a02);
      a03 = a02 * a0;
      d0 = zc*a03 + e2*s03;
      f0 = pn*a03 - e2*c03;

   /* Prepare Halley correction factor. */
      b0 = e4t * s02 * c02 * pn * (a0 - ec);
      s1 = d0*f0 - b0*s0;
      cc = ec * (f0*f0 - b0*c0);

   /* Evaluate latitude and height. */
      *phi = atan(s1/cc);
      s12 = s1 * s1;
      cc2 = cc * cc;
      *height = (p*cc + absz*s1 - a * sqrt(ec2*s12 + cc2)) /
                                                        sqrt(s12 + cc2);
   } else {

   /* Exception: pole. */
      *phi = DPI / 2.0;
      *height = absz - b;
   }

/* Restore sign of latitude. */
   if ( z < 0 ) *phi = -*phi;

/* OK status. */
   return 0;

/* Finished. */

/*----------------------------------------------------------------------
**
**  Copyright (C) 2023
**  Standards of Fundamental Astronomy Board
**  of the International Astronomical Union.
**
**  =====================
**  SOFA Software License
**  =====================
**
**  NOTICE TO USER:
**
**  BY USING THIS SOFTWARE YOU ACCEPT THE FOLLOWING SIX TERMS AND
**  CONDITIONS WHICH APPLY TO ITS USE.
**
**  1. The Software is owned by the IAU SOFA Board ("SOFA").
**
**  2. Permission is granted to anyone to use the SOFA software for any
**     purpose, including commercial applications, free of charge and
**     without payment of royalties, subject to the conditions and
**     restrictions listed below.
**
**  3. You (the user) may copy and distribute SOFA source code to others,
**     and use and adapt its code and algorithms in your own software,
**     on a world-wide, royalty-free basis.  That portion of your
**     distribution that does not consist of intact and unchanged copies
**     of SOFA source code files is a "derived work" that must comply
**     with the following requirements:
**
**     a) Your work shall be marked or carry a statement that it
**        (i) uses routines and computations derived by you from
**        software provided by SOFA under license to you; and
**        (ii) does not itself constitute software provided by and/or
**        endorsed by SOFA.
**
**     b) The source code of your derived work must contain descriptions
**        of how the derived work is based upon, contains and/or differs
**        from the original SOFA software.
**
**     c) The names of all routines in your derived work shall not
**        include the prefix "iau" or "sofa" or trivial modifications
**        thereof such as changes of case.
**
**     d) The origin of the SOFA components of your derived work must
**        not be misrepresented;  you must not claim that you wrote the
**        original software, nor file a patent application for SOFA
**        software or algorithms embedded in the SOFA software.
**
**     e) These requirements must be reproduced intact in any source
**        distribution and shall apply to anyone to whom you have
**        granted a further right to modify the source code of your
**        derived work.
**
**     Note that, as originally distributed, the SOFA software is
**     intended to be a definitive implementation of the IAU standards,
**     and consequently third-party modifications are discouraged.  All
**     variations, no matter how minor, must be explicitly marked as
**     such, as explained above.
**
**  4. You shall not cause the SOFA software to be brought into
**     disrepute, either by misuse, or use for inappropriate tasks, or
**     by inappropriate modification.
**
**  5. The SOFA software is provided "as is" and SOFA makes no warranty
**     as to its use or performance.   SOFA does not and cannot warrant
**     the performance or results which the user may obtain by using the
**     SOFA software.  SOFA makes no warranties, express or implied, as
**     to non-infringement of third party rights, merchantability, or
**     fitness for any particular purpose.  In no event will SOFA be
**     liable to the user for any consequential, incidental, or special
**     damages, including any lost profits or lost savings, even if a
**     SOFA representative has been advised of such damages, or for any
**     claim by any third party.
**
**  6. The provision of any version of the SOFA software under the terms
**     and conditions specified herein does not imply that future
**     versions will also be made available under the same terms and
**     conditions.
*
**  In any published work or commercial product which uses the SOFA
**  software directly, acknowledgement (see www.iausofa.org) is
**  appreciated.
**
**  Correspondence concerning SOFA software should be addressed as
**  follows:
**
**      By email:  sofa@ukho.gov.uk
**      By post:   IAU SOFA Center
**                 HM Nautical Almanac Office
**                 UK Hydrographic Office
**                 Admiralty Way, Taunton
**                 Somerset, TA1 2DN
**                 United Kingdom
**
**--------------------------------------------------------------------*/
}
