#include "sofa.h"

void iauIcrs2g ( double dr, double dd, double *dl, double *db )
/*
**  - - - - - - - - - -
**   i a u I c r s 2 g
**  - - - - - - - - - -
**
**  Transformation from ICRS to Galactic coordinates.
**
**  This function is part of the International Astronomical Union's
**  SOFA (Standards of Fundamental Astronomy) software collection.
**
**  Status:  support function.
**
**  Given:
**     dr     double      ICRS right ascension (radians)
**     dd     double      ICRS declination (radians)
**
**  Returned:
**     dl     double      Galactic longitude (radians)
**     db     double      Galactic latitude (radians)
**
**  Notes:
**
**  1) The IAU 1958 system of Galactic coordinates was defined with
**     respect to the now obsolete reference system FK4 B1950.0.  When
**     interpreting the system in a modern context, several factors have
**     to be taken into account:
**
**     . The inclusion in FK4 positions of the E-terms of aberration.
**
**     . The distortion of the FK4 proper motion system by differential
**       Galactic rotation.
**
**     . The use of the B1950.0 equinox rather than the now-standard
**       J2000.0.
**
**     . The frame bias between ICRS and the J2000.0 mean place system.
**
**     The Hipparcos Catalogue (Perryman & ESA 1997) provides a rotation
**     matrix that transforms directly between ICRS and Galactic
**     coordinates with the above factors taken into account.  The
**     matrix is derived from three angles, namely the ICRS coordinates
**     of the Galactic pole and the longitude of the ascending node of
**     the Galactic equator on the ICRS equator.  They are given in
**     degrees to five decimal places and for canonical purposes are
**     regarded as exact.  In the Hipparcos Catalogue the matrix
**     elements are given to 10 decimal places (about 20 microarcsec).
**     In the present SOFA function the matrix elements have been
**     recomputed from the canonical three angles and are given to 30
**     decimal places.
**
**  2) The inverse transformation is performed by the function iauG2icrs.
**
**  Called:
**     iauAnp       normalize angle into range 0 to 2pi
**     iauAnpm      normalize angle into range +/- pi
**     iauS2c       spherical coordinates to unit vector
**     iauRxp       product of r-matrix and p-vector
**     iauC2s       p-vector to spherical
**
**  Reference:
**     Perryman M.A.C. & ESA, 1997, ESA SP-1200, The Hipparcos and Tycho
**     catalogues.  Astrometric and photometric star catalogues
**     derived from the ESA Hipparcos Space Astrometry Mission.  ESA
**     Publications Division, Noordwijk, Netherlands.
**
**  This revision:   2023 April 16
**
**  SOFA release 2023-10-11
**
**  Copyright (C) 2023 IAU SOFA Board.  See notes at end.
*/
{
   double v1[3], v2[3];

/*
**  L2,B2 system of Galactic coordinates in the form presented in the
**  Hipparcos Catalogue.  In degrees:
**
**  P = 192.85948    right ascension of the Galactic north pole in ICRS
**  Q =  27.12825    declination of the Galactic north pole in ICRS
**  R =  32.93192    Galactic longitude of the ascending node of
**                   the Galactic equator on the ICRS equator
**
**  ICRS to Galactic rotation matrix, obtained by computing
**  R_3(-R) R_1(pi/2-Q) R_3(pi/2+P) to the full precision shown:
*/
   double r[3][3] = { { -0.054875560416215368492398900454,
                        -0.873437090234885048760383168409,
                        -0.483835015548713226831774175116 },
                      { +0.494109427875583673525222371358,
                        -0.444829629960011178146614061616,
                        +0.746982244497218890527388004556 },
                      { -0.867666149019004701181616534570,
                        -0.198076373431201528180486091412,
                        +0.455983776175066922272100478348 } };


/* Spherical to Cartesian. */
   iauS2c(dr, dd, v1);

/* ICRS to Galactic. */
   iauRxp(r, v1, v2);

/* Cartesian to spherical. */
   iauC2s(v2, dl, db);

/* Express in conventional ranges. */
   *dl = iauAnp(*dl);
   *db = iauAnpm(*db);

/* 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
**
**--------------------------------------------------------------------*/
}
