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

double iauGmst82(double dj1, double dj2)
/*
**  - - - - - - - - - -
**   i a u G m s t 8 2
**  - - - - - - - - - -
**
**  Universal Time to Greenwich mean sidereal time (IAU 1982 model).
**
**  This function is part of the International Astronomical Union's
**  SOFA (Standards of Fundamental Astronomy) software collection.
**
**  Status:  canonical model.
**
**  Given:
**     dj1,dj2    double    UT1 Julian Date (see note)
**
**  Returned (function value):
**                double    Greenwich mean sidereal time (radians)
**
**  Notes:
**
**  1) The UT1 date dj1+dj2 is a Julian Date, apportioned in any
**     convenient way between the arguments dj1 and dj2.  For example,
**     JD(UT1)=2450123.7 could be expressed in any of these ways,
**     among others:
**
**             dj1            dj2
**
**         2450123.7          0          (JD method)
**          2451545        -1421.3       (J2000 method)
**         2400000.5       50123.2       (MJD method)
**         2450123.5         0.2         (date & time method)
**
**     The JD method is the most natural and convenient to use in
**     cases where the loss of several decimal digits of resolution
**     is acceptable.  The J2000 and MJD methods are good compromises
**     between resolution and convenience.  The date & time method is
**     best matched to the algorithm used:  maximum accuracy (or, at
**     least, minimum noise) is delivered when the dj1 argument is for
**     0hrs UT1 on the day in question and the dj2 argument lies in the
**     range 0 to 1, or vice versa.
**
**  2) The algorithm is based on the IAU 1982 expression.  This is
**     always described as giving the GMST at 0 hours UT1.  In fact, it
**     gives the difference between the GMST and the UT, the steady
**     4-minutes-per-day drawing-ahead of ST with respect to UT.  When
**     whole days are ignored, the expression happens to equal the GMST
**     at 0 hours UT1 each day.
**
**  3) In this function, the entire UT1 (the sum of the two arguments
**     dj1 and dj2) is used directly as the argument for the standard
**     formula, the constant term of which is adjusted by 12 hours to
**     take account of the noon phasing of Julian Date.  The UT1 is then
**     added, but omitting whole days to conserve accuracy.
**
**  Called:
**     iauAnp       normalize angle into range 0 to 2pi
**
**  References:
**
**     Transactions of the International Astronomical Union,
**     XVIII B, 67 (1983).
**
**     Aoki et al., Astron.Astrophys., 105, 359-361 (1982).
**
**  This revision:  2021 May 11
**
**  SOFA release 2023-10-11
**
**  Copyright (C) 2023 IAU SOFA Board.  See notes at end.
*/
{
/* Coefficients of IAU 1982 GMST-UT1 model */
   double A = 24110.54841  -  DAYSEC / 2.0;
   double B = 8640184.812866;
   double C = 0.093104;
   double D =  -6.2e-6;

/* The first constant, A, has to be adjusted by 12 hours because the */
/* UT1 is supplied as a Julian date, which begins at noon.           */

   double d1, d2, t, f, gmst;


/* Julian centuries since fundamental epoch. */
   if (dj1 < dj2) {
      d1 = dj1;
      d2 = dj2;
   } else {
      d1 = dj2;
      d2 = dj1;
   }
   t = (d1 + (d2 - DJ00)) / DJC;

/* Fractional part of JD(UT1), in seconds. */
   f = DAYSEC * (fmod(d1, 1.0) + fmod(d2, 1.0));

/* GMST at this UT1. */
   gmst = iauAnp(DS2R * ((A + (B + (C + D * t) * t) * t) + f));

   return gmst;

/* 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
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**
**  5. The SOFA software is provided "as is" and SOFA makes no warranty
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**     to non-infringement of third party rights, merchantability, or
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**     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
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**
**--------------------------------------------------------------------*/
}
