#include "sofa.h"

int iauUt1utc(double ut11, double ut12, double dut1,
              double *utc1, double *utc2)
/*
**  - - - - - - - - - -
**   i a u U t 1 u t c
**  - - - - - - - - - -
**
**  Time scale transformation:  Universal Time, UT1, to Coordinated
**  Universal Time, UTC.
**
**  This function is part of the International Astronomical Union's
**  SOFA (Standards of Fundamental Astronomy) software collection.
**
**  Status:  canonical.
**
**  Given:
**     ut11,ut12  double   UT1 as a 2-part Julian Date (Note 1)
**     dut1       double   Delta UT1: UT1-UTC in seconds (Note 2)
**
**  Returned:
**     utc1,utc2  double   UTC as a 2-part quasi Julian Date (Notes 3,4)
**
**  Returned (function value):
**                int      status: +1 = dubious year (Note 5)
**                                  0 = OK
**                                 -1 = unacceptable date
**
**  Notes:
**
**  1) ut11+ut12 is Julian Date, apportioned in any convenient way
**     between the two arguments, for example where ut11 is the Julian
**     Day Number and ut12 is the fraction of a day.  The returned utc1
**     and utc2 form an analogous pair, except that a special convention
**     is used, to deal with the problem of leap seconds - see Note 3.
**
**  2) Delta UT1 can be obtained from tabulations provided by the
**     International Earth Rotation and Reference Systems Service.  The
**     value changes abruptly by 1s at a leap second;  however, close to
**     a leap second the algorithm used here is tolerant of the "wrong"
**     choice of value being made.
**
**  3) JD cannot unambiguously represent UTC during a leap second unless
**     special measures are taken.  The convention in the present
**     function is that the returned quasi JD day UTC1+UTC2 represents
**     UTC days whether the length is 86399, 86400 or 86401 SI seconds.
**
**  4) The function iauD2dtf can be used to transform the UTC quasi-JD
**     into calendar date and clock time, including UTC leap second
**     handling.
**
**  5) The warning status "dubious year" flags UTCs that predate the
**     introduction of the time scale or that are too far in the future
**     to be trusted.  See iauDat for further details.
**
**  Called:
**     iauJd2cal    JD to Gregorian calendar
**     iauDat       delta(AT) = TAI-UTC
**     iauCal2jd    Gregorian calendar to JD
**
**  References:
**
**     McCarthy, D. D., Petit, G. (eds.), IERS Conventions (2003),
**     IERS Technical Note No. 32, BKG (2004)
**
**     Explanatory Supplement to the Astronomical Almanac,
**     P. Kenneth Seidelmann (ed), University Science Books (1992)
**
**  This revision:  2019 June 20
**
**  SOFA release 2020-07-21
**
**  Copyright (C) 2020 IAU SOFA Board.  See notes at end.
*/
{
   int big1;
   int i, iy, im, id, js;
   double duts, u1, u2, d1, dats1, d2, fd, dats2, ddats, us1, us2, du;


/* UT1-UTC in seconds. */
   duts = dut1;

/* Put the two parts of the UT1 into big-first order. */
   big1 = ( fabs(ut11) >= fabs(ut12) );
   if ( big1 ) {
      u1 = ut11;
      u2 = ut12;
   } else {
      u1 = ut12;
      u2 = ut11;
   }

/* See if the UT1 can possibly be in a leap-second day. */
   d1 = u1;
   dats1 = 0;
   for ( i = -1; i <= 3; i++ ) {
      d2 = u2 + (double) i;
      if ( iauJd2cal(d1, d2, &iy, &im, &id, &fd) ) return -1;
      js = iauDat(iy, im, id, 0.0, &dats2);
      if ( js < 0 ) return -1;
      if ( i == - 1 ) dats1 = dats2;
      ddats = dats2 - dats1;
      if ( fabs(ddats) >= 0.5 ) {

      /* Yes, leap second nearby: ensure UT1-UTC is "before" value. */
         if ( ddats * duts >= 0 ) duts -= ddats;

      /* UT1 for the start of the UTC day that ends in a leap. */
         if ( iauCal2jd(iy, im, id, &d1, &d2) ) return -1;
         us1 = d1;
         us2 = d2 - 1.0 + duts/DAYSEC;

      /* Is the UT1 after this point? */
         du = u1 - us1;
         du += u2 - us2;
         if ( du > 0 ) {

         /* Yes:  fraction of the current UTC day that has elapsed. */
            fd = du * DAYSEC / ( DAYSEC + ddats );

         /* Ramp UT1-UTC to bring about SOFA's JD(UTC) convention. */
            duts += ddats * ( fd <= 1.0 ? fd : 1.0 );
         }

      /* Done. */
         break;
      }
      dats1 = dats2;
   }

/* Subtract the (possibly adjusted) UT1-UTC from UT1 to give UTC. */
   u2 -= duts / DAYSEC;

/* Result, safeguarding precision. */
   if ( big1 ) {
      *utc1 = u1;
      *utc2 = u2;
   } else {
      *utc1 = u2;
      *utc2 = u1;
   }

/* Status. */
   return js;

/*----------------------------------------------------------------------
**
**  Copyright (C) 2020
**  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
**
**--------------------------------------------------------------------*/
}
