rfa 0.5.9

A port ERFA to Rust.
Documentation 
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use crate::rfam::*;
use crate::vector_matrix::angle_ops::anp::*;
///  Universal Time to Greenwich mean sidereal time (IAU 1982 model).
///
///  Given:
///   * dj1,dj2 UT1 Julian Date (see note)
///
///  Returned (function value):
///   * 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    |       0.0    |  (JD method)         |
///     |2451545.0    |   -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:
///   * anp       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
pub fn gmst82(dj1: f64, dj2: f64)->f64
{
/* Coefficients of IAU 1982 GMST-UT1 model */
   const A: f64 = 24110.54841  -  URSA_DAYSEC / 2.0;
   const B: f64 = 8640184.812866;
   const C: f64 = 0.093104;
   const D: f64 =  -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.           */

   let d1: f64; let d2: f64; 


/* Julian centuries since fundamental epoch. */
   if dj1 < dj2 {
      d1 = dj1;
      d2 = dj2;
   } else {
      d1 = dj2;
      d2 = dj1;
   }
   let t = (d1 + (d2 - URSA_DJ00)) / URSA_DJC;

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

/* GMST at this UT1. */
   anp(URSA_DS2R * ((A + (B + (C + D * t) * t) * t) + f))
/* Finished. */

}