sofars 0.6.0

Pure Rust implementation of the IAU SOFA library
Documentation 
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use crate::consts::{DAS2R, DJ00, DJC};
use crate::vm::anp;

use super::era00;

///  Greenwich mean sidereal time, IAU 2000
///
///  Greenwich mean sidereal time (model consistent with IAU 2000
///  resolutions).
///
///  This function is part of the International Astronomical Union's
///  SOFA (Standards of Fundamental Astronomy) software collection.
///
///  Status:  canonical model.
///
///  Given:
///  ```
///     uta,utb    double    UT1 as a 2-part Julian Date (Notes 1,2)
///     tta,ttb    double    TT as a 2-part Julian Date (Notes 1,2)
///  ```
///  Returned (function value):
///  ```
///                double    Greenwich mean sidereal time (radians)
///  ```
///  Notes:
///
///  1) The UT1 and TT dates uta+utb and tta+ttb respectively, are both
///     Julian Dates, apportioned in any convenient way between the
///     argument pairs.  For example, JD(UT1)=2450123.7 could be
///     expressed in any of these ways, among others:
///  ```
///            Part A         Part B
///
///         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 (in the case of UT;  the TT is not at all critical
///     in this respect).  The J2000 and MJD methods are good compromises
///     between resolution and convenience.  For UT, the date & time
///     method is best matched to the algorithm that is used by the Earth
///     Rotation Angle function, called internally:  maximum precision is
///     delivered when the uta argument is for 0hrs UT1 on the day in
///     question and the utb argument lies in the range 0 to 1, or vice
///     versa.
///
///  2) Both UT1 and TT are required, UT1 to predict the Earth rotation
///     and TT to predict the effects of precession.  If UT1 is used for
///     both purposes, errors of order 100 microarcseconds result.
///
///  3) This GMST is compatible with the IAU 2000 resolutions and must be
///     used only in conjunction with other IAU 2000 compatible
///     components such as precession-nutation and equation of the
///     equinoxes.
///
///  4) The result is returned in the range 0 to 2pi.
///
///  5) The algorithm is from Capitaine et al. (2003) and IERS
///     Conventions 2003.
///
///  Called:
///  ```
///     iauEra00     Earth rotation angle, IAU 2000
///     iauAnp       normalize angle into range 0 to 2pi
///  ```
///  References:
///
///     Capitaine, N., Wallace, P.T. and McCarthy, D.D., "Expressions to
///     implement the IAU 2000 definition of UT1", Astronomy &
///     Astrophysics, 406, 1135-1149 (2003)
///
///     McCarthy, D. D., Petit, G. (eds.), IERS Conventions (2003),
///     IERS Technical Note No. 32, BKG (2004)
pub fn gmst00(uta: f64, utb: f64, tta: f64, ttb: f64) -> f64 {
    // TT Julian centuries since J2000.0.
    let t = ((tta - DJ00) + ttb) / DJC;

    // Greenwich Mean Sidereal Time, IAU 2000.
    let gmst = anp(era00(uta, utb)
        + (0.014506
            + (4612.15739966 + (1.39667721 + (-0.00009344 + (0.00001882) * t) * t) * t) * t)
            * DAS2R);

    gmst
}