sofars 0.6.0

Pure Rust implementation of the IAU SOFA library
Documentation 
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use std::ops::Rem;

use crate::consts::{D2PI, DJ00};
use crate::vm::anp;

///  Earth rotation angle (IAU 2000 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 as a 2-part Julian Date (see note)
///  ```
///  Returned (function value):
///  ```
///               double    Earth rotation angle (radians), range 0-2pi
///  ```
///  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 precision 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 adapted from Expression 22 of Capitaine et al.
///     2000.  The time argument has been expressed in days directly,
///     and, to retain precision, integer contributions have been
///     eliminated.  The same formulation is given in IERS Conventions
///     (2003), Chap. 5, Eq. 14.
///
///  Called:
///  ```
///     iauAnp       normalize angle into range 0 to 2pi
///  ```
///  References:
///
///     Capitaine N., Guinot B. and McCarthy D.D, 2000, Astron.
///     Astrophys., 355, 398-405.
///
///     McCarthy, D. D., Petit, G. (eds.), IERS Conventions (2003),
///     IERS Technical Note No. 32, BKG (2004)
pub fn era00(dj1: f64, dj2: f64) -> f64 {
    let d1: f64;
    let d2: f64;

    // Days since fundamental epoch.
    if dj1 < dj2 {
        d1 = dj1;
        d2 = dj2;
    } else {
        d1 = dj2;
        d2 = dj1;
    }

    let t = d1 + (d2 - DJ00);

    // Fractional part of T (days).
    let f = d1.rem(1.0) + d2.rem(1.0);

    // Earth rotation angle at this UT1.
    let theta = anp(D2PI * (f + 0.7790572732640 + 0.00273781191135448 * t));

    theta
}