sofars 0.6.0

Pure Rust implementation of the IAU SOFA library
Documentation 
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use crate::coords::ecm06;
use crate::vm::{anp, anpm, c2s, rxp, s2c};

/// Transformation from ICRS equatorial coordinates to ecliptic
/// coordinates (mean equinox and ecliptic of date) using IAU 2006
/// precession model.
///
/// Status:  support function.
///
/// Given:
///    date1,date2 f64 TT as a 2-part Julian date (Note 1)
///    dr,dd       f64 ICRS right ascension and declination (radians)
///
/// Returned:
///    dl,db       f64 ecliptic longitude and latitude (radians)
///
/// 1) The TT date date1+date2 is a Julian Date, apportioned in any
///    convenient way between the two arguments.  For example,
///    JD(TT)=2450123.7 could be expressed in any of these ways,
///    among others:
///
///           date1          date2
///
///        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 method is best matched to the way
///    the argument is handled internally and will deliver the
///    optimum resolution.  The MJD method and the date & time methods
///    are both good compromises between resolution and convenience.
///
/// 2) No assumptions are made about whether the coordinates represent
///    starlight and embody astrometric effects such as parallax or
///    aberration.
///
/// 3) The transformation is approximately that from mean J2000.0 right
///    ascension and declination to ecliptic longitude and latitude
///    (mean equinox and ecliptic of date), with only frame bias (always
///    less than 25 mas) to disturb this classical picture.
pub fn eqec06(date1: f64, date2: f64, dr: f64, dd: f64) -> (f64, f64) {
    let mut v2 = [0.0; 3];

    /* Spherical to Cartesian. */
    let v1 = s2c(dr, dd);

    /* Rotation matrix, ICRS equatorial to ecliptic. */
    let rm = ecm06(date1, date2);

    /* The transformation from ICRS to ecliptic. */
    rxp(&rm, &v1, &mut v2);

    /* Cartesian to spherical. */
    let (a, b) = c2s(&v2);

    /* Express in conventional ranges. */
    (anp(a), anpm(b))
}