sofars 0.6.0

Pure Rust implementation of the IAU SOFA library
Documentation 
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use super::{bpn2xy, pnm06a, s06};

/// For a given TT date, compute the X,Y coordinates of the Celestial
/// Intermediate Pole and the CIO locator s, using the IAU 2006
/// precession and IAU 2000A nutation models.
///
/// This function is part of the International Astronomical Union's
/// SOFA (Standards of Fundamental Astronomy) software collection.
///
/// Status:  support function.
///
/// Given:
///    date1,date2  f64   TT as a 2-part Julian Date (Note 1)
///
/// Returned:
///    (x, y, s)    (f64, f64, f64)
///                 x,y: Celestial Intermediate Pole (Note 2)
///                 s: the CIO locator s (Note 3)
///
/// Notes:
///
/// 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:
///
/// ```text
///           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) The Celestial Intermediate Pole coordinates are the x,y
///    components of the unit vector in the Geocentric Celestial
///    Reference System.
///
/// 3) The CIO locator s (in radians) positions the Celestial
///    Intermediate Origin on the equator of the CIP.
///
/// 4) Use of this function provides results of sub-mas accuracy for
///    the interval 1995-2050.
///
/// References:
///
///    McCarthy, D. D., Petit, G. (eds.), IERS Conventions (2003),
///    IERS Technical Note No. 32, BKG (2004)
pub fn xys06a(date1: f64, date2: f64) -> (f64, f64, f64) {
    /* Form the bias-precession-nutation matrix, IAU 2006/2000A. */
    let rbpn = pnm06a(date1, date2);

    /* Extract X,Y. */
    let (x, y) = bpn2xy(&rbpn);

    /* Obtain s. */
    let s = s06(date1, date2, x, y);

    (x, y, s)
}