rfa 0.5.9

A port ERFA to Rust.
Documentation 
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use super::{pnm06a::*, bpn2xy::*, s06::*};
///  The CIO locator s, positioning the Celestial Intermediate Origin on
///  the equator of the Celestial Intermediate Pole, using the IAU 2006
///  precession and IAU 2000A nutation models.
///
///  Given:
///     date1,date2  double    TT as a 2-part Julian Date (Note 1)
///
///  Returned (function value):
///                  double    the CIO locator s in radians (Note 2)
///
///  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:
///
///            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 CIO locator s is the difference between the right ascensions
///     of the same point in two systems.  The two systems are the GCRS
///     and the CIP,CIO, and the point is the ascending node of the
///     CIP equator.  The CIO locator s remains a small fraction of
///     1 arcsecond throughout 1900-2100.
///
///  3) The series used to compute s is in fact for s+XY/2, where X and Y
///     are the x and y components of the CIP unit vector;  this series is
///     more compact than a direct series for s would be.  The present
///     function uses the full IAU 2000A nutation model when predicting
///     the CIP position.
///
///  Called:
///     eraPnm06a    classical NPB matrix, IAU 2006/2000A
///     eraBpn2xy    extract CIP X,Y coordinates from NPB matrix
///     eraS06       the CIO locator s, given X,Y, IAU 2006
///
///  References:
///
///     Capitaine, N., Chapront, J., Lambert, S. and Wallace, P.,
///     "Expressions for the Celestial Intermediate Pole and Celestial
///     Ephemeris Origin consistent with the IAU 2000A precession-
///     nutation model", Astron.Astrophys. 400, 1145-1154 (2003)
///
///     n.b. The celestial ephemeris origin (CEO) was renamed "celestial
///          intermediate origin" (CIO) by IAU 2006 Resolution 2.
///
///     Capitaine, N. & Wallace, P.T., 2006, Astron.Astrophys. 450, 855
///
///     McCarthy, D. D., Petit, G. (eds.), 2004, IERS Conventions (2003),
///     IERS Technical Note No. 32, BKG
///
///     Wallace, P.T. & Capitaine, N., 2006, Astron.Astrophys. 459, 981
///
///  This revision:  2021 May 11

pub fn s06a(date1: f64, date2: f64)->f64
{
    let mut rnpb = [[0.0; 3]; 3]; 
    let mut x =0.0; let mut y =0.0;
 
 
 /* Bias-precession-nutation-matrix, IAU 20006/2000A. */
    pnm06a(date1, date2, &mut rnpb);
 
 /* Extract the CIP coordinates. */
    bpn2xy(&rnpb, &mut x, &mut y);
 
 /* Compute the CIO locator s, given the CIP coordinates. */
    s06(date1, date2, x, y)
 
 /* Finished. */
 
 }