use super::{bpn2xy, pnm06a, 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.

///

/// 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 (function value):

///                 f64    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:

///

/// ```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 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.

///

/// 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

pub fn s06a(date1: f64, date2: f64) -> f64 {

    /* Bias-precession-nutation-matrix, IAU 20006/2000A. */

    let rnpb = pnm06a(date1, date2);

    /* Extract the CIP coordinates. */

    let (x, y) = bpn2xy(&rnpb);

    /* Compute the CIO locator s, given the CIP coordinates. */

    s06(date1, date2, x, y)

}