use super::era00;

use crate::pnp::{bpn2xy, eors, s06};

use crate::vm::anp;

///  Greenwich apparent ST, IAU 2006, given NPB matrix

///

///  Greenwich apparent sidereal time, IAU 2006, given the NPB matrix.

///

///  This function is part of the International Astronomical Union's

///  SOFA (Standards of Fundamental Astronomy) software collection.

///

///  Status:  support function.

///

///  Given:

///  ```

///     uta,utb  double        UT1 as a 2-part Julian Date (Notes 1,2)

///     tta,ttb  double        TT as a 2-part Julian Date (Notes 1,2)

///     rnpb     double[3][3]  nutation x precession x bias matrix

///  ```

///  Returned (function value):

///  ```

///              double        Greenwich apparent sidereal time (radians)

///  ```

///  Notes:

///

///  1) The UT1 and TT dates uta+utb and tta+ttb respectively, are both

///     Julian Dates, apportioned in any convenient way between the

///     argument pairs.  For example, JD(UT1)=2450123.7 could be

///     expressed in any of these ways, among others:

///  ```

///             uta            utb

///

///         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 (in the case of UT;  the TT is not at all critical

///     in this respect).  The J2000 and MJD methods are good compromises

///     between resolution and convenience.  For UT, the date & time

///     method is best matched to the algorithm that is used by the Earth

///     rotation angle function, called internally:  maximum precision is

///     delivered when the uta argument is for 0hrs UT1 on the day in

///     question and the utb argument lies in the range 0 to 1, or vice

///     versa.

///

///  2) Both UT1 and TT are required, UT1 to predict the Earth rotation

///     and TT to predict the effects of precession-nutation.  If UT1 is

///     used for both purposes, errors of order 100 microarcseconds

///     result.

///

///  3) Although the function uses the IAU 2006 series for s+XY/2, it is

///     otherwise independent of the precession-nutation model and can in

///     practice be used with any equinox-based NPB matrix.

///

///  4) The result is returned in the range 0 to 2pi.

///

///  Called:

///  ```

///     iauBpn2xy    extract CIP X,Y coordinates from NPB matrix

///     iauS06       the CIO locator s, given X,Y, IAU 2006

///     iauAnp       normalize angle into range 0 to 2pi

///     iauEra00     Earth rotation angle, IAU 2000

///     iauEors      equation of the origins, given NPB matrix and s

///  ```

///  Reference:

///

///     Wallace, P.T. & Capitaine, N., 2006, Astron.Astrophys. 459, 981

///

pub fn gst06(uta: f64, utb: f64, tta: f64, ttb: f64, rnpb: &[[f64; 3]; 3]) -> f64 {

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

    // The CIO locator, s.

    let s = s06(tta, ttb, x, y);

    // Greenwich apparent sidereal time.

    let era = era00(uta, utb);

    let eors = eors(rnpb, s);

    let gst = anp(era - eors);

    gst

}