use crate::vm::{cr, rxr, rz};

///  Assemble the celestial to terrestrial matrix from equinox-based

///  components (the celestial-to-true matrix, the Greenwich Apparent

///  Sidereal Time and the polar motion matrix).

///

///  Given:

///     rbpn   [[f64; 3]; 3]  celestial-to-true matrix

///     gst    f64            Greenwich (apparent) Sidereal Time (radians)

///     rpom   [[f64; 3]; 3]  polar-motion matrix

///

///  Returned (function value):

///                 [[f64; 3]; 3]  celestial-to-terrestrial matrix (Note 2)

///

///  Notes:

///

///  1) This function constructs the rotation matrix that transforms

///     vectors in the celestial system into vectors in the terrestrial

///     system.  It does so starting from precomputed components, namely

///     the matrix which rotates from celestial coordinates to the

///     true equator and equinox of date, the Greenwich Apparent Sidereal

///     Time and the polar motion matrix.  One use of the present function

///     is when generating a series of celestial-to-terrestrial matrices

///     where only the Sidereal Time changes, avoiding the considerable

///     overhead of recomputing the precession-nutation more often than

///     necessary to achieve given accuracy objectives.

///

///  2) The relationship between the arguments is as follows:

///

///        [TRS] = rpom * R_3(gst) * rbpn * [CRS]

///

///              = rc2t * [CRS]

///

///     where [CRS] is a vector in the Geocentric Celestial Reference

///     System and [TRS] is a vector in the International Terrestrial

///     Reference System (see IERS Conventions 2003).

///

///  Called:

///     iauCr        copy r-matrix

///     iauRz        rotate around Z-axis

///     iauRxr       product of two r-matrices

///

///  Reference:

///

///     McCarthy, D. D., Petit, G. (eds.), IERS Conventions (2003),

///     IERS Technical Note No. 32, BKG (2004)

pub fn c2teqx(rbpn: &[[f64; 3]; 3], gst: f64, rpom: &[[f64; 3]; 3]) -> [[f64; 3]; 3] {

    let mut r = [[0.0; 3]; 3];

    let mut rc2t = [[0.0; 3]; 3];

    /* Construct the matrix. */

    cr(rbpn, &mut r);

    rz(gst, &mut r);

    rxr(rpom, &r, &mut rc2t);

    rc2t

}