sofars 0.6.0

Pure Rust implementation of the IAU SOFA library
Documentation 
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use crate::pnp::{bpn2xy, c2ixy};

///  Form the celestial-to-intermediate matrix for a given date given
///  the bias-precession-nutation matrix.  IAU 2000.
///
///  Given:
///     date1,date2 f64          TT as a 2-part Julian Date (Note 1)
///     rbpn        [[f64; 3]; 3] celestial-to-true matrix (Note 2)
///
///  Returned (function value):
///                 [[f64; 3]; 3] celestial-to-intermediate matrix (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:
///
///            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 matrix rbpn transforms vectors from GCRS to true equator (and
///     CIO or equinox) of date.  Only the CIP (bottom row) is used.
///
///  3) The matrix rc2i is the first stage in the transformation from
///     celestial to terrestrial coordinates:
///
///        [TRS] = RPOM * R_3(ERA) * rc2i * [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), ERA is the Earth
///     Rotation Angle and RPOM is the polar motion matrix.
///
///  4) Although its name does not include "00", This function is in fact
///     specific to the IAU 2000 models.
///
///  Called:
///     iauBpn2xy    extract CIP X,Y coordinates from NPB matrix
///     iauC2ixy     celestial-to-intermediate matrix, given X,Y
///
///  References:
///     "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.
///
///     McCarthy, D. D., Petit, G. (eds.), IERS Conventions (2003),
///     IERS Technical Note No. 32, BKG (2004)
pub fn c2ibpn(date1: f64, date2: f64, rbpn: &[[f64; 3]; 3]) -> [[f64; 3]; 3] {
    /* Extract the X,Y coordinates. */
    let (x, y) = bpn2xy(rbpn);

    /* Form the celestial-to-intermediate matrix (n.b. IAU 2000 specific). */
    c2ixy(date1, date2, x, y)
}