sofars 0.6.0

Pure Rust implementation of the IAU SOFA library
Documentation 
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use super::{nutm80, pmat76};
use crate::vm::rxr;

/// Form the matrix of precession/nutation for a given date, IAU 1976
/// precession model, IAU 1980 nutation model.
///
/// 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:
///    rmatpn      f64[3][3]    combined precession/nutation matrix
///
/// 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 matrix operates in the sense V(date) = rmatpn * V(J2000),
///    where the p-vector V(date) is with respect to the true equatorial
///    triad of date date1+date2 and the p-vector V(J2000) is with
///    respect to the mean equatorial triad of epoch J2000.0.
///
/// References:
///
///    Explanatory Supplement to the Astronomical Almanac,
///    P. Kenneth Seidelmann (ed), University Science Books (1992),
///    Section 3.3 (p145).
pub fn pnm80(date1: f64, date2: f64) -> [[f64; 3]; 3] {
    let mut rmatpn = [[0.0; 3]; 3];

    /* Precession matrix, J2000.0 to date. */
    let rmatp = pmat76(date1, date2);

    /* Nutation matrix. */
    let rmatn = nutm80(date1, date2);

    /* Combine the matrices:  PN = N x P. */
    rxr(&rmatn, &rmatp, &mut rmatpn);

    rmatpn
}