use crate::consts::{DAS2R, DJ00, DJC};

///  Precession-rate part of the IAU 2000 precession-nutation models

///  (part of MHB2000).

///

///  Given:

///     date1,date2    f64      TT as a 2-part Julian Date (Note 1)

///

///  Returned:

///     (dpsipr,depspr) (f64,f64) precession corrections (Notes 2,3)

///

///  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 precession adjustments are expressed as "nutation

///     components", corrections in longitude and obliquity with respect

///     to the J2000.0 equinox and ecliptic.

///

///  3) Although the precession adjustments are stated to be with respect

///     to Lieske et al. (1977), the MHB2000 model does not specify which

///     set of Euler angles are to be used and how the adjustments are to

///     be applied.  The most literal and straightforward procedure is to

///     adopt the 4-rotation epsilon_0, psi_A, omega_A, xi_A option, and

///     to add dpsipr to psi_A and depspr to both omega_A and eps_A.

///

///  4) This is an implementation of one aspect of the IAU 2000A nutation

///     model, formally adopted by the IAU General Assembly in 2000,

///     namely MHB2000 (Mathews et al. 2002).

///

///  References:

///

///     Lieske, J.H., Lederle, T., Fricke, W. & Morando, B., "Expressions

///     for the precession quantities based upon the IAU (1976) System of

///     Astronomical Constants", Astron.Astrophys., 58, 1-16 (1977)

///

///     Mathews, P.M., Herring, T.A., Buffet, B.A., "Modeling of nutation

///     and precession   New nutation series for nonrigid Earth and

///     insights into the Earth's interior", J.Geophys.Res., 107, B4,

///     2002.

///

///     Wallace, P.T., "Software for Implementing the IAU 2000

///     Resolutions", in IERS Workshop 5.1 (2002).

pub fn pr00(date1: f64, date2: f64) -> (f64, f64) {

    /* Precession and obliquity corrections (radians per century) */

    const PRECOR: f64 = -0.29965 * DAS2R;

    const OBLCOR: f64 = -0.02524 * DAS2R;

    /* Interval between fundamental epoch J2000.0 and given date (JC). */

    let t = ((date1 - DJ00) + date2) / DJC;

    /* Precession rate contributions with respect to IAU 1976/80. */

    let dpsipr = PRECOR * t;

    let depspr = OBLCOR * t;

    (dpsipr, depspr)

}