use std::ops::Rem;

use crate::consts::{D2PI, DJ00};

use crate::vm::anp;

///  Earth rotation angle (IAU 2000 model).

///

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

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

///

///  Status:  canonical model.

///

///  Given:

///  ```

///     dj1,dj2   double    UT1 as a 2-part Julian Date (see note)

///  ```

///  Returned (function value):

///  ```

///               double    Earth rotation angle (radians), range 0-2pi

///  ```

///  Notes:

///

///  1) The UT1 date dj1+dj2 is a Julian Date, apportioned in any

///     convenient way between the arguments dj1 and dj2.  For example,

///     JD(UT1)=2450123.7 could be expressed in any of these ways,

///     among others:

///  ```

///             dj1            dj2

///

///         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 and MJD methods are good compromises

///     between resolution and convenience.  The date & time method is

///     best matched to the algorithm used:  maximum precision is

///     delivered when the dj1 argument is for 0hrs UT1 on the day in

///     question and the dj2 argument lies in the range 0 to 1, or vice

///     versa.

///

///  2) The algorithm is adapted from Expression 22 of Capitaine et al.

///     2000.  The time argument has been expressed in days directly,

///     and, to retain precision, integer contributions have been

///     eliminated.  The same formulation is given in IERS Conventions

///     (2003), Chap. 5, Eq. 14.

///

///  Called:

///  ```

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

///  ```

///  References:

///

///     Capitaine N., Guinot B. and McCarthy D.D, 2000, Astron.

///     Astrophys., 355, 398-405.

///

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

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

pub fn era00(dj1: f64, dj2: f64) -> f64 {

    let d1: f64;

    let d2: f64;

    // Days since fundamental epoch.

    if dj1 < dj2 {

        d1 = dj1;

        d2 = dj2;

    } else {

        d1 = dj2;

        d2 = dj1;

    }

    let t = d1 + (d2 - DJ00);

    // Fractional part of T (days).

    let f = d1.rem(1.0) + d2.rem(1.0);

    // Earth rotation angle at this UT1.

    let theta = anp(D2PI * (f + 0.7790572732640 + 0.00273781191135448 * t));

    theta

}