sofars 0.6.0

Pure Rust implementation of the IAU SOFA library
Documentation 
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use crate::consts::D2PI;

///  (hour angle, declination) to (azimuth, altitude)
///
///  Equatorial to horizon coordinates:  transform hour angle and
///  declination to azimuth and altitude.
///
///  This function is part of the International Astronomical Union's
///  SOFA (Standards of Fundamental Astronomy) software collection.
///
///  Status:  support function.
///
///  Given:
///     ha       double       hour angle (local)
///     dec      double       declination
///     phi      double       site latitude
///
///  Returned:
///     *az      double       azimuth
///     *el      double       altitude (informally, elevation)
///
///  Notes:
///
///  1)  All the arguments are angles in radians.
///
///  2)  Azimuth is returned in the range 0-2pi;  north is zero, and east
///      is +pi/2.  Altitude is returned in the range +/- pi/2.
///
///  3)  The latitude phi is pi/2 minus the angle between the Earth's
///      rotation axis and the adopted zenith.  In many applications it
///      will be sufficient to use the published geodetic latitude of the
///      site.  In very precise (sub-arcsecond) applications, phi can be
///      corrected for polar motion.
///
///  4)  The returned azimuth az is with respect to the rotational north
///      pole, as opposed to the ITRS pole, and for sub-arcsecond
///      accuracy will need to be adjusted for polar motion if it is to
///      be with respect to north on a map of the Earth's surface.
///
///  5)  Should the user wish to work with respect to the astronomical
///      zenith rather than the geodetic zenith, phi will need to be
///      adjusted for deflection of the vertical (often tens of
///      arcseconds), and the zero point of the hour angle ha will also
///      be affected.
///
///  6)  The transformation is the same as Vh = Rz(pi)*Ry(pi/2-phi)*Ve,
///      where Vh and Ve are lefthanded unit vectors in the (az,el) and
///      (ha,dec) systems respectively and Ry and Rz are rotations about
///      first the y-axis and then the z-axis.  (n.b. Rz(pi) simply
///      reverses the signs of the x and y components.)  For efficiency,
///      the algorithm is written out rather than calling other utility
///      functions.  For applications that require even greater
///      efficiency, additional savings are possible if constant terms
///      such as functions of latitude are computed once and for all.
///
///  7)  Again for efficiency, no range checking of arguments is carried
///      out.
///
///  Last revision:   2021 February 24
///
///  SOFA release 2023-10-11
///
///  Copyright (C) 2023 IAU SOFA Board.  See notes at end.
///
pub fn hd2ae(ha: f64, dec: f64, phi: f64) -> [f64; 2] {
    /* Useful trig functions. */
    let sh = ha.sin();
    let ch = ha.cos();
    let sd = dec.sin();
    let cd = dec.cos();
    let sp = phi.sin();
    let cp = phi.cos();

    /* Useful trig functions. */
    let x = -ch * cd * sp + sd * cp;
    let y = -sh * cd;
    let z = ch * cd * cp + sd * sp;

    /* To spherical. */
    let r = (x * x + y * y).sqrt();
    let a = if r != 0.0 { y.atan2(x) } else { 0.0 };
    let az = if a < 0.0 { a + D2PI } else { a };
    let el = z.atan2(r);

    [az, el]
}