//! Keplerian orbital elements and two-body propagation.
//!
//! A Keplerian element set describes an orbit as an ellipse, a parabola, or
//! a hyperbola with a fixed orientation in space. From the six elements and
//! the gravitational parameter, this crate computes the position and
//! velocity at any time under the two-body assumption, and converts in the
//! other direction.
//!
//! The two-body model includes only the point mass gravity of the central
//! body. It omits the oblateness of the Earth, drag, and third body
//! perturbations, so it drifts from reality over time. It is the right model
//! for analysing an orbit whose elements are already known, for example to
//! find the radius at a true anomaly, and the wrong model for propagating a
//! two-line element set, which is the job of `apsis-sgp4`.
//!
//! Angles are in radians. Lengths are kilometers and the gravitational
//! parameter is in cubic kilometers per second squared, matching the rest of
//! the orbit layer.
//!
//! ```
//! use apsis_kepler::{GravityParameter, KeplerianElements};
//!
//! // A circular orbit at the Earth's equatorial radius has the speed
//! // sqrt(mu / r), about 7.9 km/s.
//! let elements = KeplerianElements {
//!     semi_major_axis: 6_378.137,
//!     eccentricity: 0.0,
//!     inclination: 0.0,
//!     right_ascension: 0.0,
//!     argument_of_perigee: 0.0,
//!     true_anomaly: 0.0,
//! };
//! let state = elements.to_state(GravityParameter::WGS84).expect("valid elements");
//! assert!((state.velocity.norm() - 7.909).abs() < 0.01);
//! ```

#![forbid(unsafe_code)]
#![deny(missing_docs)]

mod anomaly;
mod elements;
mod parameter;

pub use anomaly::{
    eccentric_from_mean, eccentric_from_true, mean_from_eccentric, mean_from_true,
    true_from_eccentric, true_from_mean, wrap_to_pi,
};
pub use elements::{KeplerianElements, State};
pub use parameter::GravityParameter;