sidereon-core 0.8.0

use crate::astro::constants::{J2_EARTH, MU_EARTH, RE_EARTH};
use crate::astro::error::PropagationError;
use crate::astro::forces::r#trait::ForceModel;
use crate::astro::propagator::api::PropagationContext;
use crate::astro::state::CartesianState;
use nalgebra::Vector3;

pub struct J2Gravity {
    pub mu: f64,
    pub re: f64,
    pub j2: f64,
}

impl Default for J2Gravity {
    fn default() -> Self {
        Self {
            mu: MU_EARTH,
            re: RE_EARTH,
            j2: J2_EARTH,
        }
    }
}

impl ForceModel for J2Gravity {
    fn acceleration(
        &self,
        state: &CartesianState,
        _ctx: &PropagationContext,
    ) -> Result<Vector3<f64>, PropagationError> {
        let r_mag2 = state.position_km.norm_squared();
        if r_mag2 == 0.0 {
            return Err(PropagationError::NumericalFailure(
                "Zero position magnitude".to_string(),
            ));
        }
        let r_mag = r_mag2.sqrt();

        let f = 1.5 * self.j2 * (self.mu / r_mag2) * (self.re / r_mag).powi(2);
        let z_r2 = (state.position_km.z * state.position_km.z) / r_mag2;

        Ok(Vector3::new(
            f * (state.position_km.x / r_mag) * (5.0 * z_r2 - 1.0),
            f * (state.position_km.y / r_mag) * (5.0 * z_r2 - 1.0),
            f * (state.position_km.z / r_mag) * (5.0 * z_r2 - 3.0),
        ))
    }
}

#[cfg(test)]
mod tests {
    use super::*;
    use crate::astro::propagator::api::PropagationContext;
    use crate::astro::state::CartesianState;

    #[test]
    fn acceleration_matches_orbis_force_wrapper_bits() {
        let state = CartesianState::new(0.0, [7000.0, -1210.0, 1300.0], [0.0, 0.0, 0.0]);
        let acceleration = J2Gravity::default()
            .acceleration(&state, &PropagationContext::default())
            .unwrap();

        assert_eq!(acceleration.x.to_bits(), 13_754_131_348_549_160_135);
        assert_eq!(acceleration.y.to_bits(), 4_519_025_615_523_880_849);
        assert_eq!(acceleration.z.to_bits(), 13_750_824_904_549_515_386);
    }
}