orts 0.1.0

//! Simulation event detection for orbital mechanics.
//!
//! Provides event checkers compatible with `Integrator::integrate_with_events`.

use std::ops::ControlFlow;

use crate::OrbitalState;

/// Events that can terminate a simulation early.
#[derive(Debug, Clone, PartialEq)]
pub enum SimulationEvent {
    /// Satellite impacted the central body surface.
    Collision {
        /// Altitude at impact [km] (negative = below surface).
        altitude_km: f64,
    },
    /// Satellite entered the atmosphere (below the effective atmosphere boundary).
    AtmosphericEntry {
        /// Altitude at detection [km].
        altitude_km: f64,
    },
}

/// Create an event checker for surface collision and atmospheric entry detection.
///
/// - Returns `Break(Collision)` when `|r| < body_radius`.
/// - Returns `Break(AtmosphericEntry)` when `|r| < body_radius + atmosphere_altitude`.
/// - The atmospheric entry check fires first (higher altitude).
pub fn collision_check(
    body_radius: f64,
    atmosphere_altitude: Option<f64>,
) -> impl Fn(f64, &OrbitalState) -> ControlFlow<SimulationEvent> {
    move |_t: f64, state: &OrbitalState| {
        let r = state.position().magnitude();
        if r < body_radius {
            ControlFlow::Break(SimulationEvent::Collision {
                altitude_km: r - body_radius,
            })
        } else if let Some(atm_alt) = atmosphere_altitude {
            if r < body_radius + atm_alt {
                ControlFlow::Break(SimulationEvent::AtmosphericEntry {
                    altitude_km: r - body_radius,
                })
            } else {
                ControlFlow::Continue(())
            }
        } else {
            ControlFlow::Continue(())
        }
    }
}

#[cfg(test)]
mod tests {
    use super::*;
    use arika::earth::{MU as MU_EARTH, R as R_EARTH};
    use nalgebra::vector;

    #[test]
    fn collision_check_above_surface() {
        let check = collision_check(R_EARTH, None);
        let state = OrbitalState::new(vector![R_EARTH + 400.0, 0.0, 0.0], vector![0.0, 7.66, 0.0]);
        assert!(matches!(check(0.0, &state), ControlFlow::Continue(())));
    }

    #[test]
    fn collision_check_below_surface() {
        let check = collision_check(R_EARTH, None);
        let state = OrbitalState::new(vector![R_EARTH - 10.0, 0.0, 0.0], vector![0.0, 7.0, 0.0]);
        match check(0.0, &state) {
            ControlFlow::Break(SimulationEvent::Collision { altitude_km }) => {
                assert!(altitude_km < 0.0);
                assert!((altitude_km - (-10.0)).abs() < 1e-10);
            }
            other => panic!("Expected collision, got {other:?}"),
        }
    }

    #[test]
    fn collision_check_at_surface() {
        let check = collision_check(R_EARTH, None);
        // Exactly at surface: r == body_radius, should NOT trigger (not < body_radius)
        let state = OrbitalState::new(vector![R_EARTH, 0.0, 0.0], vector![0.0, 7.0, 0.0]);
        assert!(matches!(check(0.0, &state), ControlFlow::Continue(())));
    }

    #[test]
    fn collision_check_3d_position() {
        let check = collision_check(R_EARTH, None);
        // Position magnitude = sqrt(3000^2 * 3) ≈ 5196 km < R_EARTH (6378 km)
        let state = OrbitalState::new(vector![3000.0, 3000.0, 3000.0], vector![0.0, 0.0, 0.0]);
        assert!(matches!(
            check(0.0, &state),
            ControlFlow::Break(SimulationEvent::Collision { .. })
        ));
    }

    #[test]
    fn atmospheric_entry_above_karman() {
        // At 200 km altitude with 100 km atmosphere → no event
        let check = collision_check(R_EARTH, Some(100.0));
        let state = OrbitalState::new(vector![R_EARTH + 200.0, 0.0, 0.0], vector![0.0, 7.66, 0.0]);
        assert!(matches!(check(0.0, &state), ControlFlow::Continue(())));
    }

    #[test]
    fn atmospheric_entry_below_karman() {
        // At 50 km altitude with 100 km atmosphere → AtmosphericEntry
        let check = collision_check(R_EARTH, Some(100.0));
        let state = OrbitalState::new(vector![R_EARTH + 50.0, 0.0, 0.0], vector![0.0, 7.0, 0.0]);
        match check(0.0, &state) {
            ControlFlow::Break(SimulationEvent::AtmosphericEntry { altitude_km }) => {
                assert!((altitude_km - 50.0).abs() < 1e-10);
            }
            other => panic!("Expected AtmosphericEntry, got {other:?}"),
        }
    }

    #[test]
    fn collision_takes_priority_over_atmospheric_entry() {
        // Below surface with atmosphere set → Collision, not AtmosphericEntry
        let check = collision_check(R_EARTH, Some(100.0));
        let state = OrbitalState::new(vector![R_EARTH - 5.0, 0.0, 0.0], vector![0.0, 7.0, 0.0]);
        assert!(matches!(
            check(0.0, &state),
            ControlFlow::Break(SimulationEvent::Collision { .. })
        ));
    }

    #[test]
    fn suborbital_trajectory_terminates_on_collision() {
        use crate::orbital::two_body::TwoBodySystem;
        use utsuroi::{IntegrationOutcome, Integrator, Rk4};

        let system = TwoBodySystem { mu: MU_EARTH };

        // Start at 100 km altitude with only 80% of circular velocity → will fall back
        let r = R_EARTH + 100.0;
        let v_circular = (MU_EARTH / r).sqrt();
        let initial = OrbitalState::new(vector![r, 0.0, 0.0], vector![0.0, v_circular * 0.8, 0.0]);

        // No atmosphere → hits surface directly
        let event_checker = collision_check(R_EARTH, None);
        let outcome = Rk4.integrate_with_events(
            &system,
            initial,
            0.0,
            100_000.0,
            1.0,
            |_t, _state| {},
            event_checker,
        );

        match outcome {
            IntegrationOutcome::Terminated {
                reason: SimulationEvent::Collision { altitude_km },
                t,
                ..
            } => {
                assert!(
                    altitude_km < 0.0,
                    "Altitude should be negative at collision"
                );
                assert!(t < 100_000.0, "Should terminate before t_end");
                assert!(t > 0.0, "Should not terminate immediately");
            }
            other => panic!("Expected collision termination, got {other:?}"),
        }
    }

    #[test]
    fn suborbital_trajectory_terminates_on_atmospheric_entry() {
        use crate::orbital::two_body::TwoBodySystem;
        use utsuroi::{IntegrationOutcome, Integrator, Rk4};

        let system = TwoBodySystem { mu: MU_EARTH };

        // Start at 200 km altitude with 80% of circular velocity → will fall back
        let r = R_EARTH + 200.0;
        let v_circular = (MU_EARTH / r).sqrt();
        let initial = OrbitalState::new(vector![r, 0.0, 0.0], vector![0.0, v_circular * 0.8, 0.0]);

        // With 100 km atmosphere → should stop at atmosphere boundary before hitting surface
        let event_checker = collision_check(R_EARTH, Some(100.0));
        let outcome = Rk4.integrate_with_events(
            &system,
            initial,
            0.0,
            100_000.0,
            1.0,
            |_t, _state| {},
            event_checker,
        );

        match outcome {
            IntegrationOutcome::Terminated {
                reason: SimulationEvent::AtmosphericEntry { altitude_km },
                t,
                ..
            } => {
                assert!(altitude_km < 100.0, "Should be below atmosphere boundary");
                assert!(altitude_km > 0.0, "Should still be above surface");
                assert!(t > 0.0, "Should not terminate immediately");
            }
            other => panic!("Expected AtmosphericEntry, got {other:?}"),
        }
    }
}