Astronome/backend/src/astronomy/visibility.rs

use chrono::{DateTime, Duration, Utc};
use serde::{Deserialize, Serialize};

use crate::config::{LAT, LON, MIN_ALT_DEG};
use super::{
    coords::{airmass, extinction_mag, radec_to_altaz},
    horizon::{horizon_alt, HorizonPoint},
    lunar::{moon_altitude, moon_separation},
    time::{julian_date, local_sidereal_time},
};

#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct CurvePoint {
    pub utc: DateTime<Utc>,
    pub alt_deg: f64,
    pub az_deg: f64,
    pub airmass: f64,
    pub above_custom_horizon: bool,
    pub moon_alt_deg: f64,
}

#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct VisibilitySummary {
    pub max_alt_deg: f64,
    pub transit_utc: Option<DateTime<Utc>>,
    pub rise_utc: Option<DateTime<Utc>>,
    pub set_utc: Option<DateTime<Utc>>,
    pub best_start_utc: Option<DateTime<Utc>>,
    pub best_end_utc: Option<DateTime<Utc>>,
    pub usable_min: u32,
    pub is_visible_tonight: bool,
    pub meridian_flip_utc: Option<DateTime<Utc>>,
    pub airmass_at_transit: f64,
    pub extinction_at_transit: f64,
    pub moon_sep_deg: f64,
    pub curve: Vec<CurvePoint>,
}

pub struct TonightWindow {
    pub dusk: DateTime<Utc>,
    pub dawn: DateTime<Utc>,
}

pub struct MoonState {
    pub ra_deg: f64,
    pub dec_deg: f64,
    pub illumination: f64,
    pub alt_at_midnight: f64,
}

/// Compute full visibility summary for a catalog object during tonight's window.
/// step_minutes: resolution for the altitude curve (1 for detailed view, 10 for precompute cache).
pub fn compute_visibility(
    ra_deg: f64,
    dec_deg: f64,
    window: &TonightWindow,
    horizon: &[HorizonPoint],
    moon: &MoonState,
) -> VisibilitySummary {
    compute_visibility_with_step(ra_deg, dec_deg, window, horizon, moon, 10)
}

pub fn compute_visibility_with_step(
    ra_deg: f64,
    dec_deg: f64,
    window: &TonightWindow,
    horizon: &[HorizonPoint],
    moon: &MoonState,
    step_minutes: i64,
) -> VisibilitySummary {
    let step = Duration::minutes(step_minutes);
    let mut t = window.dusk;

    let mut curve = Vec::new();
    let mut max_alt = f64::NEG_INFINITY;
    let mut transit_utc: Option<DateTime<Utc>> = None;
    let mut rise_utc: Option<DateTime<Utc>> = None;
    let mut set_utc: Option<DateTime<Utc>> = None;
    let mut best_start: Option<DateTime<Utc>> = None;
    let mut best_end: Option<DateTime<Utc>> = None;
    let mut usable_min = 0u32;
    let mut prev_alt = f64::NEG_INFINITY;

    while t <= window.dawn {
        let jd = julian_date(t);
        let lst = local_sidereal_time(jd, LON);
        let (alt, az) = radec_to_altaz(ra_deg, dec_deg, lst, LAT);
        let am = airmass(alt);
        let h_alt = horizon_alt(az, horizon);
        let above = alt > h_alt.max(MIN_ALT_DEG);
        let moon_alt = moon_altitude(jd, LAT, LON);

        curve.push(CurvePoint {
            utc: t,
            alt_deg: alt,
            az_deg: az,
            airmass: am,
            above_custom_horizon: above,
            moon_alt_deg: moon_alt,
        });

        if alt > max_alt {
            max_alt = alt;
            transit_utc = Some(t);
        }

        // Rise: first crossing above effective horizon
        if prev_alt <= h_alt.max(MIN_ALT_DEG) && alt > h_alt.max(MIN_ALT_DEG) && rise_utc.is_none() {
            rise_utc = Some(t);
        }
        // Set: last time we were above horizon
        if alt > h_alt.max(MIN_ALT_DEG) {
            set_utc = Some(t);
        }

        // Best window: above 30°
        if alt > 30.0 {
            if best_start.is_none() {
                best_start = Some(t);
            }
            best_end = Some(t);
            usable_min += step_minutes as u32;
        }

        prev_alt = alt;
        t += step;
    }

    let is_visible = usable_min > 0 || rise_utc.is_some();

    let airmass_transit = transit_utc
        .map(|tr| {
            let jd = julian_date(tr);
            let lst = local_sidereal_time(jd, LON);
            let (alt, _) = radec_to_altaz(ra_deg, dec_deg, lst, LAT);
            airmass(alt)
        })
        .unwrap_or(40.0);

    let extinction_transit = extinction_mag(
        transit_utc
            .map(|tr| {
                let jd = julian_date(tr);
                let lst = local_sidereal_time(jd, LON);
                let (alt, _) = radec_to_altaz(ra_deg, dec_deg, lst, LAT);
                alt
            })
            .unwrap_or(0.0),
    );

    let moon_sep = moon_separation(moon.ra_deg, moon.dec_deg, ra_deg, dec_deg);

    // Meridian flip: transit + time for HA to reach +5°
    // 5° of HA = 5/360 * 86400 = 1200 seconds
    let meridian_flip = transit_utc.map(|tr| tr + Duration::seconds(1200));

    VisibilitySummary {
        max_alt_deg: if max_alt == f64::NEG_INFINITY { 0.0 } else { max_alt },
        transit_utc,
        rise_utc,
        set_utc,
        best_start_utc: best_start,
        best_end_utc: best_end,
        usable_min,
        is_visible_tonight: is_visible,
        meridian_flip_utc: meridian_flip,
        airmass_at_transit: airmass_transit,
        extinction_at_transit: extinction_transit,
        moon_sep_deg: moon_sep,
        curve,
    }
}

/// Find the longest continuous true-dark window (sun < -18° AND moon below horizon).
pub fn true_dark_window(
    dusk: DateTime<Utc>,
    dawn: DateTime<Utc>,
    lat: f64,
    lon: f64,
) -> Option<(DateTime<Utc>, DateTime<Utc>)> {
    let step = Duration::minutes(5);
    let mut t = dusk;
    let mut best: Option<(DateTime<Utc>, DateTime<Utc>)> = None;
    let mut current_start: Option<DateTime<Utc>> = None;
    let mut best_duration = Duration::zero();

    while t <= dawn {
        let jd = julian_date(t);
        let moon_alt = moon_altitude(jd, lat, lon);
        let is_dark = moon_alt < 0.0;

        if is_dark {
            if current_start.is_none() {
                current_start = Some(t);
            }
        } else if let Some(start) = current_start {
            let dur = t - start;
            if dur > best_duration {
                best_duration = dur;
                best = Some((start, t));
            }
            current_start = None;
        }

        t += step;
    }

    // Check if still in dark window at dawn
    if let Some(start) = current_start {
        let dur = dawn - start;
        if dur > best_duration {
            best = Some((start, dawn));
        }
    }

    best
}