earths 0.0.1

High-fidelity Earth simulation engine — orbit, atmosphere, geology, hydrology, biosphere, terrain, lighting, rendering, satellites, and temporal systems with full scientific coupling
Documentation
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use earth::biosphere::ecosystems::{boreal_forest, coral_reef, tropical_rainforest};
use earth::biosphere::fauna::{PredatorPrey, african_elephant, blue_whale};
use earth::biosphere::vegetation::{
    beer_lambert_light_extinction, coniferous_forest, temperate_grassland, tropical_broadleaf,
};
use earth::geodata::bathymetry::{BathymetryData, OceanBasin};
use earth::geodata::coordinates::{
    EARTH_FLATTENING, EARTH_SEMI_MAJOR_M, EARTH_SEMI_MINOR_M, LatLon,
};
use earth::geodata::elevation::ElevationProvider;
use earth::geodata::regions::{RegionDatabase, RegionType};
use earth::lighting::day_night::{DayNightCycle, DaylightState};
use earth::lighting::seasons::{
    AXIAL_TILT_DEG as SEASONS_TILT, Season, TROPICAL_YEAR_DAYS, VERNAL_EQUINOX_JD, season_at,
    subsolar_point,
};
use earth::lighting::solar_position::{EARTH_AXIAL_TILT_DEG, SolarPosition};
use earth::physics::collisions::{chicxulub_equivalent, tunguska_equivalent};
use earth::physics::orbit::{
    ARGUMENT_PERIHELION_DEG, ECCENTRICITY, EarthOrbit, INCLINATION_DEG,
    LONGITUDE_ASCENDING_NODE_DEG, SEMI_MAJOR_AXIS,
};
use earth::physics::rotation::{
    AXIAL_TILT_DEG, AXIAL_TILT_RAD, EarthRotation, PRECESSION_PERIOD_YEARS, SIDEREAL_DAY_S,
    SOLAR_DAY_S,
};
use earth::physics::tides::{
    TidalForce, lunar_to_solar_tide_ratio, neap_tide_amplitude, spring_tide_amplitude,
};
use earth::rendering::atmosphere_scattering::AtmosphereParams;
use earth::rendering::clouds::{CloudLayer, CloudSystem, CloudType};
use earth::rendering::materials::PbrMaterial;
use earth::rendering::ocean_rendering::OceanParams;
use earth::rendering::shaders::{ShaderData, UniformValue};
use earth::satellites::artificial::{ArtificialSatellite, Constellation, OrbitType};
use earth::satellites::moon::{
    EARTH_MOON_DISTANCE, LUNAR_MASS, LUNAR_ORBITAL_PERIOD, LUNAR_RADIUS, MoonSource, MoonState,
};
use earth::temporal::calendar::{DateTime, J2000_JD, SECONDS_PER_DAY, UNIX_EPOCH_JD};
use earth::temporal::epoch::{Epoch, J1950_EPOCH, J2000_EPOCH, MJD_OFFSET};
use earth::temporal::time_scale::TimeScale;
use earth::terrain::heightmap::Heightmap;
use earth::terrain::lod::{Face, LodConfig, LodTerrain};
use earth::terrain::mesh::TerrainMesh;
use earth::terrain::texturing::{Biome, BiomeClassifier};

fn main() {
    let orbit = EarthOrbit::new();
    let period_days = orbit.orbital_period_days();
    let v_perihelion = orbit.velocity_at_distance(orbit.perihelion_m());
    let v_aphelion = orbit.velocity_at_distance(orbit.aphelion_m());
    let energy = orbit.specific_orbital_energy();
    let ang_mom = orbit.specific_angular_momentum();
    let v_esc = EarthOrbit::escape_velocity_at_surface();
    let f_sun = orbit.gravitational_force_sun();
    let r = orbit.current_radius();
    let v_mean = orbit.mean_orbital_velocity();
    println!(
        "Orbit: P={:.2}d  v_peri={:.0}m/s  v_aph={:.0}m/s  v_mean={:.0}m/s",
        period_days, v_perihelion, v_aphelion, v_mean
    );
    println!(
        "  E={:.2e}J/kg  L={:.2e}m²/s  v_esc={:.0}m/s  F_sun={:.2e}N  r={:.3e}m",
        energy, ang_mom, v_esc, f_sun, r
    );
    println!(
        "  a={:.3e}m  e={}  i={}°  Ω={}°  ω={}°",
        SEMI_MAJOR_AXIS,
        ECCENTRICITY,
        INCLINATION_DEG,
        LONGITUDE_ASCENDING_NODE_DEG,
        ARGUMENT_PERIHELION_DEG
    );

    let rot = EarthRotation::new();
    let v_equator = rot.surface_velocity_at_latitude(0.0);
    let v_45 = rot.surface_velocity_at_latitude(45.0);
    let accel = rot.centripetal_acceleration_at_latitude(0.0);
    let coriolis = rot.coriolis_parameter(45.0);
    let moi = rot.moment_of_inertia();
    let ke = rot.rotational_kinetic_energy();
    let l_rot = rot.angular_momentum();
    let prec = rot.precession_rate_rad_per_year();
    let daylen_summer = rot.day_length_variation_due_to_tilt(172, 60.0);
    let daylen_winter = rot.day_length_variation_due_to_tilt(355, 60.0);
    println!(
        "Rotation: v_eq={:.0}m/s  v_45={:.0}m/s  a_c={:.4}m/s²  f_45={:.5e}",
        v_equator, v_45, accel, coriolis
    );
    println!(
        "  I={:.3e}kg·m²  KE={:.3e}J  L={:.3e}kg·m²/s  prec={:.2e}rad/yr",
        moi, ke, l_rot, prec
    );
    println!(
        "  day@60°N: summer={:.1}h  winter={:.1}h",
        daylen_summer, daylen_winter
    );
    println!(
        "  sidereal={:.4}s  solar={}s  tilt={}°={:.4}rad  prec_period={}yr",
        SIDEREAL_DAY_S, SOLAR_DAY_S, AXIAL_TILT_DEG, AXIAL_TILT_RAD, PRECESSION_PERIOD_YEARS
    );

    let moon_tide = TidalForce::from_moon();
    let sun_tide = TidalForce::from_sun();
    let a_moon = moon_tide.tidal_acceleration();
    let a_sun = sun_tide.tidal_acceleration();
    let pot = moon_tide.tidal_potential(0.0);
    let bulge_moon = moon_tide.tidal_bulge_height();
    let bulge_sun = sun_tide.tidal_bulge_height();
    let grav = moon_tide.gravitational_attraction();
    let spring = spring_tide_amplitude();
    let neap = neap_tide_amplitude();
    let ratio = lunar_to_solar_tide_ratio();
    println!(
        "Tides: a_moon={:.2e}  a_sun={:.2e}  ratio={:.2}",
        a_moon, a_sun, ratio
    );
    println!(
        "  bulge_moon={:.3}m  bulge_sun={:.3}m  spring={:.3}m  neap={:.3}m",
        bulge_moon, bulge_sun, spring, neap
    );
    println!("  potential(0)={:.2e}  F_grav={:.2e}N", pot, grav);

    let chix = chicxulub_equivalent();
    let tung = tunguska_equivalent();
    let ke_chix = chix.kinetic_energy_mt();
    let crater = chix.crater_diameter_m(2700.0);
    let fireball = chix.fireball_radius_m();
    let ejecta = chix.ejecta_volume_m3(2700.0);
    let v_imp = chix.impact_velocity();
    let ke_tung = tung.kinetic_energy_mt();
    println!(
        "Chicxulub: {:.2e}Mt  crater={:.0}m  fireball={:.0}m  ejecta={:.2e}m³  v_imp={:.0}m/s",
        ke_chix, crater, fireball, ejecta, v_imp
    );
    println!("Tunguska: {:.2e}Mt", ke_tung);

    let mut moon = MoonState::new();
    let source = match moon.source.get() {
        MoonSource::Binary => "binary",
        MoonSource::Simulation => "simulated",
    };
    let pos0 = moon.position();
    moon.step(3600.0);
    let pos1 = moon.position();
    let g_moon = moon.gravity_at(EARTH_MOON_DISTANCE);
    println!(
        "Moon({}): pos0=({:.0},{:.0})  pos1h=({:.0},{:.0})  g@dist={:.4e}m/s²",
        source, pos0.0, pos0.1, pos1.0, pos1.1, g_moon
    );
    println!(
        "  M={:.3e}kg  R={:.4e}m  d={:.3e}m  P={:.0}s",
        LUNAR_MASS, LUNAR_RADIUS, EARTH_MOON_DISTANCE, LUNAR_ORBITAL_PERIOD
    );

    let iss = ArtificialSatellite::leo("ISS", 420_000.0, 408_000.0);
    let geo = ArtificialSatellite::geo("GEO-Sat", 300.0);
    let custom = ArtificialSatellite::new("Molniya", 1500.0, 500_000.0, 0.7, 1.1);
    let p_iss = iss.orbital_period_s();
    let v_iss = iss.orbital_velocity_ms();
    let p_geo = geo.orbital_period_s();
    let g_surf = iss.gravity_at_surface();
    let pos_iss = iss.position();
    let pos_geo = geo.position();

    let orbit_type = |sat: &ArtificialSatellite| -> &str {
        match sat.orbit_type {
            OrbitType::LEO => "LEO",
            OrbitType::MEO => "MEO",
            OrbitType::GEO => "GEO",
            OrbitType::HEO => "HEO",
            OrbitType::Custom => "Custom",
        }
    };

    println!(
        "ISS({}): P={:.0}s  v={:.0}m/s  g_surf={:.2}  pos=({:.0},{:.0},{:.0})",
        orbit_type(&iss),
        p_iss,
        v_iss,
        g_surf,
        pos_iss.0,
        pos_iss.1,
        pos_iss.2
    );
    println!(
        "GEO({}): P={:.0}s  pos=({:.0},{:.0},{:.0})",
        orbit_type(&geo),
        p_geo,
        pos_geo.0,
        pos_geo.1,
        pos_geo.2
    );
    println!(
        "Custom({}): e={:.1}  i={:.1}rad",
        orbit_type(&custom),
        custom.eccentricity,
        custom.inclination_rad
    );

    let mut constellation = Constellation::new("Starlink-test");
    constellation.add(ArtificialSatellite::leo("S1", 260.0, 550_000.0));
    constellation.add(ArtificialSatellite::leo("S2", 260.0, 550_000.0));
    let mut sat_stepped = iss;
    sat_stepped.step(60.0);
    constellation.step_all(60.0);
    let positions = constellation.positions();
    println!(
        "Constellation: {} sats  positions={}",
        positions.len(),
        positions.len()
    );

    let dt = DateTime::new(2024, 6, 21, 12, 0, 0.0);
    let jd = dt.to_julian_date();
    let dt_back = DateTime::from_julian_date(jd);
    let unix = dt.to_unix_timestamp();
    let dt_unix = DateTime::from_unix_timestamp(unix);
    println!(
        "DateTime: {}-{}-{} {}:{}:{:.0}  JD={:.4}  unix={:.0}",
        dt.year, dt.month, dt.day, dt.hour, dt.minute, dt.second, jd, unix
    );
    println!(
        "  roundtrip JD: {}-{}-{}  roundtrip unix: {}-{}-{}",
        dt_back.year, dt_back.month, dt_back.day, dt_unix.year, dt_unix.month, dt_unix.day
    );
    println!(
        "  J2000_JD={} UNIX_JD={} SPD={}",
        J2000_JD, UNIX_EPOCH_JD, SECONDS_PER_DAY
    );

    let mut epoch = Epoch::j2000();
    let epoch_mjd = Epoch::from_mjd(51544.5);
    let centuries = epoch.centuries_since_j2000();
    let days = epoch.days_since_j2000();
    let gmst = epoch.gmst_degrees();
    let mjd = epoch.to_mjd();
    epoch.advance_days(365.25);
    let centuries_1yr = epoch.centuries_since_j2000();
    let mut epoch2 = Epoch::from_jd(J2000_JD);
    epoch2.advance_seconds(86400.0);
    println!(
        "Epoch: T0={:.4}  days={:.1}  GMST={:.2}°  MJD={:.1}",
        centuries, days, gmst, mjd
    );
    println!(
        "  +1yr: T={:.6}  from_mjd.jd={:.1}  epoch2.days={:.1}",
        centuries_1yr,
        epoch_mjd.julian_date,
        epoch2.days_since_j2000()
    );
    println!(
        "  J2000={} J1950={} MJD_OFF={}",
        J2000_EPOCH, J1950_EPOCH, MJD_OFFSET
    );

    let mut ts = TimeScale::realtime();
    ts.step(1.0);
    let sim_dt = ts.simulation_dt(1.0);
    let hours = ts.sim_hours();
    let days_ts = ts.sim_days();
    let years = ts.sim_years();
    ts.pause();
    ts.resume();
    ts.toggle_pause();
    ts.set_speed(100.0);
    let mut ff = TimeScale::fast_forward(10.0);
    ff.step(1.0);
    let sm = TimeScale::slow_motion(2.0);
    println!(
        "TimeScale: dt={:.1}  h={:.6}  d={:.8}  yr={:.10}  ff_sim={:.1}s  sm_speed={:.1}",
        sim_dt, hours, days_ts, years, ff.simulation_time_s, sm.speed_multiplier
    );

    let sun_pos = SolarPosition::compute(jd, 48.8, 2.3);
    let above = sun_pos.is_above_horizon();
    let dist_sun = sun_pos.distance_m();
    println!(
        "Solar: el={:.1}°  az={:.1}°  above={}  dist={:.3e}m  tilt={}°",
        sun_pos.elevation_deg, sun_pos.azimuth_deg, above, dist_sun, EARTH_AXIAL_TILT_DEG
    );

    let dnc = DayNightCycle::new(jd);
    let state_paris = dnc.state_at(48.8, 2.3);
    let state_text = match state_paris {
        DaylightState::Day => "day",
        DaylightState::Night => "night",
        DaylightState::CivilTwilight => "civil_twilight",
        DaylightState::NauticalTwilight => "nautical_twilight",
        DaylightState::AstronomicalTwilight => "astro_twilight",
    };
    let terminator = dnc.terminator_points(36);
    let ambient = dnc.ambient_light(48.8, 2.3);
    println!(
        "DayNight: Paris={}  ambient={:.2}  terminator_pts={}",
        state_text,
        ambient,
        terminator.len()
    );

    let season_state = season_at(jd, 48.8);
    let season_name = match season_state.season_north {
        Season::Spring => "spring",
        Season::Summer => "summer",
        Season::Autumn => "autumn",
        Season::Winter => "winter",
    };
    let south_name = match season_state.season_south {
        Season::Spring => "spring",
        Season::Summer => "summer",
        Season::Autumn => "autumn",
        Season::Winter => "winter",
    };
    let (sub_lat, sub_lon) = subsolar_point(jd);
    println!(
        "Season: N={}  S={}  decl={:.2}°  day_h={:.2}  subsolar=({:.1},{:.1})",
        season_name,
        south_name,
        season_state.solar_declination_deg,
        season_state.day_length_hours,
        sub_lat,
        sub_lon
    );
    println!(
        "  tilt={}°  trop_year={}d  vernal_jd={}",
        SEASONS_TILT, TROPICAL_YEAR_DAYS, VERNAL_EQUINOX_JD
    );

    let paris = LatLon::new(48.8566, 2.3522, 35.0);
    let tokyo = LatLon::new(35.6762, 139.6503, 40.0);
    let ecef_paris = paris.to_ecef();
    let cart = paris.to_cartesian_simple();
    let dist_pt = paris.distance_to(&tokyo);
    let latlon_back = ecef_paris.to_latlon();
    println!(
        "Paris->ECEF: ({:.0},{:.0},{:.0})  cart=({:.0},{:.0},{:.0})",
        ecef_paris.x, ecef_paris.y, ecef_paris.z, cart[0], cart[1], cart[2]
    );
    println!(
        "  dist(Paris-Tokyo)={:.0}m  roundtrip_lat={:.4}°",
        dist_pt, latlon_back.lat_deg
    );
    println!(
        "  f={:.9}  a={:.0}m  b={:.3}m",
        EARTH_FLATTENING, EARTH_SEMI_MAJOR_M, EARTH_SEMI_MINOR_M
    );

    let regions = RegionDatabase::continents();
    let region = regions.point_in_region(48.8, 2.3);
    if let Some(r) = region {
        let rtype = match r.region_type {
            RegionType::Continent => "continent",
            RegionType::Ocean => "ocean",
            RegionType::Sea => "sea",
            RegionType::Country => "country",
            RegionType::Island => "island",
        };
        println!(
            "Region(48.8,2.3): {} ({})  area={:.0}km²",
            r.name, rtype, r.area_km2
        );
    }

    let elev_prov = ElevationProvider::global(30.0);
    let elev_everest = elev_prov.sample(27.988, 86.925);
    let notables = ElevationProvider::notable_elevations();
    println!(
        "Elevation(Everest)={:.0}m  notables={}",
        elev_everest,
        notables.len()
    );

    let bathy = BathymetryData::global(60.0);
    let depth_mariana = bathy.sample(11.35, 142.2);
    let is_ocean = bathy.is_ocean(0.0, -30.0);
    let stats = BathymetryData::ocean_stats();
    let basins = OceanBasin::major_basins();
    println!(
        "Bathy(Mariana)={:.0}m  is_ocean(0,-30)={}  basins={}  avg_depth={:.0}m",
        depth_mariana,
        is_ocean,
        basins.len(),
        stats.avg_depth_m
    );

    let mut hmap = Heightmap::new(64);
    hmap.generate_procedural(42, 6, 0.5, 2.0);
    let h_sample = hmap.sample(45.0, 10.0);
    let r_at = hmap.radius_at(45.0, 10.0);
    println!(
        "Heightmap(64): sample(45,10)={:.1}m  radius={:.0}m",
        h_sample, r_at
    );

    let config = LodConfig::default();
    let mut lod = LodTerrain::new(config);
    lod.update([6_371_000.0 + 1000.0, 0.0, 0.0]);

    let mesh = TerrainMesh::from_region(0.0, 10.0, 0.0, 10.0, 8, &|lat, lon| hmap.sample(lat, lon));
    println!(
        "Mesh: {} vertices  {} triangles",
        mesh.vertex_count(),
        mesh.triangle_count()
    );

    let classifier = BiomeClassifier::default();
    let biome = classifier.classify(2000.0, 45.0, 0.6);
    let biome_name = match biome {
        Biome::Ocean => "ocean",
        Biome::Beach => "beach",
        Biome::Desert => "desert",
        Biome::Grassland => "grassland",
        Biome::Forest => "forest",
        Biome::Tundra => "tundra",
        Biome::Snow => "snow",
        Biome::Mountain => "mountain",
        Biome::Volcanic => "volcanic",
        Biome::Taiga => "taiga",
    };
    let splat = classifier.splat(2000.0, 45.0, 0.6);
    println!(
        "Biome(2000m,45°,0.6)={}  splat_top={:.2}",
        biome_name, splat.weights[0].1
    );

    let face = Face::PosZ;
    println!("LOD face={:?}", face);

    let mat_ocean = PbrMaterial::ocean();
    let mat_grass = PbrMaterial::grassland();
    let mat_desert = PbrMaterial::desert();
    let mat_snow = PbrMaterial::snow();
    let mat_rock = PbrMaterial::rock();
    let mat_volc = PbrMaterial::volcanic();
    let mat_forest = PbrMaterial::forest();
    let mat_ice = PbrMaterial::ice();
    println!(
        "Materials: ocean_rough={:.2}  grass_rough={:.2}  desert_metal={:.2}  \
        snow_rough={:.2}  rock={:.2}  volc={:.2}  forest={:.2}  ice={:.2}",
        mat_ocean.roughness,
        mat_grass.roughness,
        mat_desert.metallic,
        mat_snow.roughness,
        mat_rock.roughness,
        mat_volc.roughness,
        mat_forest.roughness,
        mat_ice.roughness
    );

    let sh_terrain = ShaderData::terrain();
    let sh_atm = ShaderData::atmosphere();
    let sh_ocean = ShaderData::ocean();
    println!(
        "Shaders: terrain={} uniforms  atm={}  ocean={}",
        sh_terrain.uniforms.len(),
        sh_atm.uniforms.len(),
        sh_ocean.uniforms.len()
    );
    for u in &sh_terrain.uniforms {
        let val = match &u.value {
            UniformValue::Float(f) => format!("f{:.2}", f),
            UniformValue::Vec3(v) => format!("v3({:.1},{:.1},{:.1})", v[0], v[1], v[2]),
            UniformValue::Vec4(v) => format!("v4({:.1},{:.1},{:.1},{:.1})", v[0], v[1], v[2], v[3]),
            UniformValue::Mat4(_) => "mat4".to_string(),
            UniformValue::Int(i) => format!("i{}", i),
        };
        println!("  {}={}", u.name, val);
    }

    let atm_params = AtmosphereParams::default();
    let ray_dens = atm_params.rayleigh_density(4000.0);
    let mie_dens = atm_params.mie_density(600.0);
    let scat_r = atm_params.scatter_rayleigh(0.5);
    let scat_m = atm_params.scatter_mie(0.5);
    let sky = atm_params.sky_color([0.0, 1.0, 0.0], [0.0, 0.5, 0.866], 0.0);
    println!(
        "Atmosphere: ρ_ray(4km)={:.4}  ρ_mie(600m)={:.4}  scat_r={:.4}  scat_m={:.4}",
        ray_dens, mie_dens, scat_r, scat_m
    );
    println!("  sky=({:.4},{:.4},{:.4})", sky[0], sky[1], sky[2]);

    let ocean_rdr = OceanParams::default();
    let phillips = ocean_rdr.phillips_spectrum(0.1, 0.05);
    let spectrum = ocean_rdr.generate_spectrum();
    let disp = ocean_rdr.dispersion(0.1);
    println!(
        "Ocean render: phillips(0.1,0.05)={:.6}  spectrum_size={}  disp={:.4}",
        phillips,
        spectrum.heights.len(),
        disp
    );

    let cumulus = CloudLayer::cumulus();
    let stratus = CloudLayer::stratus();
    let cirrus = CloudLayer::cirrus();
    let cb = CloudLayer::cumulonimbus();
    let cloud_type = |ct: &CloudType| match ct {
        CloudType::Cumulus => "Cu",
        CloudType::Stratus => "St",
        CloudType::Cirrus => "Ci",
        CloudType::Cumulonimbus => "Cb",
        CloudType::Altocumulus => "Ac",
    };
    println!(
        "Clouds: {} base={:.0}m  {} base={:.0}m  {} base={:.0}m  {} base={:.0}m",
        cloud_type(&cumulus.cloud_type),
        cumulus.base_altitude_m,
        cloud_type(&stratus.cloud_type),
        stratus.base_altitude_m,
        cloud_type(&cirrus.cloud_type),
        cirrus.base_altitude_m,
        cloud_type(&cb.cloud_type),
        cb.base_altitude_m
    );

    let mut clouds = CloudSystem::earth_default();
    clouds.step(3600.0);
    let density = clouds.sample_density(5000.0, 45.0, 10.0);
    println!("CloudSystem: density@5km={:.4}", density);

    let rainforest = tropical_rainforest();
    let boreal = boreal_forest();
    let reef = coral_reef();
    println!("Ecosystems:");
    for eco in [&rainforest, &boreal, &reef] {
        let sp = eco.total_species();
        let ind = eco.total_individuals();
        let shannon = eco.shannon_index();
        let simpson = eco.simpson_diversity();
        let area_sp = eco.expected_species_from_area(0.25, 100.0);
        let npp = eco.total_npp_gc_yr();
        let turnover = eco.biomass_turnover_time_yr(15_000.0);
        println!(
            "  {} species  {} ind  H'={:.2}  D={:.4}  SAR={:.0}  NPP={:.0}  τ={:.1}yr",
            sp, ind, shannon, simpson, area_sp, npp, turnover
        );
    }

    let tb = tropical_broadleaf();
    let tg = temperate_grassland();
    let cf = coniferous_forest();
    let photo = tb.photosynthesis_rate(400.0, 1500.0, 28.0);
    let canopy = tb.canopy_photosynthesis(photo);
    let transp = tg.transpiration_mm_day(1.5, 0.3);
    let npp_veg = cf.npp_kgc_m2_yr(15.0);
    let crt = cf.carbon_residence_time_yr(npp_veg);
    let light = beer_lambert_light_extinction(2000.0, 4.0, 0.5);
    println!(
        "Vegetation: photo={:.2}  canopy={:.2}  transp={:.2}mm/d  NPP={:.3}  CRT={:.1}yr  light={:.1}",
        photo, canopy, transp, npp_veg, crt, light
    );

    let elephant = african_elephant();
    let whale = blue_whale();
    let gr_el = elephant.growth_rate();
    let proj = elephant.project_forward(10.0);
    let metab = whale.metabolic_rate_w();
    let range = elephant.home_range_km2();
    let gen_time = whale.generation_time_years();
    let lifespan = elephant.max_lifespan_years();
    println!(
        "Elephant: r={:.3}  N(+10yr)={:.0}  range={:.1}km²  lifespan={:.0}yr",
        gr_el, proj, range, lifespan
    );
    println!("Whale: metab={:.0}W  gen={:.1}yr", metab, gen_time);

    let mut pp = PredatorPrey {
        prey: african_elephant(),
        predator: blue_whale(),
        attack_rate: 0.01,
        conversion_efficiency: 0.1,
        predator_death_rate: 0.05,
    };
    let prey_r = pp.prey_growth_rate();
    let pred_r = pp.predator_growth_rate();
    pp.step(0.1);
    println!(
        "Lotka-Volterra: prey_r={:.1}  pred_r={:.1}  -> prey={:.1}  pred={:.2}",
        prey_r, pred_r, pp.prey.count, pp.predator.count
    );
}