path: root/src/space/survey.rs

//! What a ship's survey says about a place: how hot it is and how that
//! swings over its year, how long its days are, how hard it pulls, what its
//! air is like, what hangs in its sky, and how many people live there.
//! None of it changes how the game plays. It's there so each place feels
//! like somewhere.
//!
//! Temperatures are worked out, not made up: the starlight a body gets at
//! its distance from the star (which changes along an eccentric orbit),
//! less what it reflects, plus what its air traps or its insides give off.
//! Moons share their planet's distance. Everything else is drawn from a
//! hash of the place, so it's the same every visit and system generation
//! (and every universe already made) is left alone.

use std::f64::consts::TAU;

use super::orbit::{EARTH_MASS, Orbit};
use super::system::{Climate, Landing, PlaceKind, StarKind, System, SystemKind};
use crate::names::Culture;
use crate::scenes::sky::hash01;
use crate::seed;

/// How warm a bare, black, quickly turning rock gets 1 AU from a star of
/// one sun, in kelvin.
const WARMTH_AT_1AU: f64 = 278.6;
/// Water freezes at this many kelvin.
pub const FREEZING: f64 = 273.15;
/// How wide the Sun looks from 1 AU, in degrees.
const SUN_AT_1AU: f64 = 0.533;
const EARTH_RADIUS_KM: f64 = 6371.0;
const AU_KM: f64 = 149.6e6;
/// Rocks in the belt: dark, and no air to keep them warm.
const BELT_CLIMATE: Climate = Climate { albedo: 0.1, warmth: 0.0, air: &[], breathable: false };

pub struct Survey {
    /// At the surface, or a giant's cloud tops. None aboard a station,
    /// where it's whatever they set it to.
    pub temperature: Option<Temperature>,
    /// How long a day lasts, or how fast a station spins.
    pub day: Option<String>,
    /// What its tilt does to its year, if it has days to speak of.
    pub seasons: Option<&'static str>,
    /// In g: at the surface, at a giant's cloud tops, or from a station's spin.
    pub gravity: Option<f64>,
    /// What the air is like, and whether you can breathe it. None if
    /// there isn't any.
    pub air: Option<(&'static str, bool)>,
    /// What you'd see overhead.
    pub sky: String,
    /// How many live there, if anyone does.
    pub population: Option<u64>,
}

/// In kelvin.
#[derive(Clone, Copy, Debug)]
pub struct Temperature {
    pub now: f64,
    /// At the far end of its orbit.
    pub coldest: f64,
    /// At the near end.
    pub hottest: f64,
}

impl Temperature {
    fn along(orbit: &Orbit, day: f64, star: &StarKind, climate: &Climate) -> Temperature {
        let at = |distance: f64| {
            WARMTH_AT_1AU * (star.luminosity * (1.0 - climate.albedo)).powf(0.25) / distance.sqrt()
                + climate.warmth
        };
        Temperature {
            now: at(orbit.position(day).length()),
            coldest: at(orbit.apoapsis_distance()),
            hottest: at(orbit.periapsis_distance()),
        }
    }

    /// One word for how it feels, going by the temperature now.
    pub fn word(self) -> &'static str {
        match self.now - FREEZING {
            c if c < -150.0 => "deep-frozen",
            c if c < -60.0 => "frozen",
            c if c < -20.0 => "bitter",
            c if c < 5.0 => "cold",
            c if c < 15.0 => "cool",
            c if c < 28.0 => "temperate",
            c if c < 45.0 => "hot",
            c if c < 120.0 => "scorching",
            c if c < 400.0 => "searing",
            _ => "molten",
        }
    }
}

/// Kelvin to whole degrees Celsius, for showing.
pub fn celsius(kelvin: f64) -> i64 {
    (kelvin - FREEZING).round() as i64
}

pub fn survey(s: &System, place: usize, day: f64) -> Survey {
    let key = seed::derive(s.seed, "survey", place as u64);
    let h = |i: i64| hash01(key, i, 0);
    let star = s.star;
    let landing = s.landing(place);
    match s.places[place].kind {
        PlaceKind::Planet(p) => {
            let planet = &s.planets[p];
            let kind = planet.kind;
            let temperature = Temperature::along(&planet.orbit, day, star, &kind.climate);
            // Close to a small star, tides stop a world turning.
            let locked = !kind.giant && planet.orbit.a < 0.4 * star.mass;
            let (day_length, seasons) = if locked {
                ("locked: one face in endless day".to_string(), None)
            } else {
                let hours = if kind.giant {
                    lerp(9.0, 18.0, h(1))
                } else if h(2) < 0.08 {
                    24.0 * log_lerp(5.0, 120.0, h(1))
                } else {
                    log_lerp(10.0, 80.0, h(1))
                };
                (duration(hours), Some(seasons(h(3), h(4))))
            };
            let earths = planet.mass / EARTH_MASS;
            let radius = radius(earths, kind.giant);
            let moons = match planet.moons.len() {
                0 => String::new(),
                1 => " and a moon".to_string(),
                n => format!(" and {n} moons"),
            };
            Survey {
                temperature: Some(temperature),
                day: Some(day_length),
                seasons,
                gravity: Some(earths / (radius * radius)),
                air: air(&kind.climate, temperature, h(5)),
                sky: format!("{}{moons}", sun(star, planet.orbit.position(day).length())),
                population: population(landing, s.kind, p == s.home, false, h(6)),
            }
        }
        PlaceKind::Moon(p, m) => {
            let planet = &s.planets[p];
            let moon = &planet.moons[m];
            let temperature = Temperature::along(&planet.orbit, day, star, &moon.kind.climate);
            let earths = log_lerp(0.004, 0.12, h(1));
            // How wide its planet looks, from down on the surface.
            let planet_km = radius(planet.mass / EARTH_MASS, planet.kind.giant) * EARTH_RADIUS_KM;
            let across = 2.0 * (planet_km / (moon.orbit.a * AU_KM)).atan().to_degrees();
            let size = if across < 1.0 { format!("{across:.1}") } else { format!("{across:.0}") };
            let looms = if across >= 3.0 { "looms" } else { "hangs" };
            Survey {
                temperature: Some(temperature),
                day: Some(format!(
                    "{}, always facing {}",
                    duration(24.0 * moon.orbit.period),
                    planet.name
                )),
                seasons: None,
                gravity: Some(earths.powf(0.46)),
                air: air(&moon.kind.climate, temperature, h(5)),
                sky: format!(
                    "{} {looms} {size}° wide; {}",
                    planet.name,
                    sun(star, planet.orbit.position(day).length())
                ),
                population: population(landing, s.kind, false, true, h(6)),
            }
        }
        PlaceKind::Station(k) => {
            let station = &s.stations[k];
            let planet = &s.planets[station.planet];
            // Spun for gravity: the bigger the wheel, the slower it turns.
            let g = lerp(0.35, 1.0, h(1));
            let wheel = lerp(150.0, 900.0, h(2));
            let seconds = TAU * (wheel / (g * 9.81)).sqrt();
            Survey {
                temperature: None,
                day: Some(format!("turns once every {seconds:.0} seconds")),
                seasons: None,
                gravity: Some(g),
                air: Some((station_air(station.culture), true)),
                sky: format!(
                    "{}, and {} a bright star",
                    sun(star, planet.orbit.position(day).length()),
                    planet.name
                ),
                population: population(landing, s.kind, false, false, h(6)),
            }
        }
        PlaceKind::Belt => {
            let region = &s.belt.region;
            Survey {
                temperature: Some(Temperature::along(region, day, star, &BELT_CLIMATE)),
                day: None,
                seasons: None,
                gravity: None,
                air: None,
                sky: format!("{}, and rocks tumbling all round", sun(star, region.a)),
                population: None,
            }
        }
    }
}

/// In Earth radii, for a body of `earths` Earth masses: rocky worlds grow
/// slowly with mass, and giants stop growing at about Jupiter's size.
fn radius(earths: f64, giant: bool) -> f64 {
    if giant { earths.powf(0.48).min(11.3) } else { earths.powf(0.27) }
}

fn air(climate: &Climate, t: Temperature, h: f64) -> Option<(&'static str, bool)> {
    if climate.air.is_empty() {
        return None;
    }
    let what = climate.air[(h * climate.air.len() as f64) as usize % climate.air.len()];
    let bearable = (FREEZING - 40.0..=FREEZING + 55.0).contains(&t.now);
    Some((what, climate.breathable && bearable))
}

fn station_air(culture: Culture) -> &'static str {
    match culture {
        Culture::Qaari => "warm and dry, scented with spice-tea",
        Culture::Cant => "recycled; hot solder and incense",
        Culture::Houses => "perfumed, and filtered twice over",
        Culture::Drift => "stale, and patched like the hulls",
    }
}

/// The star as it looks from `distance` AU.
fn sun(star: &StarKind, distance: f64) -> String {
    let colour = star.name.trim_end_matches(" dwarf").trim_end_matches(" star");
    // Near enough for a star in the prime of its life.
    let radius = star.mass.powf(0.8);
    let degrees = SUN_AT_1AU * radius / distance;
    let size = match degrees {
        d if d < 0.12 => return format!("a {colour} sun, shrunk to a point"),
        d if d < 0.35 => "small ",
        d if d < 0.9 => "",
        d if d < 2.5 => "broad ",
        d if d < 6.0 => "vast ",
        _ => "sky-filling ",
    };
    format!("a {size}{colour} sun")
}

/// How much its tilt changes its year. Most worlds lean a little; a few
/// roll round their star on their side.
fn seasons(h: f64, h2: f64) -> &'static str {
    let tilt = if h < 0.05 { 80.0 + 20.0 * h2 } else { 40.0 * h2 * h2 };
    match tilt {
        t if t < 4.0 => "no seasons",
        t if t < 15.0 => "mild seasons",
        t if t < 30.0 => "strong seasons",
        t if t < 60.0 => "harsh seasons",
        _ => "spins on its side",
    }
}

/// How many people live somewhere: billions on a seat's homeworld, far
/// fewer out on the frontier or among the wrecks.
fn population(landing: Landing, kind: SystemKind, main: bool, moon: bool, h: f64) -> Option<u64> {
    let (low, high) = match landing {
        Landing::Capital(_) if main => match kind {
            SystemKind::Seat => (6e8, 4e9),
            SystemKind::Settled => (4e7, 6e8),
            SystemKind::Rubble => (3e6, 6e7),
            _ => (2e5, 8e6),
        },
        Landing::Capital(_) if moon => (2e5, 2e7),
        Landing::Capital(_) => (5e6, 3e8),
        Landing::Aloft(_) => (1e6, 4e7),
        Landing::Market(_) => (3e3, 8e4),
        Landing::Wilds | Landing::Nowhere => return None,
    };
    // Away from the settled heart of things, everywhere is emptier.
    let thin = matches!(kind, SystemKind::Frontier | SystemKind::Graveyard) && !main;
    let (low, high) = if thin { (low * 0.1, high * 0.1) } else { (low, high) };
    // To two figures: nobody counts heads more closely than that.
    let n = log_lerp(low, high, h);
    let unit = 10f64.powi(n.log10().floor() as i32 - 1);
    Some(((n / unit).round() * unit) as u64)
}

/// "26 hours", "3.5 days", "41 days".
pub fn duration(hours: f64) -> String {
    let days = hours / 24.0;
    if hours < 72.0 {
        format!("{hours:.0} hours")
    } else if days < 10.0 {
        format!("{days:.1} days")
    } else {
        format!("{days:.0} days")
    }
}

fn lerp(low: f64, high: f64, k: f64) -> f64 {
    low + (high - low) * k
}

/// From `low` to `high`, evenly in orders of magnitude.
fn log_lerp(low: f64, high: f64, k: f64) -> f64 {
    (low.ln() + (high.ln() - low.ln()) * k).exp()
}

#[cfg(test)]
mod tests {
    use super::*;
    use crate::space::system::generate;

    #[test]
    fn surveys_are_the_same_every_time() {
        let s = generate(4);
        for place in 0..s.places.len() {
            let (a, b) = (survey(&s, place, 30.0), survey(&s, place, 30.0));
            assert_eq!((a.day, a.sky, a.population), (b.day, b.sky, b.population));
        }
    }

    #[test]
    fn temperatures_swing_between_the_ends_of_the_orbit() {
        for seed in 0..60 {
            let s = generate(seed);
            for place in 0..s.places.len() {
                for day in [0.0, 100.0, 1234.5] {
                    let Some(t) = survey(&s, place, day).temperature else { continue };
                    assert!(t.coldest <= t.now + 1e-9 && t.now <= t.hottest + 1e-9);
                    assert!(t.coldest > 0.0, "seed {seed}: below absolute zero");
                }
            }
        }
    }

    #[test]
    fn a_wet_world_in_the_middle_of_the_habitable_zone_is_mild() {
        let star = &crate::space::system::STARS[0];
        let hz = star.luminosity.sqrt();
        let orbit = Orbit::new(hz, 0.0, 0.0, 0.0, star.mass);
        let ocean = Climate { albedo: 0.3, warmth: 33.0, air: &["wet"], breathable: true };
        let t = Temperature::along(&orbit, 0.0, star, &ocean);
        assert_eq!(celsius(t.now), 15, "about Earth's average");
        assert_eq!(t.word(), "cool");
        assert_eq!(air(&ocean, t, 0.0), Some(("wet", true)));
    }

    #[test]
    fn only_settled_places_have_people() {
        for seed in 0..40 {
            let s = generate(seed);
            for place in 0..s.places.len() {
                let people = survey(&s, place, 0.0).population;
                assert_eq!(
                    people.is_some(),
                    !matches!(s.landing(place), Landing::Wilds | Landing::Nowhere)
                );
            }
            let home = survey(&s, s.home_place(), 0.0).population.unwrap();
            assert!(home >= 600_000_000, "seed {seed}: a homeworld of {home}");
        }
    }

    #[test]
    fn durations_read_naturally() {
        assert_eq!(duration(26.2), "26 hours");
        assert_eq!(duration(84.0), "3.5 days");
        assert_eq!(duration(24.0 * 41.2), "41 days");
    }
}