path: root/src/space/system.rs

//! Star systems: a star, planets on Kepler orbits with moons of their own,
//! an asteroid belt, stations at Lagrange points, and wrecks that come and
//! go with the calendar.
//!
//! Planets are laid out outward from the star, each orbit 1.5-2 times the
//! last (roughly how real systems space out). What a planet is depends on
//! where it sits: scorched rock close in, temperate worlds around the
//! habitable zone, ice beyond, and giants past the frost line. The belt
//! takes the first wide gap past the frost line.
//!
//! Everything you can travel to is a `Place`, listed outward from the star.
//!
//! Systems come in kinds (see `SystemKind`): a people's seat, ordinary
//! settled systems, thin frontiers, desolate ones no one lives in, rubble
//! fields with a great belt, and graveyards of old wars. The galaxy decides
//! which kind each system is, and who holds it, and hands that over as a
//! `Profile`. The system a universe first started you in is made without
//! one, exactly as it always was, so old universes and saves keep it.

use std::cell::RefCell;
use std::collections::{BTreeMap, HashSet};
use std::f64::consts::{PI, TAU};
use std::ops::RangeInclusive;

use rand::RngExt;
use rand::seq::IndexedRandom;

use super::Vec2;
use super::orbit::{EARTH_MASS, Orbit};
use crate::dungeon::crawl;
use crate::names::Culture;
use crate::seed::{self, Rng};

pub type Rgb = (u8, u8, u8);

pub struct StarKind {
    pub name: &'static str,
    pub rgb: Rgb,
    /// In suns.
    pub mass: f64,
    /// In suns; sets where the habitable zone is.
    pub luminosity: f64,
    weight: u32,
    /// How likely it is in a system no one lives in.
    dead_weight: u32,
}

#[rustfmt::skip]
pub const STARS: &[StarKind] = &[
    StarKind { name: "yellow dwarf", rgb: (255, 226, 130), mass: 1.0, luminosity: 1.0, weight: 4, dead_weight: 1 },
    StarKind { name: "orange dwarf", rgb: (255, 168, 90), mass: 0.75, luminosity: 0.35, weight: 3, dead_weight: 2 },
    StarKind { name: "red dwarf", rgb: (255, 110, 80), mass: 0.4, luminosity: 0.04, weight: 2, dead_weight: 5 },
    StarKind { name: "blue-white star", rgb: (175, 205, 255), mass: 1.9, luminosity: 9.0, weight: 1, dead_weight: 2 },
];

/// A star for a system of this kind: dim red dwarfs, mostly, where no one
/// lives.
pub fn random_star(kind: SystemKind, rng: &mut Rng) -> &'static StarKind {
    if kind == SystemKind::Desolate {
        STARS.choose_weighted(rng, |s| s.dead_weight).unwrap()
    } else {
        STARS.choose_weighted(rng, |s| s.weight).unwrap()
    }
}

/// What sort of system it is, which shapes everything in it.
#[derive(Clone, Copy, Debug, PartialEq, Eq, Hash)]
pub enum SystemKind {
    /// The seat of a people: their homeworld and its great capital.
    Seat,
    /// An ordinary system: a capital, settled worlds, a station or two.
    Settled,
    /// Thinly settled: a small capital or an outpost station.
    Frontier,
    /// No one lives here at all, and nothing is for sale.
    Desolate,
    /// Few planets, and a wide belt thick with ore, worked by mining stations.
    Rubble,
    /// An old battlefield: wrecks everywhere, few people.
    Graveyard,
}

impl SystemKind {
    pub const ALL: [SystemKind; 6] = [
        SystemKind::Seat,
        SystemKind::Settled,
        SystemKind::Frontier,
        SystemKind::Desolate,
        SystemKind::Rubble,
        SystemKind::Graveyard,
    ];

    /// "a frontier system", for "a frontier system of the Qaari".
    pub fn name(self) -> &'static str {
        match self {
            SystemKind::Seat => "the seat",
            SystemKind::Settled => "a settled system",
            SystemKind::Frontier => "a frontier system",
            SystemKind::Desolate => "a desolate system",
            SystemKind::Rubble => "a rubble field",
            SystemKind::Graveyard => "a graveyard of ships",
        }
    }

    /// What you'll find there, in a sentence.
    pub fn blurb(self) -> &'static str {
        match self {
            SystemKind::Seat => "Their homeworld and its great capital, busy stations, fuel.",
            SystemKind::Settled => "Settled worlds round a capital, a station or two, fuel.",
            SystemKind::Frontier => "Thinly settled: a small capital or an outpost, and fuel.",
            SystemKind::Desolate => "No one lives here. Nothing is for sale, fuel least of all.",
            SystemKind::Rubble => "A wide belt thick with ore, worked by mining stations.",
            SystemKind::Graveyard => "An old battlefield. Wrecks drift everywhere; few people.",
        }
    }

    /// Whether anyone in a system of this kind sells fuel. Only desolate
    /// systems have no one to.
    pub fn sells_fuel(self) -> bool {
        self != SystemKind::Desolate
    }
}

/// What the galaxy decides about a system before it's made.
pub struct Profile {
    pub name: String,
    /// Who holds it.
    pub culture: Culture,
    pub star: &'static StarKind,
    pub kind: SystemKind,
    /// The people over the border, for a system on one: they settle here
    /// too, and may keep a station.
    pub neighbour: Option<Culture>,
}

/// How a kind of system is laid out and settled.
struct Rules {
    planets: RangeInclusive<usize>,
    /// How much wider than it starts the belt is, outer edge over inner.
    belt_width: (f64, f64),
    /// Scales every world's chance of being settled.
    settle: f64,
    /// Of settled worlds, the share the system's own people hold.
    own_share: f64,
    /// Who holds the rest; anyone at all, if no one in particular.
    minority: Option<Culture>,
    /// The chance the main world has a capital.
    capital: f64,
    /// No life: oceans and jungles give way to rock and ice.
    dead: bool,
    stations: Stations,
    /// How many wrecks are calling at once, on average.
    wrecks: f64,
    /// Rocks in the belt, and the share with ore in them, against an
    /// ordinary belt's.
    density: f64,
    richness: f64,
}

#[derive(Clone, Copy, PartialEq)]
enum Stations {
    /// One at the heaviest planet, and often one at the main world.
    Usual,
    /// One at the heaviest planet and one at the main world, or another.
    Two,
    /// Exactly one.
    One,
    /// One if there's no capital to trade at; otherwise, maybe.
    Outpost,
    None,
}

impl Rules {
    /// The rules a system was always made by, before there were kinds.
    const ORIGINAL: Rules = Rules {
        planets: 4..=7,
        belt_width: (1.4, 1.7),
        settle: 1.0,
        own_share: 0.7,
        minority: None,
        capital: 1.0,
        dead: false,
        stations: Stations::Usual,
        wrecks: 2.5,
        density: 1.0,
        richness: 0.6,
    };

    fn for_profile(p: &Profile) -> Rules {
        // On a border, the neighbours settle nearly as much as the owners.
        let (own_share, minority) = match p.neighbour {
            Some(n) => (0.6, Some(n)),
            None => (0.92, None),
        };
        let base = Rules { own_share, minority, ..Rules::ORIGINAL };
        match p.kind {
            SystemKind::Seat => Rules { stations: Stations::Two, ..base },
            SystemKind::Settled => base,
            SystemKind::Frontier => Rules {
                planets: 3..=6,
                settle: 0.35,
                capital: 0.55,
                stations: Stations::Outpost,
                wrecks: 2.0,
                ..base
            },
            SystemKind::Desolate => Rules {
                planets: 3..=6,
                settle: 0.0,
                capital: 0.0,
                dead: true,
                stations: Stations::None,
                wrecks: 2.0,
                ..base
            },
            SystemKind::Rubble => Rules {
                planets: 2..=4,
                belt_width: (2.2, 2.8),
                settle: 0.6,
                capital: 0.5,
                stations: Stations::Two,
                density: 1.6,
                richness: 0.85,
                ..base
            },
            SystemKind::Graveyard => {
                Rules { settle: 0.3, capital: 0.35, stations: Stations::One, wrecks: 6.5, ..base }
            }
        }
    }
}

pub struct BodyKind {
    pub name: &'static str,
    pub rgb: Rgb,
    /// Drawn with cloud bands.
    pub banded: bool,
    /// Radius when you're in orbit around it, in local map units. Not to
    /// scale; giants are just bigger.
    pub size: f64,
    pub giant: bool,
    /// The chance someone lives there.
    settled: f64,
    pub climate: Climate,
    pub terrain: Terrain,
    flavour: &'static [&'static str],
}

/// What a world's ground is, for a city built on it.
#[derive(Clone, Copy, Debug, PartialEq, Eq)]
pub enum Terrain {
    Desert,
    Ice,
    Ocean,
    Jungle,
    Barren,
    Forge,
    /// A giant's: no ground at all. No one builds a city on one.
    Clouds,
}

/// How a kind of world holds heat, and what its air is like. Only for
/// show: see `space::survey`.
pub struct Climate {
    /// How much starlight it reflects, from 0 to 1.
    pub albedo: f64,
    /// How much warmer it is than bare rock would be at the same distance,
    /// in kelvin: heat its air traps, or its insides give off.
    pub warmth: f64,
    /// A few ways of putting what the air is like. Empty if there's none.
    pub air: &'static [&'static str],
    /// Whether people could breathe it, if it weren't too hot or cold.
    pub breathable: bool,
}

macro_rules! kind {
    ($name:expr, $terrain:expr, $rgb:expr, $size:expr, $settled:expr, $climate:expr,
     $($flavour:expr),+) => {
        BodyKind { name: $name, rgb: $rgb, banded: false, size: $size, giant: false,
                   settled: $settled, climate: $climate, terrain: $terrain,
                   flavour: &[$($flavour),+] }
    };
}

macro_rules! climate {
    ($albedo:expr, $warmth:expr, $breathable:expr, [$($air:expr),*]) => {
        Climate { albedo: $albedo, warmth: $warmth, breathable: $breathable, air: &[$($air),*] }
    };
}

const DESERT: BodyKind = kind!(
    "desert world",
    Terrain::Desert,
    (214, 172, 104),
    10.0,
    0.7,
    climate!(0.25, 35.0, true, ["thin, dry and dusty", "dry; it smells of hot stone"]),
    "Wind-carved dunes run unbroken from pole to pole.",
    "Glass storms scour the plains at dusk.",
    "Under the sand lie the bones of something enormous."
);
const ICE: BodyKind = kind!(
    "ice world",
    Terrain::Ice,
    (190, 222, 240),
    9.0,
    0.3,
    climate!(0.6, 8.0, false, ["thin and bitter", "a whisper of nitrogen"]),
    "Blue glaciers grind slowly over a buried sea.",
    "Its auroras whistle and sing on the comm bands."
);
const OCEAN: BodyKind = kind!(
    "ocean world",
    Terrain::Ocean,
    (70, 124, 205),
    11.0,
    0.6,
    climate!(0.3, 33.0, true, ["thick, humid and salty", "warm, wet and briny"]),
    "One endless ocean, broken by a scatter of black islands.",
    "Kelp forests a hundred metres tall sway beneath the waves."
);
const JUNGLE: BodyKind = kind!(
    "jungle world",
    Terrain::Jungle,
    (84, 164, 84),
    11.0,
    0.5,
    climate!(0.25, 40.0, true, ["dense, wet, spore-laden", "heavy and green-smelling"]),
    "Canopy covers everything; nothing on the ground has seen the sun.",
    "The air is thick with spores and slow-drifting seed-kites."
);
const BARREN: BodyKind = kind!(
    "barren rock",
    Terrain::Barren,
    (146, 136, 124),
    7.0,
    0.15,
    climate!(0.12, 0.0, false, []),
    "Cratered grey rock, airless and silent.",
    "Ancient mine shafts riddle the crust."
);
const FORGE: BodyKind = kind!(
    "forge world",
    Terrain::Forge,
    (200, 86, 60),
    9.0,
    0.4,
    climate!(0.15, 220.0, false, ["sulphurous and choking", "thin, hot and ashy"]),
    "Rivers of lava light up the night side.",
    "The crust is thin and restless, and the ground shakes daily."
);
const GAS_GIANT: BodyKind = BodyKind {
    banded: true,
    giant: true,
    ..kind!(
        "gas giant",
        Terrain::Clouds,
        (206, 154, 112),
        20.0,
        0.25,
        climate!(0.34, 45.0, false, ["hydrogen and helium", "hydrogen, streaked with ammonia"]),
        "Storms the size of worlds turn in its banded clouds.",
        "Cloud-skimmers dive into its upper layers for fuel."
    )
};
const ICE_GIANT: BodyKind = BodyKind {
    banded: true,
    giant: true,
    ..kind!(
        "ice giant",
        Terrain::Clouds,
        (120, 200, 210),
        16.0,
        0.1,
        climate!(0.3, 15.0, false, ["hydrogen and methane haze", "cold hydrogen, faintly blue"]),
        "A pale blue-green ball of methane haze, cold and still.",
        "Diamond hail falls forever through its depths."
    )
};

/// What a planet `a` AU out is likely to be, given where the habitable zone
/// (`hz`) and the frost line are.
fn planet_kinds(a: f64, hz: f64) -> &'static [&'static BodyKind] {
    if a < 0.75 * hz {
        &[&FORGE, &BARREN, &DESERT]
    } else if a < 1.7 * hz {
        &[&OCEAN, &JUNGLE, &DESERT, &DESERT]
    } else if a < FROST_LINE * hz {
        &[&ICE, &BARREN, &DESERT]
    } else {
        &[&GAS_GIANT, &GAS_GIANT, &ICE_GIANT]
    }
}

/// The same, in a dead system: no oceans or jungles, only rock and ice.
fn dead_kinds(a: f64, hz: f64) -> &'static [&'static BodyKind] {
    if a < 0.75 * hz {
        &[&FORGE, &BARREN, &BARREN]
    } else if a < 1.7 * hz {
        &[&BARREN, &DESERT, &BARREN]
    } else if a < FROST_LINE * hz {
        &[&ICE, &BARREN]
    } else {
        &[&GAS_GIANT, &ICE_GIANT, &ICE_GIANT]
    }
}

/// Beyond this many habitable-zone distances out, water freezes and giants form.
const FROST_LINE: f64 = 2.7;

/// What a moon is likely to be, inside the frost line and beyond it. Forge
/// moons are the ones a giant kneads hot with its tides.
const MOON_KINDS: [&[&BodyKind]; 2] =
    [&[&BARREN, &BARREN, &DESERT, &FORGE], &[&ICE, &ICE, &BARREN, &FORGE]];

fn settlements(culture: Culture) -> &'static [&'static str] {
    match culture {
        Culture::Qaari => &[
            "Qaari caravan-cities crawl across the surface on great tracked platforms.",
            "The Qaari keep a water-court here, where oaths are sworn over cisterns.",
        ],
        Culture::Cant => &[
            "Foundry-cathedrals of the Cant hum day and night with litanies.",
            "The Cant strip old hulls here, and sing over the parts.",
        ],
        Culture::Houses => &[
            "A High House palace sprawls across the one temperate valley.",
            "House banners hang from the spires of a lofty court-city.",
        ],
        Culture::Drift => &[
            "A tangle of lashed-together hulls hangs in orbit: a Drift anchorage.",
            "The Drift trade here, from a market of docked ships.",
        ],
    }
}

const UNSETTLED: &[&str] = &[
    "No one lives here. At night the only lights are the stars.",
    "A survey beacon still blinks on the surface, long abandoned.",
];

pub struct Planet {
    pub name: String,
    pub kind: &'static BodyKind,
    /// Who lives there, if anyone.
    pub people: Option<Culture>,
    pub orbit: Orbit,
    /// In suns.
    pub mass: f64,
    pub moons: Vec<Moon>,
    /// A few sentences for the landing screen.
    pub description: Vec<String>,
}

pub struct Moon {
    pub name: String,
    pub kind: &'static BodyKind,
    pub people: Option<Culture>,
    /// Around its planet.
    pub orbit: Orbit,
    pub description: Vec<String>,
}

pub struct Belt {
    pub name: String,
    pub inner: f64,
    pub outer: f64,
    /// The stretch of belt you can travel to, which goes round with it.
    pub region: Orbit,
    /// How many rocks there are, against an ordinary belt.
    pub density: f64,
    /// The chance a rock has ore in it.
    pub richness: f64,
}

pub struct Station {
    pub name: String,
    pub culture: Culture,
    /// The planet whose Lagrange point it sits at.
    pub planet: usize,
    /// At L4, sixty degrees ahead of the planet, rather than L5, behind it.
    pub leading: bool,
    pub description: Vec<String>,
}

/// A derelict with its distress beacon calling. Wrecks come and go: each
/// drifts in on some day, calls for a month or so, and then its beacon fails
/// and it's lost to the dark.
#[derive(Clone, Debug, PartialEq)]
pub struct Wreck {
    /// Board it and this seeds the derelict you find.
    pub seed: u64,
    pub name: String,
    /// "Cant hauler".
    pub description: String,
    /// Which place it drifts in.
    pub place: usize,
    /// The day its beacon started calling.
    pub arrived: f64,
    /// The day its beacon fails and it's gone.
    pub leaves: f64,
}

impl Wreck {
    pub fn calling(&self, day: f64) -> bool {
        self.arrived <= day && day < self.leaves
    }
}

/// Wrecks turn up in batches, one batch for every this many days.
const WRECK_EPOCH: f64 = 10.0;
/// How long a beacon calls, in days.
const BEACON_LIFE: (f64, f64) = (20.0, 45.0);
/// The most wrecks one batch can bring.
const WRECK_TRIES: usize = 6;

#[derive(Clone, Copy, Debug, PartialEq, Eq)]
pub enum PlaceKind {
    /// Orbit around a planet.
    Planet(usize),
    /// Orbit around a planet's moon.
    Moon(usize, usize),
    Belt,
    Station(usize),
}

/// What there is for you when you land or dock somewhere.
#[derive(Clone, Copy, Debug, PartialEq, Eq)]
pub enum Landing {
    /// People live on a world with ground to land on: you set down outside
    /// their capital and walk in, to a full set of shops.
    Capital(Culture),
    /// A station: dock and sell ore, and that's all.
    Market(Culture),
    /// People live in a giant's clouds, but there's no ground to land on.
    Aloft(Culture),
    /// A world no one lives on: land and look around, with no one to trade with.
    Wilds,
    /// Nowhere to land at all: the belt, or an empty giant.
    Nowhere,
}

impl Landing {
    /// Whether anyone here buys ore.
    pub fn trades(self) -> bool {
        matches!(self, Landing::Capital(_) | Landing::Market(_))
    }
}

pub struct Place {
    pub kind: PlaceKind,
    /// "Qeth Amarul", "the Zirim Belt".
    pub name: String,
}

pub struct System {
    pub seed: u64,
    pub name: String,
    /// Whose space this is; most settled worlds here are theirs.
    pub culture: Culture,
    pub kind: SystemKind,
    pub star: &'static StarKind,
    pub planets: Vec<Planet>,
    /// The system's main world: in a seat, the homeworld of its people,
    /// where you start. In a system no one lives in, the world likeliest to
    /// have been settled.
    pub home: usize,
    pub belt: Belt,
    pub stations: Vec<Station>,
    /// Everywhere you can travel to, outward from the star.
    pub places: Vec<Place>,
    /// How many wrecks are calling at once, on average.
    wreck_rate: f64,
    /// On a battlefield, the place most wrecks turn up.
    battle: Option<usize>,
    /// Batches of wrecks already made, by epoch: naming one means
    /// generating its whole ship.
    batches: RefCell<BTreeMap<i64, Vec<Wreck>>>,
}

impl System {
    /// Where a place is on `day`, in AU from the star.
    pub fn position(&self, place: usize, day: f64) -> Vec2 {
        match self.places[place].kind {
            PlaceKind::Planet(p) => self.planets[p].orbit.position(day),
            PlaceKind::Moon(p, m) => {
                self.planets[p].orbit.position(day) + self.planets[p].moons[m].orbit.position(day)
            }
            PlaceKind::Belt => self.belt.region.position(day),
            PlaceKind::Station(s) => {
                let station = &self.stations[s];
                let lead = if station.leading { PI / 3.0 } else { -PI / 3.0 };
                self.planets[station.planet].orbit.position(day).rotated(lead)
            }
        }
    }

    /// What there is for you if you land or dock at a place.
    pub fn landing(&self, place: usize) -> Landing {
        let world = |kind: &BodyKind, people: Option<Culture>| match (people, kind.giant) {
            (Some(c), true) => Landing::Aloft(c),
            (Some(c), false) => Landing::Capital(c),
            (None, true) => Landing::Nowhere,
            (None, false) => Landing::Wilds,
        };
        match self.places[place].kind {
            PlaceKind::Planet(p) => world(self.planets[p].kind, self.planets[p].people),
            PlaceKind::Moon(p, m) => {
                let moon = &self.planets[p].moons[m];
                world(moon.kind, moon.people)
            }
            PlaceKind::Station(s) => Landing::Market(self.stations[s].culture),
            PlaceKind::Belt => Landing::Nowhere,
        }
    }

    /// The place for orbit around the homeworld, where journeys start.
    pub fn home_place(&self) -> usize {
        self.places.iter().position(|p| p.kind == PlaceKind::Planet(self.home)).unwrap()
    }

    /// Whether the main world is the homeworld of a people, not just the
    /// biggest place in the system.
    pub fn is_homeworld(&self, planet: usize) -> bool {
        planet == self.home && self.kind == SystemKind::Seat
    }

    /// Where a jump brings you out: the outermost planet, or the belt if it
    /// lies further out.
    pub fn edge_place(&self) -> usize {
        self.places
            .iter()
            .rposition(|p| matches!(p.kind, PlaceKind::Planet(_) | PlaceKind::Belt))
            .unwrap()
    }

    /// Whether you can buy fuel at a place: at a station's dock, or a
    /// city's shipyard.
    pub fn sells_fuel_at(&self, place: usize) -> bool {
        self.landing(place).trades()
    }

    /// Whether anywhere in the system sells fuel.
    pub fn sells_fuel(&self) -> bool {
        (0..self.places.len()).any(|i| self.sells_fuel_at(i))
    }

    /// Where to go for fuel: the first station, or failing that the first
    /// capital.
    pub fn fuel_place(&self) -> Option<usize> {
        let station = |i: &usize| matches!(self.landing(*i), Landing::Market(_));
        (0..self.places.len())
            .find(station)
            .or_else(|| (0..self.places.len()).find(|&i| self.sells_fuel_at(i)))
    }

    /// How far out anything goes, in AU.
    pub fn extent(&self) -> f64 {
        let planets = self.planets.iter().map(|p| p.orbit.apoapsis_distance());
        planets.fold(self.belt.outer, f64::max)
    }

    /// The wrecks whose beacons are calling on `day`, oldest first.
    pub fn wrecks_on(&self, day: f64) -> Vec<Wreck> {
        let first = ((day - BEACON_LIFE.1) / WRECK_EPOCH).floor() as i64;
        let last = (day / WRECK_EPOCH).floor() as i64;
        let mut wrecks: Vec<Wreck> =
            (first..=last).flat_map(|epoch| self.batch(epoch)).filter(|w| w.calling(day)).collect();
        wrecks.sort_by(|a, b| a.arrived.total_cmp(&b.arrived));
        wrecks
    }

    /// The wrecks calling on `day` that you haven't stripped yet.
    pub fn beacons(&self, day: f64, stripped: &HashSet<u64>) -> Vec<Wreck> {
        let mut wrecks = self.wrecks_on(day);
        wrecks.retain(|w| !stripped.contains(&w.seed));
        wrecks
    }

    /// The wrecks you find at `place` on `day`, having set out for it on
    /// `departed`: those calling now, and those that were calling when you
    /// set out, even if their beacons have failed on the way, since you
    /// came by their last fix. Not those you've stripped. Oldest first.
    pub fn wrecks_at(
        &self,
        place: usize,
        day: f64,
        departed: Option<f64>,
        stripped: &HashSet<u64>,
    ) -> Vec<Wreck> {
        let mut wrecks = self.beacons(day, stripped);
        if let Some(departed) = departed.filter(|&d| d < day) {
            for w in self.beacons(departed, stripped) {
                if !wrecks.iter().any(|x| x.seed == w.seed) {
                    wrecks.push(w);
                }
            }
        }
        wrecks.retain(|w| w.place == place);
        wrecks.sort_by(|a, b| a.arrived.total_cmp(&b.arrived));
        wrecks
    }

    /// The wrecks that turned up in one epoch, whenever they leave.
    fn batch(&self, epoch: i64) -> Vec<Wreck> {
        if let Some(batch) = self.batches.borrow().get(&epoch) {
            return batch.clone();
        }
        let batch_seed = seed::derive(self.seed, "wrecks", epoch as u64);
        let mut rng = seed::rng(batch_seed);
        // Enough turn up, for as long as they last, that `wreck_rate` are
        // calling at any one time.
        let expected = self.wreck_rate * WRECK_EPOCH / ((BEACON_LIFE.0 + BEACON_LIFE.1) / 2.0);
        let chance = (expected / WRECK_TRIES as f64).min(1.0);
        let count = (0..WRECK_TRIES).filter(|_| rng.random_bool(chance)).count();
        let batch: Vec<Wreck> = (0..count)
            .map(|i| {
                let seed = seed::derive(batch_seed, "wreck", i as u64);
                let (name, description) = crawl::describe(seed);
                let place = match self.battle {
                    Some(b) if rng.random_bool(0.6) => b,
                    _ => rng.random_range(..self.places.len()),
                };
                let arrived = (epoch as f64 + rng.random_range(0.0..1.0)) * WRECK_EPOCH;
                let leaves = arrived + rng.random_range(BEACON_LIFE.0..BEACON_LIFE.1);
                Wreck { seed, name, description, place, arrived, leaves }
            })
            .collect();
        self.batches.borrow_mut().insert(epoch, batch.clone());
        batch
    }
}

/// The system a universe has always started you in, made from its seed
/// alone: who holds it, its name and its star come from the seed too.
pub fn generate(seed: u64) -> System {
    build(seed, None)
}

/// A system the galaxy has already decided things about.
pub fn generate_as(seed: u64, profile: &Profile) -> System {
    build(seed, Some(profile))
}

// Without a profile, this must draw from the RNG exactly as it always has,
// or the system every old universe started in would change under its saves.
fn build(seed: u64, profile: Option<&Profile>) -> System {
    let mut rng = seed::rng(seed);
    let (culture, system_name, star, kind, rules) = match profile {
        None => {
            let culture = random_culture(&mut rng);
            let name = culture.word(&mut rng);
            let star = STARS.choose_weighted(&mut rng, |s| s.weight).unwrap();
            (culture, name, star, SystemKind::Seat, Rules::ORIGINAL)
        }
        Some(p) => (p.culture, p.name.clone(), p.star, p.kind, Rules::for_profile(p)),
    };
    let hz = star.luminosity.sqrt();

    // Orbits first, so the homeworld can be chosen before names are given.
    let count = rng.random_range(rules.planets.clone());
    let mut orbits: Vec<(Orbit, &'static BodyKind, f64)> = Vec::new();
    let mut belt_span = None;
    let mut a = hz * rng.random_range(0.3..0.55);
    let mut outermost = 0.0;
    for i in 0..count {
        if i > 0 {
            a *= rng.random_range(1.5..2.0);
        }
        if belt_span.is_none() && i > 0 && a > FROST_LINE * hz {
            let inner: f64 = outermost * 1.3;
            let outer = inner * rng.random_range(rules.belt_width.0..rules.belt_width.1);
            belt_span = Some((inner, outer));
            a = a.max(outer * 1.45);
        }
        // Keep each orbit well clear of the last, however eccentric.
        let mut e = if rng.random_bool(0.15) {
            rng.random_range(0.12..0.22)
        } else {
            rng.random_range(0.0..0.08)
        };
        while i > 0 && e > 1e-6 && a * (1.0 - e) < outermost * 1.15 {
            e *= 0.5;
        }
        let kinds = if rules.dead { dead_kinds(a, hz) } else { planet_kinds(a, hz) };
        let kind = *kinds.choose(&mut rng).unwrap();
        let mass = EARTH_MASS
            * if kind.giant { rng.random_range(15.0..320.0) } else { rng.random_range(0.05..2.5) };
        let orbit =
            Orbit::new(a, e, rng.random_range(0.0..TAU), rng.random_range(0.0..TAU), star.mass);
        outermost = orbit.apoapsis_distance();
        orbits.push((orbit, kind, mass));
    }
    let (inner, outer) = belt_span.unwrap_or_else(|| {
        let inner = outermost * 1.3;
        let width = if rules.belt_width == Rules::ORIGINAL.belt_width {
            1.5
        } else {
            rng.random_range(rules.belt_width.0..rules.belt_width.1)
        };
        (inner, inner * width)
    });
    let region = Orbit::new((inner + outer) / 2.0, 0.0, 0.0, rng.random_range(0.0..TAU), star.mass);
    let word = culture.word(&mut rng);
    let belt = Belt {
        name: if kind == SystemKind::Rubble {
            format!("the {word} Shoals")
        } else {
            format!("the {word} Belt")
        },
        inner,
        outer,
        region,
        density: rules.density,
        richness: rules.richness,
    };

    // Home is the habitable-zone world nearest the middle of the zone.
    let home = (0..orbits.len())
        .filter(|&i| !orbits[i].1.giant)
        .min_by(|&i, &j| {
            let off = |k: usize| (orbits[k].0.a / hz).ln().abs();
            off(i).total_cmp(&off(j))
        })
        .unwrap_or(0);
    // And it's always somewhere people could live, unless nothing lives here.
    if !rules.dead {
        orbits[home].1 = *[&OCEAN, &JUNGLE, &DESERT].choose(&mut rng).unwrap();
    }
    let capital = rules.capital >= 1.0 || rng.random_bool(rules.capital);

    let mut planets = Vec::new();
    for (i, (orbit, kind, mass)) in orbits.into_iter().enumerate() {
        let people = if i == home {
            capital.then_some(culture)
        } else {
            settle(kind, culture, &rules, &mut rng)
        };
        let name = match people {
            Some(c) => c.word(&mut rng),
            None => format!("{system_name} {}", roman(i + 1)),
        };
        let moon_count =
            if kind.giant { rng.random_range(1..=4) } else { usize::from(rng.random_bool(0.4)) };
        let mut moon_a = (if kind.giant { 0.002 } else { 0.0012 }) * rng.random_range(1.0..1.6);
        // Moons only stay put well inside the planet's Hill sphere, where
        // its pull beats the star's.
        let hill = orbit.periapsis_distance() * (mass / (3.0 * star.mass)).cbrt();
        let frozen = orbit.a > FROST_LINE * hz;
        let mut moons = Vec::new();
        for _ in 0..moon_count {
            if moon_a > 0.4 * hill {
                break;
            }
            let kind = *MOON_KINDS[usize::from(frozen)].choose(&mut rng).unwrap();
            let people = settle(kind, culture, &rules, &mut rng).filter(|_| rng.random_bool(0.5));
            let moon_name = match people {
                Some(c) => c.word(&mut rng),
                None => culture.word(&mut rng),
            };
            let orbit = Orbit::new(
                moon_a,
                rng.random_range(0.0..0.05),
                rng.random_range(0.0..TAU),
                rng.random_range(0.0..TAU),
                mass,
            );
            moons.push(Moon {
                name: moon_name,
                kind,
                people,
                orbit,
                description: describe(kind, people, &mut rng),
            });
            moon_a *= rng.random_range(1.6..2.2);
        }
        planets.push(Planet {
            name,
            kind,
            people,
            orbit,
            mass,
            moons,
            description: describe(kind, people, &mut rng),
        });
    }

    let stations = stations(&planets, home, culture, &rules, &mut rng);
    let places = list_places(&planets, &belt, &stations);
    // On a battlefield, most of the wrecks turn up in one place: where the
    // fighting was thickest, and still draws scavengers and the lost.
    let battle = (kind == SystemKind::Graveyard)
        .then(|| seed::rng(seed::derive(seed, "battle", 0)).random_range(..places.len()));

    System {
        seed,
        name: system_name,
        culture,
        kind,
        star,
        planets,
        home,
        belt,
        stations,
        places,
        wreck_rate: rules.wrecks,
        battle,
        batches: RefCell::new(BTreeMap::new()),
    }
}

fn settle(kind: &BodyKind, culture: Culture, rules: &Rules, rng: &mut Rng) -> Option<Culture> {
    rng.random_bool((kind.settled * rules.settle).min(1.0)).then(|| {
        if rng.random_bool(rules.own_share) {
            culture
        } else {
            rules.minority.unwrap_or_else(|| random_culture(rng))
        }
    })
}

/// Stations at Lagrange points: usually one at the heaviest planet, and
/// often one at the homeworld's too.
fn stations(
    planets: &[Planet],
    home: usize,
    culture: Culture,
    rules: &Rules,
    rng: &mut Rng,
) -> Vec<Station> {
    let heaviest =
        (0..planets.len()).max_by(|&i, &j| planets[i].mass.total_cmp(&planets[j].mass)).unwrap();
    let capital = planets[home].people.is_some() && !planets[home].kind.giant;
    let mut anchors = vec![heaviest];
    match rules.stations {
        Stations::Usual => {
            if heaviest != home && rng.random_bool(0.6) {
                anchors.push(home);
            }
        }
        Stations::Two => {
            let second = if heaviest != home {
                Some(home)
            } else {
                (0..planets.len())
                    .filter(|&p| p != heaviest)
                    .max_by_key(|&p| planets[p].moons.len())
            };
            anchors.extend(second);
        }
        Stations::One => {}
        Stations::Outpost => {
            if capital && rng.random_bool(0.5) {
                anchors.clear();
            }
        }
        Stations::None => anchors.clear(),
    }
    anchors
        .into_iter()
        .enumerate()
        .map(|(i, p)| {
            // Over a border, the neighbours may keep the second station.
            let who = match rules.minority {
                Some(n) if i > 0 && rng.random_bool(0.5) => n,
                _ => culture,
            };
            station(p, &planets[p], who, rng)
        })
        .collect()
}

fn describe(kind: &BodyKind, people: Option<Culture>, rng: &mut Rng) -> Vec<String> {
    let settlement = match people {
        Some(c) => settlements(c).choose(rng).unwrap(),
        None => UNSETTLED.choose(rng).unwrap(),
    };
    vec![kind.flavour.choose(rng).unwrap().to_string(), settlement.to_string()]
}

fn station(planet: usize, at: &Planet, culture: Culture, rng: &mut Rng) -> Station {
    let leading = rng.random_bool(0.5);
    let word = culture.word(rng);
    let name = match culture {
        Culture::Qaari => format!("the {word} Caravanserai"),
        Culture::Cant => format!("Foundry {word}"),
        Culture::Houses => format!("the {word} Court"),
        Culture::Drift => format!("{word} Anchorage"),
    };
    let (point, side) = if leading { ("L4", "ahead of") } else { ("L5", "behind") };
    let mut description = vec![format!(
        "It turns slowly at {}'s {point} point, sixty degrees {side} the planet, \
         where the pulls of {} and the star balance.",
        at.name, at.name
    )];
    description.push(
        match culture {
            Culture::Qaari => {
                "Its docks smell of spice-tea and hot metal; caravans of ore haulers queue outside."
            }
            Culture::Cant => {
                "Its hull is studded with salvaged plating, every plate engraved with a prayer."
            }
            Culture::Houses => {
                "Liveried pilots wait in its gilded docks, and everyone is watching everyone."
            }
            Culture::Drift => {
                "It's less a station than a raft of docked ships, rearranged every season."
            }
        }
        .to_string(),
    );
    Station { name, culture, planet, leading, description }
}

/// Every place, in order outward: each planet followed by its moons and any
/// station at its Lagrange points, with the belt where it falls.
fn list_places(planets: &[Planet], belt: &Belt, stations: &[Station]) -> Vec<Place> {
    let mut places = Vec::new();
    let mut belt_listed = false;
    for (p, planet) in planets.iter().enumerate() {
        if !belt_listed && planet.orbit.a > belt.region.a {
            places.push(Place { kind: PlaceKind::Belt, name: belt.name.clone() });
            belt_listed = true;
        }
        places.push(Place { kind: PlaceKind::Planet(p), name: planet.name.clone() });
        for (m, moon) in planet.moons.iter().enumerate() {
            places.push(Place { kind: PlaceKind::Moon(p, m), name: moon.name.clone() });
        }
        for (s, station) in stations.iter().enumerate().filter(|(_, s)| s.planet == p) {
            places.push(Place { kind: PlaceKind::Station(s), name: station.name.clone() });
        }
    }
    if !belt_listed {
        places.push(Place { kind: PlaceKind::Belt, name: belt.name.clone() });
    }
    places
}

fn random_culture(rng: &mut Rng) -> Culture {
    Culture::ALL[rng.random_range(..Culture::ALL.len())]
}

fn roman(n: usize) -> &'static str {
    ["0", "I", "II", "III", "IV", "V", "VI", "VII", "VIII"].get(n).copied().unwrap_or("IX")
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn systems_are_sound() {
        for seed in 0..300 {
            let s = generate(seed);
            assert!((4..=7).contains(&s.planets.len()));
            let mut outermost = 0.0;
            for p in &s.planets {
                assert!(
                    p.orbit.periapsis_distance() > outermost,
                    "seed {seed}: {} crosses",
                    p.name
                );
                outermost = p.orbit.apoapsis_distance();
                let hill = p.orbit.periapsis_distance() * (p.mass / (3.0 * s.star.mass)).cbrt();
                for m in &p.moons {
                    assert!(
                        m.orbit.a < 0.4 * hill,
                        "seed {seed}: {} strays from {}",
                        m.name,
                        p.name
                    );
                    assert!(m.orbit.period < p.orbit.period / 5.0);
                }
            }
            let crosses_belt = |p: &Planet| {
                p.orbit.apoapsis_distance() > s.belt.inner
                    && p.orbit.periapsis_distance() < s.belt.outer
            };
            assert!(
                !s.planets.iter().any(crosses_belt),
                "seed {seed}: a planet runs through the belt"
            );
            assert_eq!(s.planets[s.home].people, Some(s.culture), "seed {seed}: home is unsettled");
            assert!(!s.planets[s.home].kind.giant);
            assert!(!s.stations.is_empty());
            let belts = s.places.iter().filter(|p| p.kind == PlaceKind::Belt).count();
            assert_eq!(belts, 1);
            for w in s.wrecks_on(0.0) {
                assert!(w.place < s.places.len());
                assert!(w.calling(0.0));
            }
        }
    }

    #[test]
    fn capitals_need_people_and_ground() {
        for seed in 0..100 {
            let s = generate(seed);
            assert_eq!(s.landing(s.home_place()), Landing::Capital(s.culture));
            for (i, place) in s.places.iter().enumerate() {
                let landing = s.landing(i);
                match place.kind {
                    PlaceKind::Planet(p) if s.planets[p].kind.giant => {
                        assert!(matches!(landing, Landing::Aloft(_) | Landing::Nowhere));
                    }
                    PlaceKind::Station(_) => assert!(matches!(landing, Landing::Market(_))),
                    PlaceKind::Belt => assert_eq!(landing, Landing::Nowhere),
                    _ => {}
                }
            }
        }
    }

    #[test]
    fn places_are_listed_outward() {
        let s = generate(8);
        let distances: Vec<f64> = (0..s.places.len())
            .filter(|&i| matches!(s.places[i].kind, PlaceKind::Planet(_) | PlaceKind::Belt))
            .map(|i| match s.places[i].kind {
                PlaceKind::Belt => s.belt.region.a,
                PlaceKind::Planet(p) => s.planets[p].orbit.a,
                _ => unreachable!(),
            })
            .collect();
        assert!(distances.windows(2).all(|w| w[0] < w[1]), "{distances:?}");
    }

    #[test]
    fn stations_sit_sixty_degrees_from_their_planet() {
        let s = generate(3);
        for (i, place) in s.places.iter().enumerate() {
            if let PlaceKind::Station(k) = place.kind {
                let planet = s.planets[s.stations[k].planet].orbit.position(40.0);
                let station = s.position(i, 40.0);
                let between = (planet.dot(station) / (planet.length() * station.length())).acos();
                assert!((between - PI / 3.0).abs() < 1e-9);
            }
        }
    }

    #[test]
    fn same_seed_same_system() {
        let names = |s: System| s.places.iter().map(|p| p.name.clone()).collect::<Vec<_>>();
        assert_eq!(names(generate(5)), names(generate(5)));
    }

    #[test]
    fn wrecks_come_and_go() {
        let mut total = 0;
        let days = 0..400;
        for seed in 0..6 {
            let s = generate(seed);
            for day in days.clone() {
                let day = day as f64;
                let here = s.wrecks_on(day);
                total += here.len();
                for w in &here {
                    let life = w.leaves - w.arrived;
                    assert!((BEACON_LIFE.0..BEACON_LIFE.1).contains(&life));
                }
            }
            // A wreck calling now is long gone in a few months, and others
            // have come in its place.
            let now: Vec<u64> = s.wrecks_on(10.0).iter().map(|w| w.seed).collect();
            let later = s.wrecks_on(110.0);
            assert!(later.iter().all(|w| !now.contains(&w.seed)));
            // The same day always has the same wrecks, cached or not.
            let again: Vec<u64> = generate(seed).wrecks_on(10.0).iter().map(|w| w.seed).collect();
            assert_eq!(now, again);
        }
        let average = total as f64 / (6 * days.len()) as f64;
        assert!((2.0..3.0).contains(&average), "{average} wrecks calling at once");
    }

    #[test]
    fn a_failed_beacons_last_fix_still_finds_its_wreck() {
        let s = generate(1);
        let none = HashSet::new();
        let wreck = s.wrecks_on(0.0)[0].clone();
        let later = wreck.leaves + 2.0;
        let found = |departed| s.wrecks_at(wreck.place, later, departed, &none);
        assert!(!found(None).contains(&wreck), "its beacon has failed");
        assert!(found(Some(0.0)).contains(&wreck), "but it called when you set out");
        assert!(!found(Some(wreck.leaves)).contains(&wreck), "not if it had failed already");
        assert!(found(Some(0.0)).iter().all(|w| w.place == wreck.place), "only those here");
        let stripped = HashSet::from([wreck.seed]);
        assert!(!s.wrecks_at(wreck.place, later, Some(0.0), &stripped).contains(&wreck));
    }

    #[test]
    fn graveyards_are_thick_with_wrecks() {
        let profile = Profile {
            name: "Ashfall".into(),
            culture: Culture::Houses,
            star: &STARS[0],
            kind: SystemKind::Graveyard,
            neighbour: None,
        };
        let mut total = 0;
        let mut at_battle = 0;
        for seed in 0..6 {
            let s = generate_as(seed, &profile);
            let battle = s.battle.unwrap();
            for day in (0..300).step_by(3) {
                let here = s.wrecks_on(day as f64);
                total += here.len();
                at_battle += here.iter().filter(|w| w.place == battle).count();
            }
        }
        let average = total as f64 / (6 * 100) as f64;
        assert!((5.0..8.0).contains(&average), "{average} wrecks calling at once");
        assert!(at_battle * 2 > total, "most lie where the fighting was");
    }
}