sim/src/world.rs

mod actors;

use actors::{
    Activity, AnimalActor, AnimalId, AnimalMsg, AnimalTurn, Area, BirthCost, Goods, Mailbox, Needs,
    PartnershipActor, PartnershipMsg, PersonActor, PersonEdit, PersonMsg, ReplyBody, RequestId,
};
use picard::actors::{ActorRef, ActorRefFactory, ActorReference, ActorSystem, Sender, Tell};
use rune::runtime::RuntimeContext;
use rune::termcolor::Buffer;
use rune::{Any, Context, Diagnostics, Module, Source, Sources, Unit, Vm};
use serde::{Deserialize, Serialize};
use std::cmp::Reverse;
use std::collections::{BTreeMap, BinaryHeap, HashSet};
use std::fmt::Write;
use std::sync::{Arc, OnceLock};

pub const W: usize = 96;
pub const H: usize = 64;
pub const DEFAULT_SCRIPT: &str = include_str!("../config.rn");

#[derive(Clone, Copy, Debug, Deserialize, Serialize, rune::FromValue)]
pub struct StageDuration {
    pub min_days: u32,
    pub max_days: u32,
    #[serde(default)]
    pub fertile: bool,
}

#[derive(Clone, Debug, Deserialize, Serialize, rune::FromValue)]
#[serde(default)]
pub struct CalendarSettings {
    pub ticks_per_day: u64,
    pub days_per_week: u32,
    pub weeks_per_month: u32,
    pub months_per_year: u32,
    pub toddler: StageDuration,
    pub child: StageDuration,
    pub teen: StageDuration,
    pub young_adult: StageDuration,
    pub adult: StageDuration,
    pub older_adult: StageDuration,
    pub elderly: StageDuration,
}

impl Default for StageDuration {
    fn default() -> Self {
        Self {
            min_days: 1,
            max_days: 2,
            fertile: false,
        }
    }
}

impl Default for CalendarSettings {
    fn default() -> Self {
        default_config().calendar
    }
}

#[derive(Clone, Debug, rune::FromValue)]
pub struct SimulationConfig {
    width: usize,
    height: usize,
    calendar: CalendarSettings,
    initial_population: usize,
    initial_young_adults: usize,
    initial_rabbits: usize,
    initial_wolves: usize,
    initial_food: u32,
    initial_person_hunger: f32,
    initial_animal_energy: f32,
    animal_energy_death_threshold: f32,
    starting_tick_fraction: u64,
    population_capacity: usize,
    house_capacity: usize,
    max_houses: usize,
    daylight_night_threshold: f32,
    daylight_base: f32,
    daylight_amplitude: f32,
    home_radius: usize,
    attack_range: usize,
    entity_update_interval: u64,
    hunger_night_at_home: f32,
    hunger_other: f32,
    hunger_eat_home_threshold: f32,
    hunger_eat_carried_threshold: f32,
    hunger_return_threshold: f32,
    hunger_starvation_threshold: f32,
    hunger_meal_reduction: f32,
    food_per_meal: u32,
    resource_search_radius: usize,
    person_hunt_success_chance: f32,
    person_hunt_food_yield: u32,
    person_gather_yield: u16,
    person_tooled_gather_yield: u16,
    partnership_distance: usize,
    birth_food_threshold: u32,
    birth_food_cost: u32,
    tool_wood_cost: u32,
    tool_stone_cost: u32,
    house_wood_cost: u32,
    house_stone_cost: u32,
    house_placement_radius: usize,
    resource_regrow_attempts: usize,
    initial_tree_chance: f32,
    initial_tree_amount: u16,
    initial_berry_chance: f32,
    initial_berry_amount: u16,
    initial_stone_chance: f32,
    initial_stone_amount: u16,
    tree_regrow_chance: f32,
    tree_regrow_amount: u16,
    berry_regrow_chance: f32,
    berry_regrow_amount: u16,
    stone_regrow_chance: f32,
    stone_regrow_amount: u16,
    animal_population_interval: u64,
    rabbit_spawn_cap: usize,
    wolf_spawn_cap: usize,
    wolf_spawn_rabbit_threshold: usize,
    wolf_min_home_distance: usize,
    rabbit_energy_drain: f32,
    rabbit_energy_regen: f32,
    rabbit_energy_max: f32,
    rabbit_wolf_detection_range: usize,
    rabbit_breed_energy: f32,
    rabbit_max_population: usize,
    rabbit_breed_chance: f32,
    rabbit_breed_energy_cost: f32,
    rabbit_newborn_energy: f32,
    wolf_energy_drain: f32,
    wolf_prey_range: usize,
    wolf_hunt_success_chance: f32,
    wolf_hunt_energy_gain: f32,
    wolf_energy_max: f32,
    wolf_breed_energy: f32,
    wolf_max_population: usize,
    wolf_breed_chance: f32,
    wolf_breed_energy_cost: f32,
    wolf_newborn_energy: f32,
    terrain_noise_octaves: usize,
    terrain_noise_scale: f32,
    terrain_edge_height: f32,
    terrain_edge_falloff: f32,
    terrain_edge_distance_cap: f32,
    terrain_moisture_offset: f32,
    terrain_moisture_seed_offset: u64,
    terrain_deep_water_threshold: f32,
    terrain_water_threshold: f32,
    terrain_sand_threshold: f32,
    terrain_forest_grass_threshold: f32,
    terrain_hills_threshold: f32,
    terrain_forest_moisture_threshold: f32,
    random_land_attempts: usize,
    log_entry_limit: usize,
    wander_attempts: usize,
    wander_radius: usize,
    manual_spawn_limit: usize,
    max_simulation_speed: u32,
    max_stock_edit: u32,
    max_person_inventory_edit: u32,
    max_hunger_edit: i64,
    max_animal_energy_edit: f32,
    max_resource_edit: u16,
}

impl SimulationConfig {
    fn validate(&self) -> Result<(), String> {
        self.validate_limits()?;
        self.validate_calendar()?;
        self.validate_chances()?;
        self.validate_world()?;
        self.validate_amounts()
    }

    fn validate_limits(&self) -> Result<(), String> {
        if !(8..=256).contains(&self.width) || !(8..=256).contains(&self.height) {
            return Err("world width and height must be between 8 and 256".into());
        }
        if self.initial_young_adults > self.initial_population {
            return Err("initial_young_adults cannot exceed initial_population".into());
        }
        if self.food_per_meal == 0 {
            return Err("food_per_meal must be at least 1".into());
        }
        if self.population_capacity > 1_000_000 || self.house_capacity > 1_000_000 {
            return Err("population capacity values exceed supported limits".into());
        }
        if self.initial_population > 10_000
            || self.initial_rabbits > 100_000
            || self.initial_wolves > 100_000
        {
            return Err("initial population counts exceed supported limits".into());
        }
        if self.max_simulation_speed == 0
            || self.max_simulation_speed > 32
            || self.max_stock_edit == 0
            || self.max_stock_edit > 1_000_000_000
            || self.max_person_inventory_edit == 0
            || self.max_person_inventory_edit > 1_000_000_000
            || self.max_hunger_edit <= 0
            || self.max_hunger_edit > 1_000_000_000
            || self.max_animal_energy_edit <= 0.0
            || self.max_animal_energy_edit > 1_000_000.0
            || self.max_resource_edit == 0
            || self.max_resource_edit > 1_000
        {
            return Err("simulation edit limits must be positive".into());
        }
        if self.starting_tick_fraction == 0 {
            return Err("starting_tick_fraction must be at least 1".into());
        }
        Ok(())
    }

    fn validate_calendar(&self) -> Result<(), String> {
        let calendar = &self.calendar;
        if calendar.ticks_per_day == 0
            || calendar.ticks_per_day > 1_000_000
            || calendar.days_per_week == 0
            || calendar.days_per_week > 100
            || calendar.weeks_per_month == 0
            || calendar.weeks_per_month > 100
            || calendar.months_per_year == 0
            || calendar.months_per_year > 100
        {
            return Err("calendar values are outside their supported ranges".into());
        }
        for stage in [
            &calendar.toddler,
            &calendar.child,
            &calendar.teen,
            &calendar.young_adult,
            &calendar.adult,
            &calendar.older_adult,
            &calendar.elderly,
        ] {
            if stage.min_days == 0 || stage.min_days > stage.max_days || stage.max_days > 1_000_000
            {
                return Err("life-stage durations must be in the range 1..=1000000".into());
            }
        }
        Ok(())
    }

    fn validate_chances(&self) -> Result<(), String> {
        for (name, chance) in [
            (
                "person_hunt_success_chance",
                self.person_hunt_success_chance,
            ),
            ("initial_tree_chance", self.initial_tree_chance),
            ("initial_berry_chance", self.initial_berry_chance),
            ("initial_stone_chance", self.initial_stone_chance),
            ("tree_regrow_chance", self.tree_regrow_chance),
            ("berry_regrow_chance", self.berry_regrow_chance),
            ("stone_regrow_chance", self.stone_regrow_chance),
            ("rabbit_breed_chance", self.rabbit_breed_chance),
            ("wolf_breed_chance", self.wolf_breed_chance),
            ("wolf_hunt_success_chance", self.wolf_hunt_success_chance),
        ] {
            if !(0.0..=1.0).contains(&chance) {
                return Err(format!("{name} must be between 0 and 1"));
            }
        }
        Ok(())
    }

    fn validate_world(&self) -> Result<(), String> {
        if self.animal_population_interval == 0 || self.entity_update_interval < 2 {
            return Err(
                "animal_population_interval must be positive and entity_update_interval at least 2"
                    .into(),
            );
        }
        if self.terrain_noise_octaves == 0
            || self.terrain_noise_octaves > 16
            || !self.terrain_noise_scale.is_finite()
            || self.terrain_noise_scale <= 0.0
        {
            return Err("terrain_noise_octaves and terrain_noise_scale must be positive".into());
        }
        if self.house_placement_radius > self.width.max(self.height)
            || self.resource_regrow_attempts > self.width * self.height
            || self.random_land_attempts == 0
            || self.wander_attempts == 0
            || self.wander_radius == 0
            || self.wander_radius > 32
            || self.log_entry_limit == 0
        {
            return Err("world search and regrowth values exceed supported limits".into());
        }
        if self.terrain_deep_water_threshold >= self.terrain_water_threshold
            || self.terrain_water_threshold >= self.terrain_sand_threshold
            || self.terrain_sand_threshold >= self.terrain_forest_grass_threshold
            || self.terrain_forest_grass_threshold >= self.terrain_hills_threshold
        {
            return Err("terrain thresholds must be in ascending order".into());
        }
        for (name, value) in [
            ("terrain_noise_scale", self.terrain_noise_scale),
            ("terrain_edge_height", self.terrain_edge_height),
            ("terrain_edge_falloff", self.terrain_edge_falloff),
            ("terrain_edge_distance_cap", self.terrain_edge_distance_cap),
            ("terrain_moisture_offset", self.terrain_moisture_offset),
            (
                "terrain_deep_water_threshold",
                self.terrain_deep_water_threshold,
            ),
            ("terrain_water_threshold", self.terrain_water_threshold),
            ("terrain_sand_threshold", self.terrain_sand_threshold),
            (
                "terrain_forest_grass_threshold",
                self.terrain_forest_grass_threshold,
            ),
            ("terrain_hills_threshold", self.terrain_hills_threshold),
            (
                "terrain_forest_moisture_threshold",
                self.terrain_forest_moisture_threshold,
            ),
        ] {
            if !value.is_finite() {
                return Err(format!("{name} must be finite"));
            }
        }
        Ok(())
    }

    fn validate_amounts(&self) -> Result<(), String> {
        if self.birth_food_cost > self.birth_food_threshold {
            return Err("birth_food_cost cannot exceed birth_food_threshold".into());
        }
        for (name, value) in [
            ("initial_person_hunger", self.initial_person_hunger),
            ("initial_animal_energy", self.initial_animal_energy),
            (
                "animal_energy_death_threshold",
                self.animal_energy_death_threshold,
            ),
            ("daylight_night_threshold", self.daylight_night_threshold),
            ("daylight_base", self.daylight_base),
            ("daylight_amplitude", self.daylight_amplitude),
            ("hunger_night_at_home", self.hunger_night_at_home),
            ("hunger_other", self.hunger_other),
            ("hunger_eat_home_threshold", self.hunger_eat_home_threshold),
            (
                "hunger_eat_carried_threshold",
                self.hunger_eat_carried_threshold,
            ),
            ("hunger_return_threshold", self.hunger_return_threshold),
            (
                "hunger_starvation_threshold",
                self.hunger_starvation_threshold,
            ),
            ("hunger_meal_reduction", self.hunger_meal_reduction),
            ("rabbit_energy_drain", self.rabbit_energy_drain),
            ("rabbit_energy_regen", self.rabbit_energy_regen),
            ("rabbit_energy_max", self.rabbit_energy_max),
            ("wolf_energy_drain", self.wolf_energy_drain),
            ("wolf_hunt_energy_gain", self.wolf_hunt_energy_gain),
            ("wolf_energy_max", self.wolf_energy_max),
            ("rabbit_breed_energy", self.rabbit_breed_energy),
            ("rabbit_breed_energy_cost", self.rabbit_breed_energy_cost),
            ("rabbit_newborn_energy", self.rabbit_newborn_energy),
            ("wolf_breed_energy", self.wolf_breed_energy),
            ("wolf_breed_energy_cost", self.wolf_breed_energy_cost),
            ("wolf_newborn_energy", self.wolf_newborn_energy),
        ] {
            if !value.is_finite() || value < 0.0 {
                return Err(format!("{name} must be a finite non-negative number"));
            }
        }
        Ok(())
    }
}

const CALENDAR_FIELDS: &[&str] = &[
    "ticks_per_day",
    "days_per_week",
    "weeks_per_month",
    "months_per_year",
    "toddler",
    "child",
    "teen",
    "young_adult",
    "adult",
    "older_adult",
    "elderly",
];

/// `SimulationConfig` fields (except `calendar`), each read from the `pub const`
/// of the same name in upper case.
const CONFIG_FIELDS: &[&str] = &[
    "width",
    "height",
    "initial_population",
    "initial_young_adults",
    "initial_rabbits",
    "initial_wolves",
    "initial_food",
    "initial_person_hunger",
    "initial_animal_energy",
    "animal_energy_death_threshold",
    "starting_tick_fraction",
    "population_capacity",
    "house_capacity",
    "max_houses",
    "daylight_night_threshold",
    "daylight_base",
    "daylight_amplitude",
    "home_radius",
    "attack_range",
    "entity_update_interval",
    "hunger_night_at_home",
    "hunger_other",
    "hunger_eat_home_threshold",
    "hunger_eat_carried_threshold",
    "hunger_return_threshold",
    "hunger_starvation_threshold",
    "hunger_meal_reduction",
    "food_per_meal",
    "resource_search_radius",
    "person_hunt_success_chance",
    "person_hunt_food_yield",
    "person_gather_yield",
    "person_tooled_gather_yield",
    "partnership_distance",
    "birth_food_threshold",
    "birth_food_cost",
    "tool_wood_cost",
    "tool_stone_cost",
    "house_wood_cost",
    "house_stone_cost",
    "house_placement_radius",
    "resource_regrow_attempts",
    "initial_tree_chance",
    "initial_tree_amount",
    "initial_berry_chance",
    "initial_berry_amount",
    "initial_stone_chance",
    "initial_stone_amount",
    "tree_regrow_chance",
    "tree_regrow_amount",
    "berry_regrow_chance",
    "berry_regrow_amount",
    "stone_regrow_chance",
    "stone_regrow_amount",
    "animal_population_interval",
    "rabbit_spawn_cap",
    "wolf_spawn_cap",
    "wolf_spawn_rabbit_threshold",
    "wolf_min_home_distance",
    "rabbit_energy_drain",
    "rabbit_energy_regen",
    "rabbit_energy_max",
    "rabbit_wolf_detection_range",
    "rabbit_breed_energy",
    "rabbit_max_population",
    "rabbit_breed_chance",
    "rabbit_breed_energy_cost",
    "rabbit_newborn_energy",
    "wolf_energy_drain",
    "wolf_prey_range",
    "wolf_hunt_success_chance",
    "wolf_hunt_energy_gain",
    "wolf_energy_max",
    "wolf_breed_energy",
    "wolf_max_population",
    "wolf_breed_chance",
    "wolf_breed_energy_cost",
    "wolf_newborn_energy",
    "terrain_noise_octaves",
    "terrain_noise_scale",
    "terrain_edge_height",
    "terrain_edge_falloff",
    "terrain_edge_distance_cap",
    "terrain_moisture_offset",
    "terrain_moisture_seed_offset",
    "terrain_deep_water_threshold",
    "terrain_water_threshold",
    "terrain_sand_threshold",
    "terrain_forest_grass_threshold",
    "terrain_hills_threshold",
    "terrain_forest_moisture_threshold",
    "random_land_attempts",
    "log_entry_limit",
    "wander_attempts",
    "wander_radius",
    "manual_spawn_limit",
    "max_simulation_speed",
    "max_stock_edit",
    "max_person_inventory_edit",
    "max_hunger_edit",
    "max_animal_energy_edit",
    "max_resource_edit",
];

const CONFIG_ACCESSOR: &str = "__sim_config";

/// Rune does not expose unit constants to the host, so a generated function
/// gathers the script's `pub const` items into one object.
fn config_accessor_source() -> String {
    let fields = |names: &[&str]| {
        names
            .iter()
            .map(|name| format!("{name}: {}", name.to_uppercase()))
            .collect::<Vec<_>>()
            .join(", ")
    };
    format!(
        "pub fn {CONFIG_ACCESSOR}() {{ #{{ {}, calendar: #{{ {} }} }} }}",
        fields(CONFIG_FIELDS),
        fields(CALENDAR_FIELDS)
    )
}

/// A compiled simulation script: `pub const` defaults plus `pub fn` behaviours.
#[derive(Clone)]
pub struct SimScript {
    source: String,
    config: SimulationConfig,
    runtime: Arc<RuntimeContext>,
    unit: Arc<Unit>,
}

impl SimScript {
    /// Compiles the script, reads and validates its constants, and smoke-tests
    /// every behaviour so errors surface at load time.
    pub fn load(src: &str) -> Result<Self, String> {
        let mut module = Module::new();
        module.ty::<Ctx>().map_err(|e| e.to_string())?;
        let mut context = Context::with_default_modules().map_err(|e| e.to_string())?;
        context.install(module).map_err(|e| e.to_string())?;
        let mut sources = Sources::new();
        for (name, text) in [
            ("config", src.to_string()),
            ("generated", config_accessor_source()),
        ] {
            sources
                .insert(Source::new(name, text).map_err(|e| e.to_string())?)
                .map_err(|e| e.to_string())?;
        }
        let mut diagnostics = Diagnostics::new();
        let unit = rune::prepare(&mut sources)
            .with_context(&context)
            .with_diagnostics(&mut diagnostics)
            .build();
        let Ok(unit) = unit else {
            let mut buffer = Buffer::no_color();
            let _ = diagnostics.emit(&mut buffer, &sources);
            return Err(String::from_utf8_lossy(buffer.as_slice()).into_owned());
        };
        let runtime = Arc::new(context.runtime().map_err(|e| e.to_string())?);
        let unit = Arc::new(unit);
        let mut vm = Vm::new(Arc::clone(&runtime), Arc::clone(&unit));
        let value = vm
            .call([CONFIG_ACCESSOR], ())
            .map_err(|e| format!("reading constants: {e}"))?;
        let config = rune::from_value::<SimulationConfig>(value)
            .map_err(|e| format!("invalid constant: {e}"))?;
        config.validate()?;
        for f in ["person", "rabbit", "wolf"] {
            let v = vm
                .call([f], (Ctx::default(),))
                .map_err(|e| format!("{f}(): {e}"))?;
            rune::from_value::<String>(v).map_err(|_| format!("{f}() must return a string"))?;
        }
        Ok(Self {
            source: src.to_string(),
            config,
            runtime,
            unit,
        })
    }

    pub fn source(&self) -> &str {
        &self.source
    }

    fn vm(&self) -> Vm {
        Vm::new(Arc::clone(&self.runtime), Arc::clone(&self.unit))
    }
}

fn default_script() -> SimScript {
    static SCRIPT: OnceLock<SimScript> = OnceLock::new();
    SCRIPT
        .get_or_init(|| SimScript::load(DEFAULT_SCRIPT).expect("config.rn failed to load"))
        .clone()
}

fn default_config() -> SimulationConfig {
    default_script().config
}

impl CalendarSettings {
    fn duration(&self, stage: &str) -> Option<&StageDuration> {
        match stage {
            "toddler" => Some(&self.toddler),
            "child" => Some(&self.child),
            "teen" => Some(&self.teen),
            "young_adult" => Some(&self.young_adult),
            "adult" => Some(&self.adult),
            "older_adult" => Some(&self.older_adult),
            "elderly" => Some(&self.elderly),
            _ => None,
        }
    }

    fn normalize(&mut self) {
        self.ticks_per_day = self.ticks_per_day.clamp(1, 1_000_000);
        self.days_per_week = self.days_per_week.clamp(1, 100);
        self.weeks_per_month = self.weeks_per_month.clamp(1, 100);
        self.months_per_year = self.months_per_year.clamp(1, 100);
        for stage in [
            "toddler",
            "child",
            "teen",
            "young_adult",
            "adult",
            "older_adult",
            "elderly",
        ] {
            let range = self.duration_mut(stage).expect("known life stage");
            range.min_days = range.min_days.clamp(1, 1_000_000);
            range.max_days = range.max_days.clamp(range.min_days, 1_000_000);
        }
    }

    fn duration_mut(&mut self, stage: &str) -> Option<&mut StageDuration> {
        match stage {
            "toddler" => Some(&mut self.toddler),
            "child" => Some(&mut self.child),
            "teen" => Some(&mut self.teen),
            "young_adult" => Some(&mut self.young_adult),
            "adult" => Some(&mut self.adult),
            "older_adult" => Some(&mut self.older_adult),
            "elderly" => Some(&mut self.elderly),
            _ => None,
        }
    }
}

#[derive(Clone, Copy, Debug, Default, Deserialize, Serialize, PartialEq, Eq)]
#[serde(rename_all = "snake_case")]
enum LifeStage {
    #[default]
    Toddler,
    Child,
    Teen,
    YoungAdult,
    Adult,
    OlderAdult,
    Elderly,
    Dead,
}

impl LifeStage {
    fn next(self) -> Option<Self> {
        match self {
            Self::Toddler => Some(Self::Child),
            Self::Child => Some(Self::Teen),
            Self::Teen => Some(Self::YoungAdult),
            Self::YoungAdult => Some(Self::Adult),
            Self::Adult => Some(Self::OlderAdult),
            Self::OlderAdult => Some(Self::Elderly),
            Self::Elderly => Some(Self::Dead),
            Self::Dead => None,
        }
    }

    fn key(self) -> &'static str {
        match self {
            Self::Toddler => "toddler",
            Self::Child => "child",
            Self::Teen => "teen",
            Self::YoungAdult => "young_adult",
            Self::Adult => "adult",
            Self::OlderAdult => "older_adult",
            Self::Elderly => "elderly",
            Self::Dead => "dead",
        }
    }

    fn can_partner(self) -> bool {
        matches!(
            self,
            Self::YoungAdult | Self::Adult | Self::OlderAdult | Self::Elderly
        )
    }
}

const WATER_DEEP: u8 = 0;
const WATER: u8 = 1;
const SAND: u8 = 2;
const GRASS: u8 = 3;
const FOREST: u8 = 4;
const HILLS: u8 = 5;
const MOUNTAIN: u8 = 6;

const TREE: u8 = 1;
const BERRY: u8 = 2;
const STONE: u8 = 3;
pub struct Rng(u64);

impl Rng {
    fn next(&mut self) -> u64 {
        let mut x = self.0;
        x ^= x >> 12;
        x ^= x << 25;
        x ^= x >> 27;
        self.0 = x;
        x.wrapping_mul(0x2545_F491_4F6C_DD1D)
    }
    fn f(&mut self) -> f32 {
        unit_f32(self.next())
    }
    fn chance(&mut self, p: f32) -> bool {
        self.f() < p
    }
    fn below(&mut self, n: usize) -> usize {
        let n = u64::try_from(n).expect("usize fits in u64");
        usize::try_from(self.next() % n).expect("value below a usize bound fits in usize")
    }
}

/// Maps the top 24 bits of `bits` to `[0, 1)`.
#[allow(clippy::cast_precision_loss)]
fn unit_f32(bits: u64) -> f32 {
    // Values below 2^24 are exactly representable in f32.
    (bits >> 40) as f32 / (1u64 << 24) as f32
}

/// Converts a map coordinate or dimension (at most 256) to `f32` exactly.
fn coord_f32(n: usize) -> f32 {
    f32::from(u16::try_from(n).expect("map coordinates fit in u16"))
}

/// Parses the integer argument at `i` of an edit command.
fn arg(t: &[&str], i: usize) -> Option<i64> {
    t.get(i).and_then(|s| s.parse().ok())
}

/// Clamps `v` into `lo..=hi` and converts it; the bounds must fit in `T`.
fn clamp_to<T: TryFrom<i64>>(v: i64, lo: i64, hi: i64) -> T
where
    T::Error: std::fmt::Debug,
{
    T::try_from(v.clamp(lo, hi)).expect("edit bounds fit the target type")
}

/// Converts an edit value to `f32`, rounding values beyond 2^24 like `as` does.
#[allow(clippy::cast_precision_loss)]
fn small_f32(v: i64) -> f32 {
    v as f32
}

/// Sign of the step from `from` towards `to` along one axis.
fn direction(from: usize, to: usize) -> isize {
    match to.cmp(&from) {
        std::cmp::Ordering::Greater => 1,
        std::cmp::Ordering::Less => -1,
        std::cmp::Ordering::Equal => 0,
    }
}

fn hash(x: i32, y: i32, seed: u64) -> f32 {
    let mut h = i64::from(x)
        .cast_unsigned()
        .wrapping_mul(0x9E37_79B9_7F4A_7C15)
        ^ i64::from(y)
            .cast_unsigned()
            .wrapping_mul(0xC2B2_AE3D_27D4_EB4F)
        ^ seed.wrapping_mul(0x1656_67B1_9E37_79F9);
    h ^= h >> 33;
    h = h.wrapping_mul(0xFF51_AFD7_ED55_8CCD);
    h ^= h >> 33;
    unit_f32(h)
}

/// Lattice cell containing `v`.
#[allow(clippy::cast_possible_truncation)]
fn lattice(v: f32) -> i32 {
    // Noise inputs are bounded map coordinates; `as` saturates for anything larger.
    v.floor() as i32
}

fn value_noise(x: f32, y: f32, seed: u64) -> f32 {
    let smooth = |t: f32| t * t * (3.0 - 2.0 * t);
    let (xi, yi) = (lattice(x), lattice(y));
    let (u, v) = (smooth(x - x.floor()), smooth(y - y.floor()));
    let top_left = hash(xi, yi, seed);
    let top_right = hash(xi + 1, yi, seed);
    let bottom_left = hash(xi, yi + 1, seed);
    let bottom_right = hash(xi + 1, yi + 1, seed);
    let top = top_left + (top_right - top_left) * u;
    let bottom = bottom_left + (bottom_right - bottom_left) * u;
    top + (bottom - top) * v
}

fn fbm(x: f32, y: f32, seed: u64, octaves: usize, scale: f32) -> f32 {
    let (mut sum, mut amp, mut freq, mut norm) = (0.0, 1.0, 1.0 / scale, 0.0);
    for o in 0..octaves {
        sum += amp * value_noise(x * freq, y * freq, seed + o as u64);
        norm += amp;
        amp *= 0.5;
        freq *= 2.0;
    }
    sum / norm
}

/// What an entity senses this tick; passed to the Rune behaviour scripts.
// A flat record of independent sensor flags keeps field access simple in `config.rn`.
#[allow(clippy::struct_excessive_bools)]
#[derive(Any, Default, Clone)]
struct Ctx {
    #[rune(get)]
    night: bool,
    #[rune(get)]
    at_home: bool,
    #[rune(get)]
    hunger: f64,
    #[rune(get)]
    carry: i64,
    #[rune(get)]
    carried_food: i64,
    #[rune(get)]
    stock_food: i64,
    #[rune(get)]
    stock_wood: i64,
    #[rune(get)]
    stock_stone: i64,
    #[rune(get)]
    berry_near: bool,
    #[rune(get)]
    tree_near: bool,
    #[rune(get)]
    stone_near: bool,
    #[rune(get)]
    rabbit_near: bool,
    #[rune(get)]
    wolf_near: bool,
    #[rune(get)]
    prey_near: bool,
    #[rune(get)]
    roll: f64,
}

#[derive(Clone, Debug, Serialize, Deserialize)]
pub struct Person {
    id: u32,
    x: usize,
    y: usize,
    hunger: f32,
    food: u32,
    wood: u32,
    stone: u32,
    action: String,
    dead: bool,
    #[serde(default)]
    age_ticks: u64,
    #[serde(default)]
    stage: LifeStage,
    #[serde(default)]
    stage_elapsed_ticks: u64,
    #[serde(default = "default_stage_duration_days")]
    stage_duration_days: u32,
    #[serde(default)]
    partner_id: Option<u32>,
    /// State of the person's own random stream, used for their life-stage durations.
    #[serde(default)]
    rng: u64,
}

fn default_stage_duration_days() -> u32 {
    1
}

impl Person {
    fn carry(&self) -> u32 {
        self.food + self.wood + self.stone
    }

    fn position(&self) -> (usize, usize) {
        (self.x, self.y)
    }
}

#[derive(Clone, Debug, Serialize, Deserialize)]
pub struct Animal {
    x: usize,
    y: usize,
    energy: f32,
    dead: bool,
}

impl Animal {
    fn position(&self) -> (usize, usize) {
        (self.x, self.y)
    }
}

#[derive(Clone, Debug, Serialize, Deserialize)]
struct Partnership {
    id: u32,
    first_person_id: u32,
    second_person_id: u32,
    formed_tick: u64,
    last_birth_tick: u64,
}

#[derive(Clone, Copy, Debug, PartialEq, Eq)]
enum Species {
    Rabbit,
    Wolf,
}

/// The entity state observed from the actors at the start of a step, kept up to date with
/// their replies while the step runs. It is a read model only; the actors own the state.
#[derive(Default)]
struct Census {
    people: Vec<Person>,
    rabbits: Vec<(AnimalId, Animal)>,
    wolves: Vec<(AnimalId, Animal)>,
}

impl Census {
    fn animals_mut(&mut self, species: Species) -> &mut Vec<(AnimalId, Animal)> {
        match species {
            Species::Rabbit => &mut self.rabbits,
            Species::Wolf => &mut self.wolves,
        }
    }
}

/// World-level state (terrain, resources, the shared stockpile, houses, calendar and the world
/// random stream) lives here. Every person, animal and partnership is a Picard actor that owns
/// its own state; the world only coordinates them, because the Rune VM that decides behaviours
/// cannot leave the simulation thread.
pub struct World {
    seed: u64,
    width: usize,
    height: usize,
    tiles: Vec<u8>,
    res: Vec<(u8, u16)>,
    person_actors: BTreeMap<u32, ActorRef<PersonMsg>>,
    rabbit_actors: BTreeMap<AnimalId, ActorRef<AnimalMsg>>,
    wolf_actors: BTreeMap<AnimalId, ActorRef<AnimalMsg>>,
    partnership_actors: BTreeMap<u32, ActorRef<PartnershipMsg>>,
    next_animal_id: AnimalId,
    next_partnership_id: u32,
    mailbox: Mailbox,
    actor_system: ActorSystem,
    houses: Vec<(usize, usize)>,
    home: (usize, usize),
    food: u32,
    wood: u32,
    stone: u32,
    tools: u32,
    rng: Rng,
    tick: u64,
    next_id: u32,
    log: Vec<String>,
    script: SimScript,
    vm: Vm,
    pub paused: bool,
    pub speed: u32,
    pub terrain_rev: u32,
    calendar: CalendarSettings,
    config: SimulationConfig,
}

#[derive(Serialize, Deserialize)]
struct WorldSnapshot {
    version: u32,
    seed: Option<u64>,
    #[serde(default = "default_width")]
    width: usize,
    #[serde(default = "default_height")]
    height: usize,
    tiles: Vec<u8>,
    res: Vec<(u8, u16)>,
    people: Vec<Person>,
    rabbits: Vec<Animal>,
    wolves: Vec<Animal>,
    houses: Vec<(usize, usize)>,
    home: (usize, usize),
    food: u32,
    wood: u32,
    stone: u32,
    tools: u32,
    rng: u64,
    tick: u64,
    next_id: u32,
    log: Vec<String>,
    paused: bool,
    speed: u32,
    terrain_rev: u32,
    #[serde(default)]
    calendar: CalendarSettings,
    #[serde(default)]
    partnerships: Vec<Partnership>,
    #[serde(default = "default_next_partnership_id")]
    next_partnership_id: u32,
}

fn default_next_partnership_id() -> u32 {
    1
}

fn default_width() -> usize {
    W
}

fn default_height() -> usize {
    H
}

fn cheb(a: (usize, usize), b: (usize, usize)) -> usize {
    a.0.abs_diff(b.0).max(a.1.abs_diff(b.1))
}

fn sample_stage_duration(calendar: &CalendarSettings, stage: LifeStage, rng: &mut Rng) -> u32 {
    let duration = match stage {
        LifeStage::Toddler => calendar.toddler,
        LifeStage::Child => calendar.child,
        LifeStage::Teen => calendar.teen,
        LifeStage::YoungAdult => calendar.young_adult,
        LifeStage::Adult => calendar.adult,
        LifeStage::OlderAdult => calendar.older_adult,
        LifeStage::Elderly => calendar.elderly,
        LifeStage::Dead => StageDuration {
            min_days: 1,
            max_days: 1,
            fertile: false,
        },
    };
    let range = duration.max_days - duration.min_days + 1;
    duration.min_days
        + u32::try_from(rng.below(range as usize)).expect("sample is below a u32 range")
}

/// Drops partnerships with a missing member and clears partner links without a partnership.
fn repair_partnerships(people: &mut [Person], partnerships: &mut Vec<Partnership>) {
    let alive: HashSet<u32> = people.iter().map(|person| person.id).collect();
    partnerships.retain(|partnership| {
        alive.contains(&partnership.first_person_id)
            && alive.contains(&partnership.second_person_id)
    });
    let partnered: HashSet<(u32, u32)> = partnerships
        .iter()
        .map(|partnership| (partnership.first_person_id, partnership.second_person_id))
        .flat_map(|(first, second)| [(first, second), (second, first)])
        .collect();
    for person in people {
        if person
            .partner_id
            .is_some_and(|partner| !partnered.contains(&(person.id, partner)))
        {
            person.partner_id = None;
        }
    }
}

/// Collects one reply per actor and sorts them by key, so the simulation stays deterministic
/// whatever order the actors answered in.
fn sorted_by_key<K: Ord, T>(mut items: Vec<(K, T)>) -> Vec<(K, T)> {
    items.sort_by(|a, b| a.0.cmp(&b.0));
    items
}

impl World {
    #[cfg(test)]
    pub fn new(seed: u64) -> World {
        Self::new_with_script(seed, default_script())
    }

    fn generate_terrain(seed: u64, config: &SimulationConfig) -> Vec<u8> {
        let (width, height) = (config.width, config.height);
        let mut tiles = vec![0u8; width * height];
        for y in 0..height {
            for x in 0..width {
                let (fx, fy) = (coord_f32(x), coord_f32(y));
                let dx = (fx / coord_f32(width) - 0.5) * 2.0;
                let dy = (fy / coord_f32(height) - 0.5) * 2.0;
                let d = (dx * dx + dy * dy).min(config.terrain_edge_distance_cap);
                let h = fbm(
                    fx,
                    fy,
                    seed,
                    config.terrain_noise_octaves,
                    config.terrain_noise_scale,
                ) + config.terrain_edge_height
                    - config.terrain_edge_falloff * d;
                let m = fbm(
                    fx + config.terrain_moisture_offset,
                    fy + config.terrain_moisture_offset,
                    seed.wrapping_add(config.terrain_moisture_seed_offset),
                    config.terrain_noise_octaves,
                    config.terrain_noise_scale,
                );
                tiles[y * width + x] = if h < config.terrain_deep_water_threshold {
                    WATER_DEEP
                } else if h < config.terrain_water_threshold {
                    WATER
                } else if h < config.terrain_sand_threshold {
                    SAND
                } else if h < config.terrain_forest_grass_threshold {
                    if m > config.terrain_forest_moisture_threshold {
                        FOREST
                    } else {
                        GRASS
                    }
                } else if h < config.terrain_hills_threshold {
                    HILLS
                } else {
                    MOUNTAIN
                };
            }
        }
        tiles
    }

    fn populate(&mut self) {
        for i in 0..self.config.initial_population {
            let mut person = self.new_person(self.home);
            let stage = if i < self.config.initial_young_adults {
                LifeStage::YoungAdult
            } else {
                LifeStage::Adult
            };
            self.make_grown_up(&mut person, stage);
            self.spawn_person_actor(person);
        }
        for _ in 0..self.config.initial_rabbits {
            if let Some(p) = self.random_land() {
                let energy = self.config.initial_animal_energy;
                self.spawn_animal(Species::Rabbit, p, energy);
            }
        }
        for _ in 0..self.config.initial_wolves {
            if let Some(p) = self.random_land()
                && cheb(p, self.home) > self.config.wolf_min_home_distance
            {
                let energy = self.config.initial_animal_energy;
                self.spawn_animal(Species::Wolf, p, energy);
            }
        }
    }

    /// Gives `person` the age of someone who has lived through every stage before `stage`.
    fn make_grown_up(&mut self, person: &mut Person, stage: LifeStage) {
        let duration_days = self.sample_stage_duration(stage);
        let prior_stages = match stage {
            LifeStage::YoungAdult => {
                [LifeStage::Toddler, LifeStage::Child, LifeStage::Teen].as_slice()
            }
            LifeStage::Adult => [
                LifeStage::Toddler,
                LifeStage::Child,
                LifeStage::Teen,
                LifeStage::YoungAdult,
            ]
            .as_slice(),
            _ => &[],
        };
        let age_days: u64 = prior_stages
            .iter()
            .map(|&past_stage| u64::from(self.sample_stage_duration(past_stage)))
            .sum();
        person.stage = stage;
        person.stage_duration_days = duration_days;
        person.stage_elapsed_ticks = 0;
        person.age_ticks = age_days * self.calendar.ticks_per_day;
    }

    fn empty(seed: u64, script: SimScript, tiles: Vec<u8>, res: Vec<(u8, u16)>) -> World {
        let config = script.config.clone();
        let (width, height) = (config.width, config.height);
        let actor_system = ActorSystem::new().expect("failed to start Picard actor system");
        World {
            seed,
            width,
            height,
            tiles,
            res,
            person_actors: BTreeMap::new(),
            rabbit_actors: BTreeMap::new(),
            wolf_actors: BTreeMap::new(),
            partnership_actors: BTreeMap::new(),
            next_animal_id: 1,
            next_partnership_id: 1,
            mailbox: Mailbox::new(&actor_system),
            actor_system,
            houses: vec![],
            home: (width / 2, height / 2),
            food: config.initial_food,
            wood: 0,
            stone: 0,
            tools: 0,
            rng: Rng(seed.wrapping_mul(0x9E37_79B9_7F4A_7C15) | 1),
            tick: (config.calendar.ticks_per_day / config.starting_tick_fraction).max(1),
            next_id: 1,
            log: vec![],
            vm: script.vm(),
            script,
            paused: false,
            speed: 1,
            terrain_rev: 0,
            calendar: config.calendar.clone(),
            config,
        }
    }

    pub fn new_with_script(seed: u64, script: SimScript) -> World {
        let tiles = Self::generate_terrain(seed, &script.config);
        let res = vec![(0, 0); tiles.len()];
        let mut w = Self::empty(seed, script, tiles, res);
        let (width, height) = (w.width, w.height);
        w.home = w
            .find_land(GRASS)
            .unwrap_or_else(|| w.find_land(SAND).unwrap_or((width / 2, height / 2)));
        for i in 0..width * height {
            let r = w.rng.f();
            w.res[i] = match w.tiles[i] {
                FOREST if r < w.config.initial_tree_chance => (TREE, w.config.initial_tree_amount),
                GRASS if r < w.config.initial_berry_chance => {
                    (BERRY, w.config.initial_berry_amount)
                }
                HILLS if r < w.config.initial_stone_chance => {
                    (STONE, w.config.initial_stone_amount)
                }
                _ => (0, 0),
            };
        }
        let (hx, hy) = w.home;
        w.res[hy * width + hx] = (0, 0);
        w.populate();
        w
    }

    fn spawn_person_actor(&mut self, person: Person) {
        let id = person.id;
        let actor = self
            .actor_system
            .actor_of_args::<PersonActor, _>(&format!("person-{id}"), person)
            .expect("failed to create Picard person actor");
        self.person_actors.insert(id, actor);
    }

    fn animal_actors(&self, species: Species) -> &BTreeMap<AnimalId, ActorRef<AnimalMsg>> {
        match species {
            Species::Rabbit => &self.rabbit_actors,
            Species::Wolf => &self.wolf_actors,
        }
    }

    fn animal_actors_mut(
        &mut self,
        species: Species,
    ) -> &mut BTreeMap<AnimalId, ActorRef<AnimalMsg>> {
        match species {
            Species::Rabbit => &mut self.rabbit_actors,
            Species::Wolf => &mut self.wolf_actors,
        }
    }

    fn spawn_animal_actor(&mut self, species: Species, animal: Animal) -> AnimalId {
        let id = self.next_animal_id;
        self.next_animal_id += 1;
        let kind = match species {
            Species::Rabbit => "rabbit",
            Species::Wolf => "wolf",
        };
        let actor = self
            .actor_system
            .actor_of_args::<AnimalActor, _>(&format!("{kind}-{id}"), (id, animal))
            .expect("failed to create Picard animal actor");
        self.animal_actors_mut(species).insert(id, actor);
        id
    }

    fn spawn_animal(
        &mut self,
        species: Species,
        (x, y): (usize, usize),
        energy: f32,
    ) -> (AnimalId, Animal) {
        let animal = Animal {
            x,
            y,
            energy,
            dead: false,
        };
        (self.spawn_animal_actor(species, animal.clone()), animal)
    }

    fn spawn_partnership_actor(&mut self, partnership: Partnership) {
        let id = partnership.id;
        let actor = self
            .actor_system
            .actor_of_args::<PartnershipActor, _>(&format!("partnership-{id}"), partnership)
            .expect("failed to create Picard partnership actor");
        self.partnership_actors.insert(id, actor);
    }

    fn retire_person(&mut self, id: u32) {
        if let Some(actor) = self.person_actors.remove(&id) {
            actor.stop();
        }
    }

    fn retire_animal(&mut self, species: Species, id: AnimalId) {
        if let Some(actor) = self.animal_actors_mut(species).remove(&id) {
            actor.stop();
        }
    }

    fn retire_partnership(&mut self, id: u32) {
        if let Some(actor) = self.partnership_actors.remove(&id) {
            actor.stop();
        }
    }

    /// Sends one message to every actor in `actors` and waits for all replies.
    fn broadcast<K, M: picard::LocalMessage>(
        &self,
        actors: &BTreeMap<K, ActorRef<M>>,
        message: impl Fn(RequestId) -> M,
    ) -> Vec<ReplyBody> {
        if actors.is_empty() {
            return Vec::new();
        }
        let request = self.mailbox.request();
        for actor in actors.values() {
            actor.tell(message(request), self.mailbox.reply_to());
        }
        self.mailbox.collect(request, actors.len())
    }

    fn observe_people(&self) -> Vec<Person> {
        let people = self
            .broadcast(&self.person_actors, PersonMsg::Observe)
            .into_iter()
            .map(|reply| match reply {
                ReplyBody::Person(person) => (person.id, person),
                other => panic!("person actor answered an observation with {other:?}"),
            })
            .collect();
        sorted_by_key(people).into_iter().map(|(_, p)| p).collect()
    }

    fn observe_animals(&self, species: Species) -> Vec<(AnimalId, Animal)> {
        let animals = self
            .broadcast(self.animal_actors(species), AnimalMsg::Observe)
            .into_iter()
            .map(|reply| match reply {
                ReplyBody::Animal(id, animal) => (id, animal),
                other => panic!("animal actor answered an observation with {other:?}"),
            })
            .collect();
        sorted_by_key(animals)
    }

    fn observe_partnerships(&self) -> Vec<Partnership> {
        let partnerships = self
            .broadcast(&self.partnership_actors, PartnershipMsg::Observe)
            .into_iter()
            .map(|reply| match reply {
                ReplyBody::Partnership(partnership) => (partnership.id, partnership),
                other => panic!("partnership actor answered an observation with {other:?}"),
            })
            .collect();
        sorted_by_key(partnerships)
            .into_iter()
            .map(|(_, p)| p)
            .collect()
    }

    fn census(&self) -> Census {
        Census {
            people: self.observe_people(),
            rabbits: self.observe_animals(Species::Rabbit),
            wolves: self.observe_animals(Species::Wolf),
        }
    }

    fn tell_person(&self, id: u32, message: PersonMsg) {
        if let Some(actor) = self.person_actors.get(&id) {
            actor.tell(message, None);
        }
    }

    /// Sends a request to one actor and waits for its reply.
    fn ask<M: picard::LocalMessage>(
        &self,
        actor: &ActorRef<M>,
        message: impl FnOnce(RequestId) -> M,
    ) -> ReplyBody {
        self.mailbox
            .ask(|request, sender: Sender| actor.tell(message(request), sender))
    }

    fn find_land(&self, kind: u8) -> Option<(usize, usize)> {
        let c = (self.width / 2, self.height / 2);
        let mut best: Option<(usize, usize)> = None;
        for y in 0..self.height {
            for x in 0..self.width {
                if self.tiles[y * self.width + x] == kind
                    && best.is_none_or(|b| cheb((x, y), c) < cheb(b, c))
                {
                    best = Some((x, y));
                }
            }
        }
        best
    }

    fn random_land(&mut self) -> Option<(usize, usize)> {
        for _ in 0..self.config.random_land_attempts {
            let (x, y) = (self.rng.below(self.width), self.rng.below(self.height));
            if self.walkable((x, y)) {
                return Some((x, y));
            }
        }
        None
    }

    fn new_person(&mut self, (x, y): (usize, usize)) -> Person {
        let stage_duration_days = self.sample_stage_duration(LifeStage::Toddler);
        let person = Person {
            id: self.next_id,
            x,
            y,
            hunger: self.config.initial_person_hunger,
            food: 0,
            wood: 0,
            stone: 0,
            action: "idle".to_string(),
            dead: false,
            age_ticks: 0,
            stage: LifeStage::Toddler,
            stage_elapsed_ticks: 0,
            stage_duration_days,
            partner_id: None,
            rng: self.rng.next() | 1,
        };
        self.next_id += 1;
        person
    }

    fn spawn_person_at(&mut self, position: (usize, usize)) -> Person {
        let person = self.new_person(position);
        self.spawn_person_actor(person.clone());
        person
    }

    fn sample_stage_duration(&mut self, stage: LifeStage) -> u32 {
        sample_stage_duration(&self.calendar, stage, &mut self.rng)
    }

    fn population_u32(&self) -> u32 {
        u32::try_from(self.person_actors.len()).unwrap_or(u32::MAX)
    }

    fn say(&mut self, s: &str) {
        self.log.push(format!(
            "[day {}] {}",
            self.tick / self.calendar.ticks_per_day + 1,
            s
        ));
        if self.log.len() > self.config.log_entry_limit {
            self.log.remove(0);
        }
    }

    /// Fraction of the current day that has elapsed, in `[0, 1)`.
    #[allow(clippy::cast_precision_loss)]
    fn day_fraction(&self) -> f32 {
        // `ticks_per_day` is validated to at most 1,000,000, which f32 represents exactly.
        let ticks_per_day = self.calendar.ticks_per_day;
        (self.tick % ticks_per_day) as f32 / ticks_per_day as f32
    }

    pub fn daylight(&self) -> f32 {
        let phase = self.day_fraction();
        let v = -(phase * std::f32::consts::TAU).cos();
        (self.config.daylight_base + v * self.config.daylight_amplitude).clamp(0.0, 1.0)
    }

    fn walkable(&self, (x, y): (usize, usize)) -> bool {
        x < self.width
            && y < self.height
            && matches!(
                self.tiles[y * self.width + x],
                SAND | GRASS | FOREST | HILLS
            )
    }

    /// The in-bounds tile at `(x + dx, y + dy)`, if any.
    fn offset(&self, (x, y): (usize, usize), dx: isize, dy: isize) -> Option<(usize, usize)> {
        let x = x.checked_add_signed(dx).filter(|&x| x < self.width)?;
        let y = y.checked_add_signed(dy).filter(|&y| y < self.height)?;
        Some((x, y))
    }

    /// The walkable tile at `(x + dx, y + dy)`, excluding the origin itself.
    fn walk_to(&self, from: (usize, usize), dx: isize, dy: isize) -> Option<(usize, usize)> {
        if (dx, dy) == (0, 0) {
            return None;
        }
        self.offset(from, dx, dy).filter(|&to| self.walkable(to))
    }

    fn move_dir(&mut self, x: usize, y: usize, dx: isize, dy: isize) -> (usize, usize) {
        [(dx, dy), (dx, 0), (0, dy)]
            .into_iter()
            .find_map(|(a, b)| self.walk_to((x, y), a, b))
            .unwrap_or_else(|| self.wander(x, y))
    }

    fn wander(&mut self, x: usize, y: usize) -> (usize, usize) {
        let radius = self.config.wander_radius;
        let diameter = radius * 2 + 1;
        let signed_radius = isize::try_from(radius).expect("wander radius is validated");
        for _ in 0..self.config.wander_attempts {
            let a = isize::try_from(self.rng.below(diameter)).expect("bounded by diameter");
            let b = isize::try_from(self.rng.below(diameter)).expect("bounded by diameter");
            if let Some(to) = self.walk_to((x, y), a - signed_radius, b - signed_radius) {
                return to;
            }
        }
        (x, y)
    }

    fn move_toward(&mut self, from: (usize, usize), to: (usize, usize)) -> (usize, usize) {
        if from == to {
            return from;
        }

        let start = from.1 * self.width + from.0;
        let target = to.1 * self.width + to.0;
        let mut frontier = BinaryHeap::from([Reverse((cheb(from, to), start))]);
        let mut costs = vec![usize::MAX; self.width * self.height];
        let mut previous = vec![usize::MAX; self.width * self.height];
        costs[start] = 0;
        previous[start] = start;

        while let Some(Reverse((_, current))) = frontier.pop() {
            if current == target {
                let mut next = target;
                while previous[next] != start {
                    next = previous[next];
                }
                return (next % self.width, next / self.width);
            }

            let position = (current % self.width, current / self.width);
            for dy in -1..=1 {
                for dx in -1..=1 {
                    if (dx, dy) == (0, 0) {
                        continue;
                    }
                    let Some((x, y)) = self.walk_to(position, dx, dy) else {
                        continue;
                    };
                    let neighbor = y * self.width + x;
                    let cost = costs[current] + 1;
                    if cost >= costs[neighbor] {
                        continue;
                    }
                    costs[neighbor] = cost;
                    previous[neighbor] = current;
                    frontier.push(Reverse((cost + cheb((x, y), to), neighbor)));
                }
            }
        }

        self.wander(from.0, from.1)
    }

    fn nearest_res(&self, from: (usize, usize), kind: u8, r: usize) -> Option<(usize, usize)> {
        let mut best: Option<(usize, usize)> = None;
        let (x0, x1) = (
            from.0.saturating_sub(r),
            from.0.saturating_add(r).min(self.width - 1),
        );
        let (y0, y1) = (
            from.1.saturating_sub(r),
            from.1.saturating_add(r).min(self.height - 1),
        );
        for y in y0..=y1 {
            for x in x0..=x1 {
                if self.res[y * self.width + x].0 == kind
                    && best.is_none_or(|b| cheb((x, y), from) < cheb(b, from))
                {
                    best = Some((x, y));
                }
            }
        }
        best
    }

    fn nearest_animal(
        animals: &[(AnimalId, Animal)],
        from: (usize, usize),
        r: usize,
    ) -> Option<usize> {
        let mut best: Option<(usize, usize)> = None;
        for (i, (_, a)) in animals.iter().enumerate() {
            let d = cheb(a.position(), from);
            if !a.dead && d <= r && best.is_none_or(|(_, bd)| d < bd) {
                best = Some((i, d));
            }
        }
        best.map(|b| b.0)
    }

    pub fn step(&mut self) {
        self.tick += 1;
        self.regrow();
        let mut census = Census {
            people: self.advance_people(),
            rabbits: self.observe_animals(Species::Rabbit),
            wolves: self.observe_animals(Species::Wolf),
        };
        if self.tick.is_multiple_of(self.config.entity_update_interval) {
            self.tick_people(&mut census);
            self.tick_rabbits(&mut census);
        } else {
            self.tick_wolves(&mut census);
        }
        if self.retire_dead(&mut census) {
            self.prune_partnerships(&mut census.people);
        }
        self.population(&mut census);
    }

    /// Ages every person by one tick and returns them, logging stage changes and deaths.
    fn advance_people(&mut self) -> Vec<Person> {
        let calendar = self.calendar.clone();
        let replies = self.broadcast(&self.person_actors, |request| {
            PersonMsg::Age(request, calendar.clone())
        });
        let aged = replies
            .into_iter()
            .map(|reply| match reply {
                ReplyBody::Aged(person, stage) => (person.id, (person, stage)),
                other => panic!("person actor answered ageing with {other:?}"),
            })
            .collect();
        let mut people = Vec::with_capacity(self.person_actors.len());
        for (id, (person, stage)) in sorted_by_key(aged) {
            match stage {
                Some(LifeStage::Dead) => self.say(&format!("Person #{id} died of old age")),
                Some(stage) => self.say(&format!(
                    "Person #{id} became a {}",
                    stage.key().replace('_', " ")
                )),
                None => {}
            }
            people.push(person);
        }
        people
    }

    /// Stops the actors of dead entities; returns whether any person died.
    fn retire_dead(&mut self, census: &mut Census) -> bool {
        let dead_people: Vec<u32> = census
            .people
            .iter()
            .filter(|person| person.dead)
            .map(|person| person.id)
            .collect();
        for &id in &dead_people {
            self.retire_person(id);
        }
        census.people.retain(|person| !person.dead);
        for species in [Species::Rabbit, Species::Wolf] {
            let dead: Vec<AnimalId> = census
                .animals_mut(species)
                .iter()
                .filter(|(_, animal)| animal.dead)
                .map(|(id, _)| *id)
                .collect();
            for id in dead {
                self.retire_animal(species, id);
            }
            census
                .animals_mut(species)
                .retain(|(_, animal)| !animal.dead);
        }
        !dead_people.is_empty()
    }

    /// Dissolves partnerships that lost a member and tells their survivors.
    fn prune_partnerships(&mut self, people: &mut [Person]) {
        let mut partnerships = self.observe_partnerships();
        let before: HashSet<u32> = partnerships.iter().map(|p| p.id).collect();
        let partners_before: Vec<Option<u32>> = people.iter().map(|p| p.partner_id).collect();
        repair_partnerships(people, &mut partnerships);
        let kept: HashSet<u32> = partnerships.iter().map(|p| p.id).collect();
        for id in before.difference(&kept) {
            self.retire_partnership(*id);
        }
        for (person, partner_before) in people.iter().zip(partners_before) {
            if person.partner_id != partner_before {
                self.tell_person(person.id, PersonMsg::SetPartner(person.partner_id));
            }
        }
    }

    fn regrow(&mut self) {
        for _ in 0..self.config.resource_regrow_attempts {
            let i = self.rng.below(self.width * self.height);
            if self.res[i].0 != 0 {
                continue;
            }
            let p = self.rng.f();
            match self.tiles[i] {
                FOREST if p < self.config.tree_regrow_chance => {
                    self.res[i] = (TREE, self.config.tree_regrow_amount);
                }
                GRASS if p < self.config.berry_regrow_chance => {
                    self.res[i] = (BERRY, self.config.berry_regrow_amount);
                }
                HILLS if p < self.config.stone_regrow_chance => {
                    self.res[i] = (STONE, self.config.stone_regrow_amount);
                }
                _ => {}
            }
        }
    }

    fn population(&mut self, census: &mut Census) {
        let ticks_per_day = self.calendar.ticks_per_day;
        if self.tick.is_multiple_of(ticks_per_day) {
            self.find_partners(&mut census.people);
        }
        let ticks_per_week = ticks_per_day * u64::from(self.calendar.days_per_week);
        if self.tick.is_multiple_of(ticks_per_week) {
            self.have_children(ticks_per_week, census);
        }
        if self
            .tick
            .is_multiple_of(self.config.animal_population_interval)
        {
            let energy = self.config.initial_animal_energy;
            if census.rabbits.len() < self.config.rabbit_spawn_cap
                && let Some(p) = self.random_land()
            {
                let rabbit = self.spawn_animal(Species::Rabbit, p, energy);
                census.rabbits.push(rabbit);
            }
            if census.wolves.len() < self.config.wolf_spawn_cap
                && census.rabbits.len() > self.config.wolf_spawn_rabbit_threshold
                && let Some(p) = self.random_land()
                && cheb(p, self.home) > self.config.wolf_min_home_distance
            {
                let wolf = self.spawn_animal(Species::Wolf, p, energy);
                census.wolves.push(wolf);
            }
        }
    }

    fn find_partners(&mut self, people: &mut [Person]) {
        let ticks_per_week = self.calendar.ticks_per_day * u64::from(self.calendar.days_per_week);
        for first in 0..people.len() {
            if people[first].partner_id.is_some()
                || !people[first].stage.can_partner()
                || people[first].dead
            {
                continue;
            }
            let first_position = people[first].position();
            let second = (0..people.len())
                .filter(|&index| {
                    index != first
                        && people[index].partner_id.is_none()
                        && people[index].stage.can_partner()
                        && !people[index].dead
                })
                .filter_map(|index| {
                    let distance = cheb(first_position, people[index].position());
                    (distance <= self.config.partnership_distance).then_some((index, distance))
                })
                .min_by_key(|(_, distance)| *distance)
                .map(|(index, _)| index);
            let Some(second) = second else {
                continue;
            };
            let first_id = people[first].id;
            let second_id = people[second].id;
            people[first].partner_id = Some(second_id);
            people[second].partner_id = Some(first_id);
            self.tell_person(first_id, PersonMsg::SetPartner(Some(second_id)));
            self.tell_person(second_id, PersonMsg::SetPartner(Some(first_id)));
            self.spawn_partnership_actor(Partnership {
                id: self.next_partnership_id,
                first_person_id: first_id,
                second_person_id: second_id,
                formed_tick: self.tick,
                last_birth_tick: self.tick / ticks_per_week * ticks_per_week,
            });
            self.next_partnership_id += 1;
            self.say(&format!(
                "People #{first_id} and #{second_id} became partners"
            ));
        }
    }

    fn have_children(&mut self, birth_interval_ticks: u64, census: &mut Census) {
        if self.food < self.config.birth_food_threshold
            || census.people.len() >= self.population_capacity()
        {
            return;
        }
        let tick = self.tick;
        let replies = self.broadcast(&self.partnership_actors, |request| {
            PartnershipMsg::BirthDue {
                request,
                tick,
                interval: birth_interval_ticks,
            }
        });
        let due = replies
            .into_iter()
            .map(|reply| match reply {
                ReplyBody::BirthDue(partnership, due) => (partnership.id, (partnership, due)),
                other => panic!("partnership actor answered a birth check with {other:?}"),
            })
            .collect();
        for (partnership_id, (partnership, due)) in sorted_by_key(due) {
            if self.food < self.config.birth_food_threshold
                || census.people.len() >= self.population_capacity()
            {
                break;
            }
            if !due {
                continue;
            }
            let (first_id, second_id) = (partnership.first_person_id, partnership.second_person_id);
            let fertile = |id: u32| {
                census
                    .people
                    .iter()
                    .find(|person| person.id == id)
                    .and_then(|person| self.calendar.duration(person.stage.key()))
                    .is_some_and(|duration| duration.fertile)
            };
            if !(fertile(first_id) && fertile(second_id)) {
                continue;
            }

            self.food -= self.config.birth_food_cost;
            let toddler = self.spawn_person_at(self.home);
            census.people.push(toddler);
            if let Some(actor) = self.partnership_actors.get(&partnership_id) {
                actor.tell(PartnershipMsg::RecordBirth(self.tick), None);
            }
            self.say(&format!(
                "People #{first_id} and #{second_id} welcomed a toddler"
            ));
        }
    }

    fn population_capacity(&self) -> usize {
        self.config.population_capacity + self.houses.len() * self.config.house_capacity
    }

    fn decide(&mut self, func: &str, c: Ctx) -> String {
        match self.vm.call([func], (c,)) {
            Ok(v) => rune::from_value::<String>(v).unwrap_or_else(|_| "wander".into()),
            Err(e) => {
                eprintln!("script error in {func}: {e}");
                "wander".into()
            }
        }
    }

    fn tick_people(&mut self, census: &mut Census) {
        let performed = self.mailbox.request();
        let mut performers = 0;
        for index in 0..census.people.len() {
            if census.people[index].dead {
                continue;
            }
            let Some(person) = self.meet_needs(&census.people[index]) else {
                continue;
            };
            census.people[index] = person.clone();
            if person.dead {
                self.say(&format!("Person #{} starved", person.id));
                continue;
            }
            let activity = self.choose_activity(&person, census);
            if let Some(actor) = self.person_actors.get(&person.id) {
                actor.tell(
                    PersonMsg::Perform(performed, activity),
                    self.mailbox.reply_to(),
                );
                performers += 1;
            }
        }
        for reply in self.mailbox.collect(performed, performers) {
            let ReplyBody::Person(person) = reply else {
                panic!("person actor answered an activity with {reply:?}");
            };
            if let Some(entry) = census.people.iter_mut().find(|p| p.id == person.id) {
                *entry = person;
            }
        }
    }

    /// Lets a person get hungry and eat, settling deposits and meals with the stockpile.
    fn meet_needs(&mut self, person: &Person) -> Option<Person> {
        let actor = self.person_actors.get(&person.id)?;
        let at_home = cheb(person.position(), self.home) <= self.config.home_radius;
        let night = self.daylight() < self.config.daylight_night_threshold;
        let needs = Needs {
            at_home,
            hunger_gain: if night && at_home {
                self.config.hunger_night_at_home
            } else {
                self.config.hunger_other
            },
            stock_food: self.food,
            home_meal_threshold: self.config.hunger_eat_home_threshold,
            carried_meal_threshold: self.config.hunger_eat_carried_threshold,
            starvation_threshold: self.config.hunger_starvation_threshold,
            meal_reduction: self.config.hunger_meal_reduction,
            food_per_meal: self.config.food_per_meal,
        };
        let reply = self.ask(actor, |request| PersonMsg::Needs(request, needs));
        let ReplyBody::Fed(person, outcome) = reply else {
            panic!("person actor answered its needs with {reply:?}");
        };
        self.food += outcome.deposited.food;
        self.wood += outcome.deposited.wood;
        self.stone += outcome.deposited.stone;
        if outcome.ate_from_stock {
            self.food -= self.config.food_per_meal;
        }
        if at_home {
            self.craft_and_build();
        }
        Some(person)
    }

    /// Harvests `kind` at `target` when standing on it, otherwise walks there.
    fn gather(
        &mut self,
        pos: (usize, usize),
        kind: u8,
        target: (usize, usize),
    ) -> ((usize, usize), Goods) {
        let idx = pos.1 * self.width + pos.0;
        if pos != target || self.res[idx].0 != kind {
            return (self.move_toward(pos, target), Goods::default());
        }
        let yield_n: u16 = if self.tools >= self.population_u32() {
            self.config.person_tooled_gather_yield
        } else {
            self.config.person_gather_yield
        };
        let take = yield_n.min(self.res[idx].1);
        self.res[idx].1 -= take;
        if self.res[idx].1 == 0 {
            self.res[idx].0 = 0;
        }
        let take = u32::from(take);
        let gathered = match kind {
            BERRY => Goods {
                food: take,
                ..Goods::default()
            },
            TREE => Goods {
                wood: take,
                ..Goods::default()
            },
            _ => Goods {
                stone: take,
                ..Goods::default()
            },
        };
        (pos, gathered)
    }

    /// Asks a rabbit actor to suffer a successful attack; returns whether it died of it.
    fn hunt_rabbit(&self, census: &mut Census, index: usize) -> bool {
        let (id, _) = census.rabbits[index];
        let Some(actor) = self.rabbit_actors.get(&id) else {
            return false;
        };
        let reply = self.ask(actor, AnimalMsg::Hunted);
        let ReplyBody::Hunted(_, killed) = reply else {
            panic!("rabbit actor answered a hunt with {reply:?}");
        };
        census.rabbits[index].1.dead = true;
        killed
    }

    /// Decides what a fed person does this tick using the `person` behaviour script.
    fn choose_activity(&mut self, p: &Person, census: &mut Census) -> Activity {
        let night = self.daylight() < self.config.daylight_night_threshold;
        let pos = p.position();
        let home = self.home;
        let at_home = cheb(pos, home) <= self.config.home_radius;
        let activity = |action: &str, position| Activity {
            action: action.to_string(),
            position,
            gathered: Goods::default(),
        };
        if p.hunger > self.config.hunger_return_threshold && self.food > 0 && !at_home {
            return activity("returning for food", self.move_toward(pos, home));
        }
        if !p.stage.can_partner() {
            let position = if at_home {
                pos
            } else {
                self.move_toward(pos, home)
            };
            return activity("growing up", position);
        }

        let berry = self.nearest_res(pos, BERRY, self.config.resource_search_radius);
        let tree = self.nearest_res(pos, TREE, self.config.resource_search_radius);
        let stone = self.nearest_res(pos, STONE, self.config.resource_search_radius);
        let rabbit = Self::nearest_animal(&census.rabbits, pos, self.config.resource_search_radius);
        let ctx = Ctx {
            night,
            at_home,
            hunger: f64::from(p.hunger),
            carry: i64::from(p.carry()),
            carried_food: i64::from(p.food),
            stock_food: i64::from(self.food),
            stock_wood: i64::from(self.wood),
            stock_stone: i64::from(self.stone),
            berry_near: berry.is_some(),
            tree_near: tree.is_some(),
            stone_near: stone.is_some(),
            rabbit_near: rabbit.is_some(),
            roll: f64::from(self.rng.f()),
            ..Default::default()
        };
        let choice = self.decide("person", ctx);
        let gather = match choice.as_str() {
            "forage" => Some((BERRY, berry, "foraging")),
            "chop" => Some((TREE, tree, "chopping wood")),
            "mine" => Some((STONE, stone, "mining stone")),
            _ => None,
        };
        match (choice.as_str(), gather, rabbit) {
            ("sleep", ..) => activity("sleeping", pos),
            ("go_home" | "haul" | "eat", ..) => {
                let action = match choice.as_str() {
                    "go_home" => "going home",
                    "haul" => "hauling goods",
                    _ => "going to eat",
                };
                let position = if at_home {
                    pos
                } else {
                    self.move_toward(pos, home)
                };
                activity(action, position)
            }
            ("hunt", _, Some(rabbit)) => {
                let target = census.rabbits[rabbit].1.position();
                if cheb(pos, target) > self.config.attack_range {
                    return activity("hunting", self.move_toward(pos, target));
                }
                let mut hunting = activity("hunting", pos);
                if self.rng.chance(self.config.person_hunt_success_chance)
                    && self.hunt_rabbit(census, rabbit)
                {
                    hunting.gathered.food = self.config.person_hunt_food_yield;
                }
                hunting
            }
            (_, Some((kind, Some(target), label)), _) => {
                let (position, gathered) = self.gather(pos, kind, target);
                Activity {
                    action: label.to_string(),
                    position,
                    gathered,
                }
            }
            _ => activity("wandering", self.wander(pos.0, pos.1)),
        }
    }

    fn craft_and_build(&mut self) {
        if self.wood >= self.config.tool_wood_cost
            && self.stone >= self.config.tool_stone_cost
            && self.tools < self.population_u32()
        {
            self.wood -= self.config.tool_wood_cost;
            self.stone -= self.config.tool_stone_cost;
            self.tools += 1;
        }
        if self.wood >= self.config.house_wood_cost
            && self.stone >= self.config.house_stone_cost
            && self.houses.len() < self.config.max_houses
        {
            let (hx, hy) = self.home;
            let radius = self.config.house_placement_radius;
            let diameter = radius * 2 + 1;
            let x = (hx + self.rng.below(diameter))
                .saturating_sub(radius)
                .min(self.width - 1);
            let y = (hy + self.rng.below(diameter))
                .saturating_sub(radius)
                .min(self.height - 1);
            if self.walkable((x, y))
                && self.res[y * self.width + x].0 == 0
                && !self.houses.contains(&(x, y))
            {
                self.wood -= self.config.house_wood_cost;
                self.stone -= self.config.house_stone_cost;
                self.houses.push((x, y));
                self.say("A new house was built");
            }
        }
    }

    /// Sends every animal of `species` its turn and returns the ids of those wanting to breed.
    fn live(
        &self,
        species: Species,
        census: &mut Census,
        turns: Vec<(AnimalId, AnimalTurn)>,
    ) -> Vec<AnimalId> {
        let request = self.mailbox.request();
        let mut sent = 0;
        for (id, turn) in turns {
            if let Some(actor) = self.animal_actors(species).get(&id) {
                actor.tell(AnimalMsg::Live(request, turn), self.mailbox.reply_to());
                sent += 1;
            }
        }
        let lived = self
            .mailbox
            .collect(request, sent)
            .into_iter()
            .map(|reply| match reply {
                ReplyBody::Lived(id, animal, wants_to_breed) => (id, (animal, wants_to_breed)),
                other => panic!("animal actor answered its turn with {other:?}"),
            })
            .collect();
        let animals = census.animals_mut(species);
        let mut breeders = Vec::new();
        for (id, (animal, wants_to_breed)) in sorted_by_key(lived) {
            if let Some(entry) = animals.iter_mut().find(|(key, _)| *key == id) {
                entry.1 = animal;
            }
            if wants_to_breed {
                breeders.push(id);
            }
        }
        breeders
    }

    /// Lets `breeders` give birth, in order, while the species is below `max_population`.
    fn breed(
        &mut self,
        species: Species,
        census: &mut Census,
        breeders: Vec<AnimalId>,
        max_population: usize,
        cost: BirthCost,
        newborn_energy: f32,
    ) {
        for id in breeders {
            let living = census
                .animals_mut(species)
                .iter()
                .filter(|(_, animal)| !animal.dead)
                .count();
            if living >= max_population {
                break;
            }
            let Some(actor) = self.animal_actors(species).get(&id) else {
                continue;
            };
            let reply = self.ask(actor, |request| AnimalMsg::GiveBirth(request, cost));
            let ReplyBody::Animal(_, parent) = reply else {
                panic!("animal actor answered a birth with {reply:?}");
            };
            let position = parent.position();
            if let Some(entry) = census
                .animals_mut(species)
                .iter_mut()
                .find(|(key, _)| *key == id)
            {
                entry.1 = parent;
            }
            let newborn = self.spawn_animal(species, position, newborn_energy);
            census.animals_mut(species).push(newborn);
        }
    }

    fn tick_rabbits(&mut self, census: &mut Census) {
        let mut turns = Vec::with_capacity(census.rabbits.len());
        for index in 0..census.rabbits.len() {
            let (id, rabbit) = census.rabbits[index].clone();
            if rabbit.dead {
                continue;
            }
            let pos = rabbit.position();
            let on_grass = matches!(self.tiles[pos.1 * self.width + pos.0], GRASS | FOREST);
            let wolf =
                Self::nearest_animal(&census.wolves, pos, self.config.rabbit_wolf_detection_range);
            let c = Ctx {
                wolf_near: wolf.is_some(),
                roll: f64::from(self.rng.f()),
                ..Default::default()
            };
            let position = match (self.decide("rabbit", c).as_str(), wolf) {
                ("flee", Some(w)) => {
                    let (wx, wy) = census.wolves[w].1.position();
                    self.move_dir(pos.0, pos.1, direction(wx, pos.0), direction(wy, pos.1))
                }
                ("wander", _) => self.wander(pos.0, pos.1),
                _ => pos,
            };
            let may_breed = self.rng.chance(self.config.rabbit_breed_chance);
            turns.push((
                id,
                AnimalTurn {
                    drain: self.config.rabbit_energy_drain,
                    gain: on_grass.then_some(self.config.rabbit_energy_regen),
                    max_energy: self.config.rabbit_energy_max,
                    position,
                    breed_above: may_breed.then_some(self.config.rabbit_breed_energy),
                    death_threshold: self.config.animal_energy_death_threshold,
                },
            ));
        }
        let breeders = self.live(Species::Rabbit, census, turns);
        let cost = BirthCost {
            energy: self.config.rabbit_breed_energy_cost,
            death_threshold: self.config.animal_energy_death_threshold,
        };
        self.breed(
            Species::Rabbit,
            census,
            breeders,
            self.config.rabbit_max_population,
            cost,
            self.config.rabbit_newborn_energy,
        );
    }

    fn tick_wolves(&mut self, census: &mut Census) {
        let mut turns = Vec::with_capacity(census.wolves.len());
        for index in 0..census.wolves.len() {
            let (id, wolf) = census.wolves[index].clone();
            if wolf.dead {
                continue;
            }
            let pos = wolf.position();
            let prey = Self::nearest_animal(&census.rabbits, pos, self.config.wolf_prey_range);
            let ctx = Ctx {
                prey_near: prey.is_some(),
                ..Default::default()
            };
            let mut gain = None;
            let position = if let ("hunt", Some(rabbit)) = (self.decide("wolf", ctx).as_str(), prey)
            {
                let target = census.rabbits[rabbit].1.position();
                if cheb(pos, target) <= self.config.attack_range {
                    if self.rng.chance(self.config.wolf_hunt_success_chance)
                        && self.hunt_rabbit(census, rabbit)
                    {
                        gain = Some(self.config.wolf_hunt_energy_gain);
                    }
                    pos
                } else {
                    self.move_toward(pos, target)
                }
            } else {
                self.wander(pos.0, pos.1)
            };
            let may_breed = self.rng.chance(self.config.wolf_breed_chance);
            turns.push((
                id,
                AnimalTurn {
                    drain: self.config.wolf_energy_drain,
                    gain,
                    max_energy: self.config.wolf_energy_max,
                    position,
                    breed_above: may_breed.then_some(self.config.wolf_breed_energy),
                    death_threshold: self.config.animal_energy_death_threshold,
                },
            ));
        }
        let breeders = self.live(Species::Wolf, census, turns);
        let cost = BirthCost {
            energy: self.config.wolf_breed_energy_cost,
            death_threshold: self.config.animal_energy_death_threshold,
        };
        self.breed(
            Species::Wolf,
            census,
            breeders,
            self.config.wolf_max_population,
            cost,
            self.config.wolf_newborn_energy,
        );
    }

    /// Replaces the running script. Behaviours and tuning constants apply
    /// immediately; map size and calendar defaults apply on reset.
    pub fn set_script(&mut self, src: &str) -> Result<(), String> {
        let script = SimScript::load(src)?;
        self.vm = script.vm();
        self.config = script.config.clone();
        self.script = script;
        Ok(())
    }

    pub fn restarted(&self, seed: u64) -> Self {
        let mut world = Self::new_with_script(seed, self.script.clone());
        world.speed = self.speed;
        world.paused = self.paused;
        world.calendar = self.calendar.clone();
        world.calendar.normalize();
        world.tick = (world.calendar.ticks_per_day / world.config.starting_tick_fraction).max(1);
        world
    }

    pub fn regenerated_seed(&self) -> u64 {
        let mut seed = rand::random::<u64>();
        if seed == self.seed {
            seed = seed.wrapping_add(1);
        }
        seed
    }

    pub fn seed(&self) -> u64 {
        self.seed
    }

    pub fn snapshot_json(&self) -> Result<String, serde_json::Error> {
        let census = self.census();
        serde_json::to_string(&WorldSnapshot {
            version: 1,
            seed: Some(self.seed),
            width: self.width,
            height: self.height,
            tiles: self.tiles.clone(),
            res: self.res.clone(),
            people: census.people,
            rabbits: census.rabbits.into_iter().map(|(_, a)| a).collect(),
            wolves: census.wolves.into_iter().map(|(_, a)| a).collect(),
            houses: self.houses.clone(),
            home: self.home,
            food: self.food,
            wood: self.wood,
            stone: self.stone,
            tools: self.tools,
            rng: self.rng.0,
            tick: self.tick,
            next_id: self.next_id,
            log: self.log.clone(),
            paused: self.paused,
            speed: self.speed,
            terrain_rev: self.terrain_rev,
            calendar: self.calendar.clone(),
            partnerships: self.observe_partnerships(),
            next_partnership_id: self.next_partnership_id,
        })
    }

    pub fn script(&self) -> &str {
        self.script.source()
    }

    #[cfg(test)]
    pub fn from_snapshot(snapshot: &str, fallback_seed: u64) -> Result<Self, String> {
        Self::from_snapshot_with_script(snapshot, fallback_seed, default_script())
    }

    /// Restores world state from a snapshot; `script` (normally `config.rn`
    /// on disk) supplies the behaviours and tuning constants.
    pub fn from_snapshot_with_script(
        snapshot: &str,
        fallback_seed: u64,
        script: SimScript,
    ) -> Result<Self, String> {
        let config = &script.config;
        let snapshot_value: serde_json::Value =
            serde_json::from_str(snapshot).map_err(|e| e.to_string())?;
        let legacy_people = snapshot_value
            .get("people")
            .and_then(serde_json::Value::as_array)
            .is_some_and(|people| {
                !people.is_empty()
                    && people.iter().all(|person| {
                        person
                            .as_object()
                            .is_some_and(|person| !person.contains_key("stage"))
                    })
            });
        let mut snapshot_value = snapshot_value;
        if snapshot_value.get("calendar").is_none() {
            let calendar = serde_json::to_value(&config.calendar).map_err(|e| e.to_string())?;
            snapshot_value
                .as_object_mut()
                .ok_or("world snapshot must be a JSON object")?
                .insert("calendar".into(), calendar);
        }
        let saved: WorldSnapshot =
            serde_json::from_value(snapshot_value).map_err(|e| e.to_string())?;
        if saved.version != 1 {
            return Err(format!(
                "unsupported world snapshot version {}",
                saved.version
            ));
        }
        if !(8..=256).contains(&saved.width)
            || !(8..=256).contains(&saved.height)
            || saved.tiles.len() != saved.width * saved.height
            || saved.res.len() != saved.width * saved.height
        {
            return Err("world snapshot has invalid terrain dimensions".into());
        }
        let mut world = Self::empty(
            saved.seed.unwrap_or(fallback_seed),
            script,
            saved.tiles,
            saved.res,
        );
        world.width = saved.width;
        world.height = saved.height;
        world.next_partnership_id = saved.next_partnership_id;
        world.houses = saved.houses;
        world.home = saved.home;
        world.food = saved.food;
        world.wood = saved.wood;
        world.stone = saved.stone;
        world.tools = saved.tools;
        world.rng = Rng(saved.rng);
        world.tick = saved.tick;
        world.next_id = saved.next_id;
        world.log = saved.log;
        world.paused = saved.paused;
        world.speed = saved.speed;
        world.terrain_rev = saved.terrain_rev;
        world.calendar = saved.calendar;
        world.calendar.normalize();
        let mut people = saved.people;
        if legacy_people {
            for (index, person) in people.iter_mut().enumerate() {
                let stage = if index < world.config.initial_young_adults {
                    LifeStage::YoungAdult
                } else {
                    LifeStage::Adult
                };
                world.make_grown_up(person, stage);
            }
        }
        let mut partnerships = saved.partnerships;
        repair_partnerships(&mut people, &mut partnerships);
        for person in people {
            world.spawn_person_actor(person);
        }
        for rabbit in saved.rabbits {
            world.spawn_animal_actor(Species::Rabbit, rabbit);
        }
        for wolf in saved.wolves {
            world.spawn_animal_actor(Species::Wolf, wolf);
        }
        for partnership in partnerships {
            world.spawn_partnership_actor(partnership);
        }
        Ok(world)
    }

    fn rect(&self, t: &[&str], i: usize) -> Option<(usize, usize, usize, usize)> {
        let n: Vec<i64> = (i..i + 4).filter_map(|k| t.get(k)?.parse().ok()).collect();
        if n.len() != 4 {
            return None;
        }
        let c = |v: i64, max: usize| usize::try_from(v.max(0)).map_or(max - 1, |v| v.min(max - 1));
        let (x0, x1) = (c(n[0].min(n[2]), self.width), c(n[0].max(n[2]), self.width));
        let (y0, y1) = (
            c(n[1].min(n[3]), self.height),
            c(n[1].max(n[3]), self.height),
        );
        Some((x0, y0, x1, y1))
    }

    /// Applies one whitespace-separated edit command; malformed commands are ignored.
    pub fn apply(&mut self, cmd: &str) {
        let t: Vec<&str> = cmd.split_whitespace().collect();
        match t.first().copied() {
            Some("pause") => self.paused = arg(&t, 1) != Some(0),
            Some("calendar") => self.edit_calendar(&t),
            Some("stage") => self.edit_stage(&t),
            Some("speed") => {
                if let Some(n) = arg(&t, 1) {
                    self.speed = clamp_to(n, 1, i64::from(self.config.max_simulation_speed));
                }
            }
            Some("stock") => self.edit_stock(&t),
            Some("person") => self.edit_person(&t),
            Some("energy") => self.edit_energy(&t),
            Some("terrain") => {
                if let (Some((x0, y0, x1, y1)), Some(kind)) = (self.rect(&t, 1), arg(&t, 5))
                    && let Ok(kind) = u8::try_from(kind)
                    && kind <= 6
                {
                    self.paint_terrain(x0, y0, x1, y1, kind);
                }
            }
            Some("res") => self.edit_resources(&t),
            Some("spawn") => self.edit_spawn(&t),
            Some("house") => self.edit_houses(&t),
            Some("remove") => self.edit_remove(&t),
            _ => {}
        }
    }

    fn edit_calendar(&mut self, t: &[&str]) {
        if let (Some(setting), Some(value)) = (t.get(1), arg(t, 2)) {
            let days: u32 = clamp_to(value, 1, 100);
            match *setting {
                "ticks_per_day" => self.calendar.ticks_per_day = clamp_to(value, 1, 1_000_000),
                "days_per_week" => self.calendar.days_per_week = days,
                "weeks_per_month" => self.calendar.weeks_per_month = days,
                "months_per_year" => self.calendar.months_per_year = days,
                _ => {}
            }
        }
    }

    fn edit_stage(&mut self, t: &[&str]) {
        let (Some(stage), Some(property)) = (t.get(1), t.get(2)) else {
            return;
        };
        if *property == "fertile" {
            if let Some(value) = arg(t, 3)
                && let Some(range) = self.calendar.duration_mut(stage)
            {
                range.fertile = value != 0;
            }
        } else if let (Some(min_days), Some(max_days)) = (arg(t, 2), arg(t, 3)) {
            let min_days: u32 = clamp_to(min_days, 1, 1_000_000);
            let max_days: u32 = clamp_to(max_days, 1, 1_000_000);
            if min_days <= max_days
                && let Some(range) = self.calendar.duration_mut(stage)
            {
                range.min_days = min_days;
                range.max_days = max_days;
            }
        }
    }

    fn edit_stock(&mut self, t: &[&str]) {
        if let (Some(name), Some(v)) = (t.get(1), arg(t, 2)) {
            let v = clamp_to(v, 0, i64::from(self.config.max_stock_edit));
            match *name {
                "food" => self.food = v,
                "wood" => self.wood = v,
                "stone" => self.stone = v,
                "tools" => self.tools = v,
                _ => {}
            }
        }
    }

    fn edit_person(&mut self, t: &[&str]) {
        let max_inventory = i64::from(self.config.max_person_inventory_edit);
        let max_hunger = self.config.max_hunger_edit;
        if let (Some(id), Some(field), Some(v)) = (arg(t, 1), t.get(2), arg(t, 3))
            && let Ok(id) = u32::try_from(id)
        {
            let u = clamp_to(v, 0, max_inventory);
            let edit = match *field {
                "hunger" => PersonEdit::Hunger(small_f32(v.clamp(0, max_hunger))),
                "food" => PersonEdit::Food(u),
                "wood" => PersonEdit::Wood(u),
                "stone" => PersonEdit::Stone(u),
                _ => return,
            };
            self.tell_person(id, PersonMsg::Edit(edit));
        }
    }

    fn edit_energy(&mut self, t: &[&str]) {
        if let (Some((x0, y0, x1, y1)), Some(v)) = (self.rect(t, 1), arg(t, 5)) {
            let max = self.config.max_animal_energy_edit.trunc().max(1.0);
            let v = small_f32(v.clamp(1, 1_000_000)).min(max);
            let area = Area { x0, y0, x1, y1 };
            for actor in self.rabbit_actors.values().chain(self.wolf_actors.values()) {
                actor.tell(AnimalMsg::SetEnergyWithin(area, v), None);
            }
        }
    }

    fn edit_resources(&mut self, t: &[&str]) {
        let (Some((x0, y0, x1, y1)), Some(kind), Some(amt)) =
            (self.rect(t, 1), arg(t, 5), arg(t, 6))
        else {
            return;
        };
        let Ok(kind) = u8::try_from(kind) else {
            return;
        };
        if kind > 3 {
            return;
        }
        let amt = clamp_to(amt, 1, i64::from(self.config.max_resource_edit));
        for y in y0..=y1 {
            for x in x0..=x1 {
                let i = y * self.width + x;
                let free =
                    self.walkable((x, y)) && (x, y) != self.home && !self.houses.contains(&(x, y));
                if kind == 0 {
                    self.res[i] = (0, 0);
                } else if free {
                    self.res[i] = (kind, amt);
                }
            }
        }
    }

    fn edit_spawn(&mut self, t: &[&str]) {
        let (Some(what), Some((x0, y0, x1, y1))) = (t.get(1), self.rect(t, 2)) else {
            return;
        };
        let mut left = self.config.manual_spawn_limit;
        for y in y0..=y1 {
            for x in x0..=x1 {
                if left == 0 || !self.walkable((x, y)) {
                    continue;
                }
                left -= 1;
                let energy = self.config.initial_animal_energy;
                match *what {
                    "person" => {
                        self.spawn_person_at((x, y));
                    }
                    "rabbit" => {
                        self.spawn_animal(Species::Rabbit, (x, y), energy);
                    }
                    "wolf" => {
                        self.spawn_animal(Species::Wolf, (x, y), energy);
                    }
                    _ => left += 1,
                }
            }
        }
    }

    fn edit_houses(&mut self, t: &[&str]) {
        let Some((x0, y0, x1, y1)) = self.rect(t, 1) else {
            return;
        };
        for y in y0..=y1 {
            for x in x0..=x1 {
                if self.houses.len() < self.config.max_houses
                    && self.walkable((x, y))
                    && self.res[y * self.width + x].0 == 0
                    && (x, y) != self.home
                    && !self.houses.contains(&(x, y))
                {
                    self.houses.push((x, y));
                }
            }
        }
    }

    fn edit_remove(&mut self, t: &[&str]) {
        let (Some((x0, y0, x1, y1)), Some(what)) = (self.rect(t, 1), t.get(5)) else {
            return;
        };
        let area = Area { x0, y0, x1, y1 };
        let all = *what == "all";
        if all || *what == "people" {
            let mut people = self.observe_people();
            let removed: Vec<u32> = people
                .iter()
                .filter(|p| area.contains(p.position()))
                .map(|p| p.id)
                .collect();
            if !removed.is_empty() {
                for &id in &removed {
                    self.retire_person(id);
                }
                people.retain(|p| !removed.contains(&p.id));
                self.prune_partnerships(&mut people);
            }
        }
        for (name, species) in [("rabbits", Species::Rabbit), ("wolves", Species::Wolf)] {
            if all || *what == name {
                for (id, animal) in self.observe_animals(species) {
                    if area.contains(animal.position()) {
                        self.retire_animal(species, id);
                    }
                }
            }
        }
        if all || *what == "houses" {
            self.houses.retain(|h| !area.contains(*h));
        }
        if all || *what == "res" {
            for y in y0..=y1 {
                for x in x0..=x1 {
                    self.res[y * self.width + x] = (0, 0);
                }
            }
        }
    }

    fn paint_terrain(&mut self, x0: usize, y0: usize, x1: usize, y1: usize, kind: u8) {
        let land = matches!(kind, SAND | GRASS | FOREST | HILLS);
        for y in y0..=y1 {
            for x in x0..=x1 {
                if !land && (x, y) == self.home {
                    continue;
                }
                self.tiles[y * self.width + x] = kind;
                if !land {
                    self.res[y * self.width + x] = (0, 0);
                    self.houses.retain(|h| *h != (x, y));
                }
            }
        }
        if !land {
            for species in [Species::Rabbit, Species::Wolf] {
                for (id, animal) in self.observe_animals(species) {
                    if !self.walkable(animal.position()) {
                        self.retire_animal(species, id);
                    }
                }
            }
            for person in self.observe_people() {
                if !self.walkable(person.position()) {
                    self.tell_person(person.id, PersonMsg::MoveTo(self.home));
                }
            }
        }
        self.terrain_rev += 1;
    }

    pub fn terrain_json(&self) -> String {
        let tiles: String = self.tiles.iter().map(|t| (b'0' + t) as char).collect();
        format!(
            "{{\"w\":{},\"h\":{},\"tiles\":\"{}\"}}",
            self.width, self.height, tiles
        )
    }

    fn write_calendar_json(&self, s: &mut String) {
        let _ = write!(
            *s,
            "\"calendar\":{{\"ticks_per_day\":{},\"days_per_week\":{},\"weeks_per_month\":{},\"months_per_year\":{},",
            self.calendar.ticks_per_day,
            self.calendar.days_per_week,
            self.calendar.weeks_per_month,
            self.calendar.months_per_year
        );
        for (index, (name, range)) in [
            ("toddler", self.calendar.toddler),
            ("child", self.calendar.child),
            ("teen", self.calendar.teen),
            ("young_adult", self.calendar.young_adult),
            ("adult", self.calendar.adult),
            ("older_adult", self.calendar.older_adult),
            ("elderly", self.calendar.elderly),
        ]
        .into_iter()
        .enumerate()
        {
            if index > 0 {
                s.push(',');
            }
            let _ = write!(
                *s,
                "\"{name}\":{{\"min_days\":{},\"max_days\":{},\"fertile\":{}}}",
                range.min_days, range.max_days, range.fertile
            );
        }
        s.push_str("},");
    }

    fn write_entities_json(&self, s: &mut String) {
        let census = self.census();
        s.push_str("\"houses\":[");
        let j: Vec<String> = self
            .houses
            .iter()
            .map(|h| format!("[{},{}]", h.0, h.1))
            .collect();
        s.push_str(&j.join(","));
        s.push_str("],\"people\":[");
        let j: Vec<String> = census
            .people
            .iter()
            .map(|p| {
                format!(
                    "[{},{},{},\"{}\",{:.0},{},{},{},\"{}\",{},{}]",
                    p.id,
                    p.x,
                    p.y,
                    p.action,
                    p.hunger,
                    p.food,
                    p.wood,
                    p.stone,
                    p.stage.key(),
                    p.age_ticks / self.calendar.ticks_per_day,
                    p.partner_id
                        .map_or_else(|| "null".to_string(), |partner| partner.to_string())
                )
            })
            .collect();
        s.push_str(&j.join(","));
        s.push_str("],\"rabbits\":[");
        let j: Vec<String> = census
            .rabbits
            .iter()
            .map(|(_, a)| format!("[{},{},{:.0}]", a.x, a.y, a.energy))
            .collect();
        s.push_str(&j.join(","));
        s.push_str("],\"wolves\":[");
        let j: Vec<String> = census
            .wolves
            .iter()
            .map(|(_, a)| format!("[{},{},{:.0}]", a.x, a.y, a.energy))
            .collect();
        s.push_str(&j.join(","));
        s.push_str("],\"res\":[");
        let mut first = true;
        for (i, r) in self.res.iter().enumerate() {
            if r.0 != 0 {
                if !first {
                    s.push(',');
                }
                first = false;
                let _ = write!(
                    *s,
                    "[{},{},{},{}]",
                    i % self.width,
                    i / self.width,
                    r.0,
                    r.1
                );
            }
        }
        s.push_str("],\"log\":[");
        let j: Vec<String> = self.log.iter().map(|l| format!("\"{l}\"")).collect();
        s.push_str(&j.join(","));
    }

    pub fn state_json(&self) -> String {
        let mut s = String::new();
        let days_per_month =
            u64::from(self.calendar.days_per_week) * u64::from(self.calendar.weeks_per_month);
        let days_per_year = days_per_month * u64::from(self.calendar.months_per_year);
        let day = self.tick / self.calendar.ticks_per_day;
        let year = day / days_per_year + 1;
        let month = (day / days_per_month) % u64::from(self.calendar.months_per_year) + 1;
        let week = (day / u64::from(self.calendar.days_per_week))
            % u64::from(self.calendar.weeks_per_month)
            + 1;
        let date_day = day % u64::from(self.calendar.days_per_week) + 1;
        let actor_stats = self.actor_system.stats();
        let _ = write!(
            s,
            "{{\"paused\":{},\"speed\":{},\"seed\":{},\"rev\":{},\"tick\":{},\"day\":{},\"year\":{},\"month\":{},\"week\":{},\"date_day\":{},\"hour\":{:.2},\"daylight\":{:.3},\"limits\":{{\"speed\":{},\"stock\":{},\"person_inventory\":{},\"hunger\":{},\"animal_energy\":{},\"resource\":{},\"spawn\":{}}},\"stock\":{{\"food\":{},\"wood\":{},\"stone\":{},\"tools\":{}}},\"home\":[{},{}],",
            self.paused,
            self.speed,
            self.seed,
            self.terrain_rev,
            self.tick,
            day + 1,
            year,
            month,
            week,
            date_day,
            self.day_fraction() * 24.0,
            self.daylight(),
            self.config.max_simulation_speed,
            self.config.max_stock_edit,
            self.config.max_person_inventory_edit,
            self.config.max_hunger_edit,
            self.config.max_animal_energy_edit,
            self.config.max_resource_edit,
            self.config.manual_spawn_limit,
            self.food,
            self.wood,
            self.stone,
            self.tools,
            self.home.0,
            self.home.1
        );
        self.write_calendar_json(&mut s);
        self.write_entities_json(&mut s);
        let _ = write!(
            s,
            "],\"actor_stats\":{{\"actors_created\":{},\"actors_terminated\":{},\"actors_restarted\":{},\"messages_sent\":{},\"messages_failed\":{},\"system_messages_sent\":{},\"live_actors\":{}}}}}",
            actor_stats.actors_created,
            actor_stats.actors_terminated,
            actor_stats.actors_restarted,
            actor_stats.messages_sent,
            actor_stats.messages_failed,
            actor_stats.system_messages_sent,
            actor_stats.live_actors()
        );
        s
    }

    #[cfg(feature = "console")]
    pub fn handle_picard_console_text(&self, request: &str) -> String {
        self.actor_system.console().handle_text(request)
    }
}

impl Drop for World {
    fn drop(&mut self) {
        // Shutdown starts eagerly; Drop must not block waiting for completion.
        drop(self.actor_system.shutdown());
    }
}

#[cfg(test)]
mod tests {
    use super::actors::{Activity, AnimalTurn, Goods, PersonMsg, ReplyBody};
    use super::{Animal, DEFAULT_SCRIPT, H, LifeStage, SimScript, W, World, cheb, default_config};

    /// Replaces the value of `pub const {name}` in `source`, whatever its current value is.
    fn with_constant(source: &str, name: &str, value: &str) -> String {
        let prefix = format!("pub const {name} = ");
        assert!(
            source.lines().any(|line| line.starts_with(&prefix)),
            "config.rn has no constant {name}"
        );
        source
            .lines()
            .map(|line| {
                if line.starts_with(&prefix) {
                    format!("{prefix}{value};")
                } else {
                    line.to_string()
                }
            })
            .collect::<Vec<_>>()
            .join("\n")
    }

    fn is_fertile(world: &World, stage: LifeStage) -> bool {
        world
            .calendar
            .duration(stage.key())
            .is_some_and(|duration| duration.fertile)
    }

    fn simulation_state(world: &World) -> serde_json::Value {
        let mut state: serde_json::Value = serde_json::from_str(&world.state_json()).unwrap();
        state.as_object_mut().unwrap().remove("actor_stats");
        state
    }

    #[test]
    fn new_people_progress_through_every_stage_and_die() {
        let mut world = World::new(42);
        world.calendar.ticks_per_day = 1;
        for stage in [
            "toddler",
            "child",
            "teen",
            "young_adult",
            "adult",
            "older_adult",
            "elderly",
        ] {
            let duration = world.calendar.duration_mut(stage).unwrap();
            duration.min_days = 1;
            duration.max_days = 1;
        }
        let id = world.spawn_person_at(world.home).id;
        assert_eq!(
            world.observe_people().last().unwrap().stage,
            LifeStage::Toddler
        );

        for expected in [
            LifeStage::Child,
            LifeStage::Teen,
            LifeStage::YoungAdult,
            LifeStage::Adult,
            LifeStage::OlderAdult,
            LifeStage::Elderly,
            LifeStage::Dead,
        ] {
            let people = world.advance_people();
            let person = people.iter().find(|person| person.id == id).unwrap();
            assert_eq!(person.stage, expected);
        }
        let people = world.observe_people();
        assert!(people.iter().find(|person| person.id == id).unwrap().dead);
    }

    #[test]
    fn eligible_people_form_partnership_actor_and_have_toddlers() {
        let mut world = World::new(42);
        world.calendar.ticks_per_day = 1;
        world.calendar.days_per_week = 1;
        world.food = 1000;
        let mut census = world.census();
        let eligible = census
            .people
            .iter()
            .filter(|person| person.stage.can_partner())
            .count();
        world.find_partners(&mut census.people);
        let partnerships = eligible / 2;
        assert!(partnerships > 0);
        assert_eq!(world.partnership_actors.len(), partnerships);
        assert_eq!(world.observe_partnerships().len(), partnerships);
        assert_eq!(
            world
                .observe_people()
                .iter()
                .filter(|person| person.partner_id.is_some())
                .count(),
            partnerships * 2
        );

        let initial_population = census.people.len();
        let births = 3;
        world.config.population_capacity = initial_population + births;
        while census.people.len() < world.population_capacity() {
            world.tick += 1;
            world.have_children(1, &mut census);
        }
        let people = world.observe_people();
        assert_eq!(people.len(), world.population_capacity());
        assert_eq!(people.len() - initial_population, births);
        let toddler = people
            .iter()
            .find(|person| person.stage == LifeStage::Toddler)
            .expect("fertile partner actors should eventually have a child");
        assert_eq!(toddler.age_ticks, 0);
        assert_eq!((toddler.x, toddler.y), world.home);
    }

    #[test]
    fn each_fertile_partnership_can_have_a_weekly_birth() {
        let mut world = World::new(42);
        world.calendar.ticks_per_day = 1;
        let mut census = world.census();
        world.find_partners(&mut census.people);
        let partnerships = world.observe_partnerships();
        let fertile_partnerships = partnerships
            .iter()
            .filter(|partnership| {
                [partnership.first_person_id, partnership.second_person_id]
                    .iter()
                    .all(|id| {
                        let person = census.people.iter().find(|person| person.id == *id);
                        person.is_some_and(|person| is_fertile(&world, person.stage))
                    })
            })
            .count();
        assert!(fertile_partnerships > 0);
        let initial_population = census.people.len();
        world.config.population_capacity = initial_population + partnerships.len();
        world.food = world.config.birth_food_threshold
            + world.config.birth_food_cost * u32::try_from(fertile_partnerships).unwrap();
        world.tick += 7;

        world.have_children(7, &mut census);

        let people = world.observe_people();
        assert_eq!(people.len(), initial_population + fertile_partnerships);
        assert_eq!(
            people
                .iter()
                .filter(|person| person.stage == LifeStage::Toddler)
                .count(),
            fertile_partnerships
        );
    }

    #[test]
    fn people_can_pathfind_back_home_around_blocked_terrain() {
        let mut world = World::new(42);
        let start = (59, 48);
        let mut position = start;

        for _ in 0..W.max(H) * 2 {
            if position == world.home {
                break;
            }
            position = world.move_toward(position, world.home);
        }

        assert_eq!(position, world.home);
        assert!(world.walkable(start));
    }

    #[test]
    fn village_survives_until_its_first_weekly_birth_check() {
        let mut world = World::new(42);
        let first_week_tick =
            world.calendar.ticks_per_day * u64::from(world.calendar.days_per_week);
        let mut daily = Vec::new();

        while world.tick < first_week_tick {
            world.step();
            if world.tick.is_multiple_of(world.calendar.ticks_per_day) {
                daily.push((world.tick, world.person_actors.len(), world.food));
            }
        }

        let initial_population = world.config.initial_population;
        let minimum_population = if initial_population < world.population_capacity() {
            initial_population + 1
        } else {
            initial_population
        };
        assert!(
            world.person_actors.len() >= minimum_population,
            "village failed to replace its initial population: daily={daily:?}, people={:?}, events={:?}",
            world
                .observe_people()
                .iter()
                .map(|person| (person.id, person.hunger, person.action.as_str()))
                .collect::<Vec<_>>(),
            world.log,
        );
        assert!(
            world.log.iter().all(|event| !event.contains("starved")),
            "people starved before the first birth check: {:?}",
            world.log
        );
    }

    #[test]
    fn legacy_snapshots_restore_the_village_as_adults() {
        let world = World::new(42);
        let mut snapshot: serde_json::Value =
            serde_json::from_str(&world.snapshot_json().unwrap()).unwrap();
        for person in snapshot["people"].as_array_mut().unwrap() {
            let person = person.as_object_mut().unwrap();
            for field in [
                "age_ticks",
                "stage",
                "stage_elapsed_ticks",
                "stage_duration_days",
                "partner_id",
            ] {
                person.remove(field);
            }
        }
        snapshot.as_object_mut().unwrap().remove("partnerships");

        let restored = World::from_snapshot(&snapshot.to_string(), 42).unwrap();

        let config = default_config();
        let people = restored.observe_people();
        assert_eq!(people.len(), config.initial_population);
        assert_eq!(people[0].stage, LifeStage::YoungAdult);
        assert!(
            people
                .iter()
                .skip(config.initial_young_adults)
                .all(|person| person.stage == LifeStage::Adult)
        );
    }

    #[test]
    fn partnership_and_life_stage_are_in_persisted_actor_state() {
        let mut world = World::new(42);
        let mut census = world.census();
        world.find_partners(&mut census.people);
        let restored = World::from_snapshot(&world.snapshot_json().unwrap(), 42).unwrap();

        assert!(!world.partnership_actors.is_empty());
        assert_eq!(
            restored.observe_partnerships().len(),
            world.partnership_actors.len()
        );
        assert_eq!(simulation_state(&restored), simulation_state(&world));
        assert_eq!(
            serde_json::from_str::<serde_json::Value>(&restored.state_json()).unwrap()["people"][0]
                [8],
            "young_adult"
        );
    }

    #[test]
    fn children_return_home_instead_of_staying_away_from_food() {
        let mut world = World::new(42);
        world.food = 100;
        let home = world.home;
        let position = (home.0.saturating_add(5), home.1.saturating_add(5));
        let id = world.spawn_person_at(position).id;

        let mut census = world.census();
        world.tick_people(&mut census);

        let child = census.people.iter().find(|person| person.id == id).unwrap();
        assert!(cheb((child.x, child.y), home) < cheb(position, home));
        assert_eq!(child.action, "growing up");
        let observed = world.observe_people();
        let observed = observed.iter().find(|person| person.id == id).unwrap();
        assert_eq!((observed.x, observed.y), (child.x, child.y));
    }

    #[test]
    fn a_person_eats_available_food_before_starving() {
        let mut world = World::new(42);
        world.food = 1;
        let id = world.observe_people()[0].id;
        world.apply(&format!("person {id} hunger 99"));

        let person = world.observe_people()[0].clone();
        let fed = world.meet_needs(&person).unwrap();

        assert!(!fed.dead);
        assert!(fed.hunger < 100.0);
        assert_eq!(world.food, 0);
    }

    #[test]
    fn snapshot_restores_state_and_rng() {
        let mut original = World::new(42);
        for _ in 0..10 {
            original.step();
        }
        original.paused = true;
        original.speed = 4;

        let snapshot = original.snapshot_json().unwrap();
        let mut restored = World::from_snapshot(&snapshot, 42).unwrap();
        assert_eq!(simulation_state(&original), simulation_state(&restored));

        original.step();
        restored.step();
        assert_eq!(simulation_state(&original), simulation_state(&restored));
    }

    #[test]
    fn reset_keeps_seed_and_settings_with_initial_population() {
        let mut original = World::new(42);
        original.apply("speed 4");
        original.paused = true;
        original.set_script(DEFAULT_SCRIPT).unwrap();
        original.step();

        let reset = original.restarted(original.seed());
        assert_eq!(reset.seed(), 42);
        assert_eq!(reset.speed, 4);
        assert!(reset.paused);
        assert_eq!(
            reset.person_actors.len(),
            default_config().initial_population
        );
        assert_eq!(
            reset.tick,
            default_config().calendar.ticks_per_day / default_config().starting_tick_fraction
        );
    }

    #[test]
    fn regenerated_seed_differs_from_current_seed() {
        let world = World::new(42);
        assert_ne!(world.regenerated_seed(), world.seed());
    }

    #[test]
    fn entity_actor_messages_round_trip_through_serde() {
        let message = PersonMsg::Perform(
            7,
            Activity {
                action: "foraging".into(),
                position: (3, 4),
                gathered: Goods {
                    food: 2,
                    wood: 0,
                    stone: 1,
                },
            },
        );
        let serialized = serde_json::to_string(&message).unwrap();
        let PersonMsg::Perform(request, activity) = serde_json::from_str(&serialized).unwrap()
        else {
            panic!("serialized person message changed variants");
        };
        assert_eq!((request, activity.position), (7, (3, 4)));
        assert_eq!(activity.gathered.stone, 1);

        let reply = ReplyBody::Lived(
            9,
            Animal {
                x: 1,
                y: 2,
                energy: 30.0,
                dead: false,
            },
            true,
        );
        let serialized = serde_json::to_string(&reply).unwrap();
        let ReplyBody::Lived(id, animal, wants_to_breed) =
            serde_json::from_str(&serialized).unwrap()
        else {
            panic!("serialized animal reply changed variants");
        };
        assert_eq!((id, animal.position(), wants_to_breed), (9, (1, 2), true));
        let turn: AnimalTurn = serde_json::from_str(
            r#"{"drain":1.0,"gain":null,"max_energy":2.0,"position":[0,0],"breed_above":null,"death_threshold":0.0}"#,
        )
        .unwrap();
        assert!(turn.gain.is_none());
    }

    #[test]
    fn picard_actors_own_living_entity_state() {
        let mut world = World::new(42);
        let person_id = world.observe_people()[0].id;

        world.apply(&format!("person {person_id} hunger 73"));

        assert!((world.observe_people()[0].hunger - 73.0).abs() < f32::EPSILON);
        let entities = world.person_actors.len()
            + world.rabbit_actors.len()
            + world.wolf_actors.len()
            + world.partnership_actors.len();
        let census = world.census();
        assert_eq!(
            census.people.len() + census.rabbits.len() + census.wolves.len(),
            entities
        );
        let state: serde_json::Value = serde_json::from_str(&world.state_json()).unwrap();
        assert!(state["actor_stats"]["live_actors"].as_u64().unwrap() > entities as u64);
        assert!(state["actor_stats"]["messages_sent"].as_u64().unwrap() > 0);
    }

    #[test]
    fn dead_entities_stop_their_actors_and_dissolve_partnerships() {
        let mut world = World::new(42);
        let mut census = world.census();
        world.find_partners(&mut census.people);
        let partnership = world.observe_partnerships()[0].clone();
        let partner = census
            .people
            .iter()
            .find(|person| person.id == partnership.first_person_id)
            .unwrap()
            .clone();
        let survivor_id = partnership.second_person_id;

        world.apply(&format!(
            "remove {x} {y} {x} {y} people",
            x = partner.x,
            y = partner.y
        ));

        assert!(!world.person_actors.contains_key(&partner.id));
        assert!(!world.partnership_actors.contains_key(&partnership.id));
        if let Some(survivor) = world
            .observe_people()
            .iter()
            .find(|person| person.id == survivor_id)
        {
            assert_eq!(survivor.partner_id, None);
        }

        let rabbit = world.observe_animals(super::Species::Rabbit)[0].clone();
        world.apply(&format!(
            "remove {x} {y} {x} {y} rabbits",
            x = rabbit.1.x,
            y = rabbit.1.y
        ));
        assert!(!world.rabbit_actors.contains_key(&rabbit.0));
    }

    #[test]
    fn calendar_and_life_stage_ranges_persist_in_snapshots() {
        let mut world = World::new(42);
        world.apply("calendar ticks_per_day 240");
        world.apply("calendar days_per_week 5");
        world.apply("stage toddler 3 9");

        let snapshot = world.snapshot_json().unwrap();
        let restored = World::from_snapshot(&snapshot, 42).unwrap();
        assert_eq!(restored.calendar.ticks_per_day, 240);
        assert_eq!(restored.calendar.days_per_week, 5);
        assert_eq!(restored.calendar.toddler.min_days, 3);
        assert_eq!(restored.calendar.toddler.max_days, 9);
        assert_eq!(simulation_state(&world), simulation_state(&restored));
    }

    #[test]
    fn snapshots_without_calendar_use_default_settings() {
        let world = World::new(42);
        let mut snapshot: serde_json::Value =
            serde_json::from_str(&world.snapshot_json().unwrap()).unwrap();
        snapshot.as_object_mut().unwrap().remove("calendar");

        let restored = World::from_snapshot(&snapshot.to_string(), 42).unwrap();
        assert_eq!(
            restored.calendar.ticks_per_day,
            default_config().calendar.ticks_per_day
        );
        assert_eq!(restored.calendar.days_per_week, 7);
    }

    #[test]
    fn rune_config_controls_initial_world_and_survives_snapshot_restore() {
        let source = [
            ("WIDTH", "48"),
            ("HEIGHT", "32"),
            ("INITIAL_POPULATION", "6"),
            ("INITIAL_YOUNG_ADULTS", "2"),
            ("INITIAL_FOOD", "125"),
            ("TICKS_PER_DAY", "240"),
        ]
        .into_iter()
        .fold(DEFAULT_SCRIPT.to_string(), |source, (name, value)| {
            with_constant(&source, name, value)
        });
        let script = SimScript::load(&source).unwrap();
        let world = World::new_with_script(42, script.clone());

        assert_eq!((world.width, world.height), (48, 32));
        assert_eq!(world.person_actors.len(), 6);
        assert_eq!(world.food, 125);
        assert_eq!(world.calendar.ticks_per_day, 240);
        assert!(world.terrain_json().contains("\"w\":48,\"h\":32"));
        let state: serde_json::Value = serde_json::from_str(&world.state_json()).unwrap();
        assert_eq!(
            state["limits"]["speed"],
            default_config().max_simulation_speed
        );

        let restored =
            World::from_snapshot_with_script(&world.snapshot_json().unwrap(), 42, script).unwrap();
        assert_eq!((restored.width, restored.height), (48, 32));
        assert_eq!(simulation_state(&restored), simulation_state(&world));
    }

    #[test]
    fn restored_snapshots_use_the_provided_script() {
        let mut world = World::new(42);
        let edited = with_constant(DEFAULT_SCRIPT, "MAX_SIMULATION_SPEED", "3");
        world.set_script(&edited).unwrap();
        assert_eq!(world.config.max_simulation_speed, 3);
        assert_eq!(world.script(), edited);

        let restored = World::from_snapshot(&world.snapshot_json().unwrap(), 42).unwrap();
        assert_eq!(restored.script(), DEFAULT_SCRIPT);
        assert_eq!(
            restored.config.max_simulation_speed,
            default_config().max_simulation_speed
        );
    }

    #[test]
    fn behaviours_read_script_constants() {
        let mut world = World::new(42);
        let all_stay = with_constant(DEFAULT_SCRIPT, "RABBIT_BEHAVIOR_WANDER_CHANCE", "0.0");
        world.set_script(&all_stay).unwrap();
        let c = super::Ctx {
            roll: 0.25,
            ..Default::default()
        };
        assert_eq!(world.decide("rabbit", c.clone()), "stay");
        let all_wander = with_constant(DEFAULT_SCRIPT, "RABBIT_BEHAVIOR_WANDER_CHANCE", "1.0");
        world.set_script(&all_wander).unwrap();
        assert_eq!(world.decide("rabbit", c), "wander");
    }

    #[test]
    fn invalid_rune_configuration_is_rejected() {
        let source = with_constant(
            &with_constant(DEFAULT_SCRIPT, "INITIAL_POPULATION", "3"),
            "INITIAL_YOUNG_ADULTS",
            "4",
        );
        assert!(
            SimScript::load(&source)
                .err()
                .unwrap()
                .contains("initial_young_adults cannot exceed initial_population")
        );
        let missing = DEFAULT_SCRIPT
            .lines()
            .filter(|line| !line.starts_with("pub const WIDTH = "))
            .collect::<Vec<_>>()
            .join("\n");
        assert!(SimScript::load(&missing).err().unwrap().contains("WIDTH"));
        let no_behaviour = DEFAULT_SCRIPT.replace("pub fn wolf(c)", "pub fn wolves(c)");
        assert!(SimScript::load(&no_behaviour).is_err());
    }
}