sim/src/world/economy.rs

//! The economy's actors (workplaces, markets and carts), the jobs people hold, and how prices
//! are set and haggled over.
//!
//! Goods only enter the economy when people gather or craft them and only leave it when they
//! are eaten or used up. Coins never appear or vanish: wages, purchases and sales move them
//! between people, workplaces and markets.

use super::actors::{PersonMsg, ReplyBody, RequestId, reply};
use super::router::{Admission, EntityKey, Kind, Router};
use picard::actors::{Actor, ActorFactoryArgs, Context as ActorContext, Sender};
use serde::{Deserialize, Serialize};
use std::sync::Arc;

#[derive(Clone, Copy, Debug, Default, Deserialize, Serialize, PartialEq, Eq)]
pub(super) struct Goods {
    pub food: u32,
    pub wood: u32,
    pub stone: u32,
    #[serde(default)]
    pub tools: u32,
}

impl Goods {
    pub fn of(good: Good, quantity: u32) -> Self {
        let mut goods = Self::default();
        *goods.get_mut(good) = quantity;
        goods
    }

    pub fn get(&self, good: Good) -> u32 {
        match good {
            Good::Food => self.food,
            Good::Wood => self.wood,
            Good::Stone => self.stone,
            Good::Tools => self.tools,
            Good::Carts => 0,
        }
    }

    /// The counter of `good`; carts are actors, not goods, so they have a scratch counter.
    pub fn get_mut(&mut self, good: Good) -> &mut u32 {
        match good {
            Good::Food => &mut self.food,
            Good::Wood => &mut self.wood,
            Good::Stone => &mut self.stone,
            Good::Tools | Good::Carts => &mut self.tools,
        }
    }

    pub fn total(&self) -> u32 {
        self.food + self.wood + self.stone + self.tools
    }

    pub fn add(&mut self, other: Goods) {
        for good in Good::GOODS {
            *self.get_mut(good) += other.get(good);
        }
    }
}

/// Something traded on a market. Carts are actors and are traded one at a time.
#[derive(Clone, Copy, Debug, Deserialize, Serialize, PartialEq, Eq, Hash)]
#[serde(rename_all = "snake_case")]
pub(super) enum Good {
    Food,
    Wood,
    Stone,
    Tools,
    Carts,
}

impl Good {
    /// The goods that are carried as [`Goods`].
    pub const GOODS: [Good; 4] = [Good::Food, Good::Wood, Good::Stone, Good::Tools];
    pub const ALL: [Good; 5] = [
        Good::Food,
        Good::Wood,
        Good::Stone,
        Good::Tools,
        Good::Carts,
    ];

    pub fn index(self) -> usize {
        self as usize
    }
}

/// What a workplace does; merchants work for markets.
#[derive(Clone, Copy, Debug, Deserialize, Serialize, PartialEq, Eq, Hash)]
#[serde(rename_all = "snake_case")]
pub(super) enum Trade {
    Farm,
    Woodcutter,
    Quarry,
    Hunter,
    Toolmaker,
    Cartwright,
    Merchant,
}

impl Trade {
    pub const WORKSHOPS: [Trade; 6] = [
        Trade::Farm,
        Trade::Woodcutter,
        Trade::Quarry,
        Trade::Hunter,
        Trade::Toolmaker,
        Trade::Cartwright,
    ];

    pub fn key(self) -> &'static str {
        match self {
            Trade::Farm => "farm",
            Trade::Woodcutter => "woodcutter",
            Trade::Quarry => "quarry",
            Trade::Hunter => "hunter",
            Trade::Toolmaker => "toolmaker",
            Trade::Cartwright => "cartwright",
            Trade::Merchant => "merchant",
        }
    }

    /// Whether workers buy inputs at the market and craft them at the workplace.
    pub fn crafts(self) -> bool {
        matches!(self, Trade::Toolmaker | Trade::Cartwright)
    }
}

/// Where a worker is in their shift.
#[derive(Clone, Copy, Debug, Deserialize, Serialize, PartialEq, Eq)]
#[serde(rename_all = "snake_case")]
pub(super) enum Phase {
    /// On the way to the workplace to start the day's shift.
    Commute,
    /// Gathering at the workplace.
    Work,
    /// A crafter buying inputs at the market.
    Fetch,
    /// A crafter turning inputs into goods at the workplace.
    Craft,
    /// Taking the day's goods to the market and settling with the employer.
    Sell,
    /// A merchant taking goods to a market elsewhere.
    Travel,
    /// A merchant bringing back what they bought.
    Return,
    Done,
}

/// A person's employment, including the employer's goods and coins the worker carries.
#[derive(Clone, Debug, Deserialize, Serialize, PartialEq)]
pub(super) struct Job {
    pub employer: EntityKey,
    pub trade: Trade,
    pub site: (usize, usize),
    pub market: EntityKey,
    pub market_site: (usize, usize),
    pub phase: Phase,
    /// The day the last shift ended.
    pub day: Option<u64>,
    pub goods: Goods,
    pub coins: u32,
    /// Carts the worker built and still has to sell.
    pub wares: Vec<u64>,
    /// The market a merchant is travelling to.
    pub destination: Option<(EntityKey, (usize, usize))>,
}

impl Job {
    pub fn new(employer: EntityKey, trade: Trade, site: (usize, usize), market: &Market) -> Self {
        Self {
            employer,
            trade,
            site,
            market: market.key(),
            market_site: market.site(),
            phase: Phase::Done,
            day: None,
            goods: Goods::default(),
            coins: 0,
            wares: Vec::new(),
            destination: None,
        }
    }

    pub fn shift_done(&self, today: u64) -> bool {
        self.phase == Phase::Done && self.day == Some(today)
    }

    pub fn on_shift(&self) -> bool {
        !matches!(self.phase, Phase::Done | Phase::Commute)
    }

    pub fn on_trip(&self) -> bool {
        matches!(self.phase, Phase::Travel | Phase::Return)
    }
}

/// Base prices and the stock each market aims to hold, per [`Good`].
#[derive(Clone, Copy, Debug, Deserialize, Serialize)]
pub(super) struct Pricing {
    pub base: [u32; 5],
    pub target: [u32; 5],
    pub rounds: u32,
}

impl Pricing {
    /// The market price: twice the base price when out of stock, the base price at the target
    /// stock, and falling towards zero as the stock grows.
    pub fn price(&self, good: Good, stock: u32) -> u32 {
        let base = u64::from(self.base[good.index()]);
        let target = u64::from(self.target[good.index()].max(1));
        let price = base * 2 * target / (u64::from(stock) + target);
        u32::try_from(price).unwrap_or(u32::MAX).max(1)
    }

    pub fn base(&self, good: Good) -> u32 {
        self.base[good.index()].max(1)
    }
}

/// One side's opening offer and the worst unit price it accepts.
#[derive(Clone, Copy, Debug, Deserialize, Serialize, PartialEq, Eq)]
pub(super) struct Stance {
    pub open: u32,
    pub limit: u32,
}

impl Stance {
    /// A seller asking half as much again as `price` and accepting no less than `limit`.
    pub fn seller(price: u32, limit: u32) -> Self {
        Self {
            open: (price + price / 2).max(limit),
            limit,
        }
    }

    /// A buyer opening at half of `price` and paying at most `limit`.
    pub fn buyer(price: u32, limit: u32) -> Self {
        Self {
            open: (price / 2).min(limit),
            limit,
        }
    }
}

/// Alternating concessions: every round, seller and buyer each give up a share of the gap
/// between their opening offer and their limit, until the bid meets the ask. They settle in
/// the middle; there is no deal when the seller's limit is above the buyer's.
pub(super) fn haggle(seller: Stance, buyer: Stance, rounds: u32) -> Option<u32> {
    if seller.limit > buyer.limit {
        return None;
    }
    let seller_open = u64::from(seller.open.max(seller.limit));
    let buyer_open = u64::from(buyer.open.min(buyer.limit));
    let (seller_limit, buyer_limit) = (u64::from(seller.limit), u64::from(buyer.limit));
    let rounds = u64::from(rounds.max(1));
    for round in 0..=rounds {
        let ask = seller_open - (seller_open - seller_limit) * round / rounds;
        let bid = buyer_open + (buyer_limit - buyer_open) * round / rounds;
        if bid >= ask {
            return u32::try_from(ask.midpoint(bid)).ok();
        }
    }
    Some(seller.limit)
}

/// The people working somewhere and what they are paid per shift.
#[derive(Clone, Debug, Default, Deserialize, Serialize)]
pub(super) struct Staff {
    pub employees: Vec<u32>,
    pub positions: usize,
    pub wage: u32,
}

impl Staff {
    fn hire(&mut self, person: u32) -> bool {
        if self.employees.contains(&person) {
            return true;
        }
        if self.employees.len() >= self.positions {
            return false;
        }
        self.employees.push(person);
        true
    }

    /// Pays `person` their wage from `coins`, as far as the coins go.
    fn pay(&self, router: &Router, coins: &mut u32, person: u32) {
        if !self.employees.contains(&person) {
            return;
        }
        let amount = self.wage.min(*coins);
        *coins -= amount;
        router.tell(
            EntityKey::person(person),
            PersonMsg::Paid {
                amount,
                owed: self.wage,
            },
        );
    }
}

#[derive(Clone, Debug, Deserialize, Serialize)]
pub(super) struct Workplace {
    pub id: u64,
    pub trade: Trade,
    pub x: usize,
    pub y: usize,
    pub market: EntityKey,
    pub market_site: (usize, usize),
    pub coins: u32,
    pub staff: Staff,
    /// Farm fields, where crops are sown and harvested.
    #[serde(default)]
    pub fields: Vec<(usize, usize)>,
}

impl Workplace {
    pub fn key(&self) -> EntityKey {
        EntityKey::new(Kind::Workplace, self.id)
    }

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

#[derive(Clone, Debug, Deserialize, Serialize)]
pub(super) enum WorkplaceMsg {
    Observe(RequestId),
    Hire(RequestId, u32),
    /// A worker leaves, handing back the coins and goods they held for the workplace.
    Resign {
        person: u32,
        coins: u32,
    },
    /// A crafter starts their shift and takes up to `budget` coins to buy inputs.
    ClockIn {
        request: RequestId,
        budget: u32,
    },
    /// A worker hands over the day's takings and gets paid.
    Settle {
        person: u32,
        coins: u32,
    },
}

pub(super) struct WorkplaceActor {
    workplace: Workplace,
    router: Arc<Router>,
}

impl ActorFactoryArgs<(Workplace, Arc<Router>)> for WorkplaceActor {
    fn create_args((workplace, router): (Workplace, Arc<Router>)) -> Self {
        Self { workplace, router }
    }
}

impl Actor for WorkplaceActor {
    type Msg = WorkplaceMsg;

    fn recv(&mut self, _ctx: &ActorContext<Self::Msg>, msg: Self::Msg, sender: Sender) {
        let workplace = &mut self.workplace;
        match msg {
            WorkplaceMsg::Observe(request) => {
                reply(sender, request, ReplyBody::Workplace(workplace.clone()));
            }
            WorkplaceMsg::Hire(request, person) => {
                let hired = workplace.staff.hire(person);
                reply(sender, request, ReplyBody::Hired(hired));
            }
            WorkplaceMsg::Resign { person, coins } => {
                workplace.coins += coins;
                workplace.staff.employees.retain(|&id| id != person);
            }
            WorkplaceMsg::ClockIn { request, budget } => {
                let advance = budget.min(workplace.coins);
                workplace.coins -= advance;
                reply(
                    sender,
                    request,
                    ReplyBody::Consigned(Goods::default(), advance),
                );
            }
            WorkplaceMsg::Settle { person, coins } => {
                workplace.coins += coins;
                workplace
                    .staff
                    .pay(&self.router, &mut workplace.coins, person);
            }
        }
    }
}

#[derive(Clone, Debug, Deserialize, Serialize)]
pub(super) struct Market {
    pub id: u64,
    pub x: usize,
    pub y: usize,
    pub coins: u32,
    pub stock: Goods,
    /// Carts for sale, parked at the market.
    pub carts: Vec<u64>,
    /// The market's merchants.
    pub staff: Staff,
}

impl Market {
    pub fn key(&self) -> EntityKey {
        EntityKey::new(Kind::Market, self.id)
    }

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

    pub fn stock_of(&self, good: Good) -> u32 {
        match good {
            Good::Carts => u32::try_from(self.carts.len()).unwrap_or(u32::MAX),
            good => self.stock.get(good),
        }
    }

    pub fn price(&self, pricing: &Pricing, good: Good) -> u32 {
        pricing.price(good, self.stock_of(good))
    }

    /// The market's stance when it buys: it opens low and pays up to 90% of its price.
    fn bid(&self, pricing: &Pricing, good: Good) -> Stance {
        let price = self.price(pricing, good);
        Stance::buyer(price, (price * 9 / 10).max(1))
    }

    /// The market's stance when it sells: it opens high and asks at least 110% of its price.
    fn ask(&self, pricing: &Pricing, good: Good) -> Stance {
        let price = self.price(pricing, good);
        Stance::seller(price, (price * 11 / 10).max(1))
    }

    pub fn quote(&self, pricing: &Pricing) -> Quote {
        Quote {
            market: self.key(),
            site: self.site(),
            prices: Good::ALL.map(|good| self.price(pricing, good)),
        }
    }

    /// Buys up to `quantity` of `good` from a seller with `stance`.
    fn buy(&mut self, pricing: &Pricing, good: Good, quantity: u32, stance: Stance) -> Traded {
        let Some(price) = haggle(stance, self.bid(pricing, good), pricing.rounds) else {
            return Traded::NONE;
        };
        let quantity = quantity.min(self.coins / price.max(1));
        self.coins -= quantity * price;
        *self.stock.get_mut(good) += quantity;
        Traded { quantity, price }
    }

    /// Sells up to `quantity` of `good` to a buyer with `stance` who has `budget` coins.
    fn sell(
        &mut self,
        pricing: &Pricing,
        good: Good,
        quantity: u32,
        stance: Stance,
        budget: u32,
    ) -> Traded {
        let Some(price) = haggle(self.ask(pricing, good), stance, pricing.rounds) else {
            return Traded::NONE;
        };
        let quantity = quantity
            .min(self.stock.get(good))
            .min(budget / price.max(1));
        self.coins += quantity * price;
        *self.stock.get_mut(good) -= quantity;
        Traded { quantity, price }
    }

    /// Hands a merchant the good the market holds the most surplus of, and coins to trade with.
    fn consign(&mut self, pricing: &Pricing, capacity: u32, escrow: u32) -> (Goods, u32) {
        let stock = self.stock;
        let surplus = |good: Good| stock.get(good).saturating_sub(pricing.target[good.index()]);
        let goods = Good::GOODS
            .into_iter()
            .max_by_key(|&good| surplus(good))
            .filter(|&good| surplus(good) > 0)
            .map(|good| {
                let quantity = surplus(good).min(capacity);
                *self.stock.get_mut(good) -= quantity;
                Goods::of(good, quantity)
            })
            .unwrap_or_default();
        let coins = escrow.min(self.coins);
        self.coins -= coins;
        (goods, coins)
    }
}

/// The outcome of a trade: how much changed hands at which unit price.
#[derive(Clone, Copy, Debug, Default, Deserialize, Serialize, PartialEq, Eq)]
pub(super) struct Traded {
    pub quantity: u32,
    pub price: u32,
}

impl Traded {
    const NONE: Traded = Traded {
        quantity: 0,
        price: 0,
    };

    pub fn total(self) -> u32 {
        self.quantity * self.price
    }
}

/// Market prices as published to every shard, so that merchants can pick where to go.
#[derive(Clone, Debug, Deserialize, Serialize, PartialEq)]
pub struct Quote {
    pub market: EntityKey,
    pub site: (usize, usize),
    pub prices: [u32; 5],
}

#[derive(Clone, Debug, Deserialize, Serialize)]
pub(super) enum MarketMsg {
    Observe(RequestId),
    /// Someone with `stance` sells the market up to `quantity` of `good`.
    Sell {
        request: RequestId,
        good: Good,
        quantity: u32,
        stance: Stance,
        pricing: Pricing,
    },
    /// Someone with `stance` and `budget` coins buys up to `quantity` of `good`.
    Buy {
        request: RequestId,
        good: Good,
        quantity: u32,
        stance: Stance,
        budget: u32,
        pricing: Pricing,
    },
    /// A cartwright sells the market a cart.
    SellCart {
        request: RequestId,
        cart: u64,
        stance: Stance,
        pricing: Pricing,
    },
    /// Someone buys one of the carts for sale.
    BuyCart {
        request: RequestId,
        stance: Stance,
        budget: u32,
        pricing: Pricing,
    },
    Hire(RequestId, u32),
    Resign {
        person: u32,
        goods: Goods,
        coins: u32,
    },
    /// A merchant starts a trip and takes goods and up to `escrow` coins.
    ClockIn {
        request: RequestId,
        capacity: u32,
        escrow: u32,
        pricing: Pricing,
    },
    /// Goods handed over without payment, such as a leaving worker's unsold takings.
    Stock(Goods),
    /// A merchant hands over what they brought back and gets paid.
    Settle {
        person: u32,
        goods: Goods,
        coins: u32,
    },
}

pub(super) struct MarketActor {
    market: Market,
    router: Arc<Router>,
}

impl ActorFactoryArgs<(Market, Arc<Router>)> for MarketActor {
    fn create_args((market, router): (Market, Arc<Router>)) -> Self {
        Self { market, router }
    }
}

impl Actor for MarketActor {
    type Msg = MarketMsg;

    fn recv(&mut self, _ctx: &ActorContext<Self::Msg>, msg: Self::Msg, sender: Sender) {
        let market = &mut self.market;
        match msg {
            MarketMsg::Observe(request) => {
                reply(sender, request, ReplyBody::Market(market.clone()));
            }
            MarketMsg::Sell {
                request,
                good,
                quantity,
                stance,
                pricing,
            } => {
                let traded = market.buy(&pricing, good, quantity, stance);
                reply(sender, request, ReplyBody::Traded(traded));
            }
            MarketMsg::Buy {
                request,
                good,
                quantity,
                stance,
                budget,
                pricing,
            } => {
                let traded = market.sell(&pricing, good, quantity, stance, budget);
                reply(sender, request, ReplyBody::Traded(traded));
            }
            MarketMsg::SellCart {
                request,
                cart,
                stance,
                pricing,
            } => {
                let bid = market.bid(&pricing, Good::Carts);
                let traded = haggle(stance, bid, pricing.rounds)
                    .filter(|&price| price <= market.coins)
                    .map_or(Traded::NONE, |price| {
                        market.coins -= price;
                        market.carts.push(cart);
                        Traded { quantity: 1, price }
                    });
                reply(sender, request, ReplyBody::Traded(traded));
            }
            MarketMsg::BuyCart {
                request,
                stance,
                budget,
                pricing,
            } => {
                let ask = market.ask(&pricing, Good::Carts);
                let bought = haggle(ask, stance, pricing.rounds)
                    .filter(|&price| price <= budget && !market.carts.is_empty())
                    .map(|price| {
                        market.coins += price;
                        (market.carts.remove(0), price)
                    });
                reply(sender, request, ReplyBody::CartBought(bought));
            }
            MarketMsg::Hire(request, person) => {
                let hired = market.staff.hire(person);
                reply(sender, request, ReplyBody::Hired(hired));
            }
            MarketMsg::Resign {
                person,
                goods,
                coins,
            } => {
                market.stock.add(goods);
                market.coins += coins;
                market.staff.employees.retain(|&id| id != person);
            }
            MarketMsg::ClockIn {
                request,
                capacity,
                escrow,
                pricing,
            } => {
                let (goods, coins) = market.consign(&pricing, capacity, escrow);
                reply(sender, request, ReplyBody::Consigned(goods, coins));
            }
            MarketMsg::Stock(goods) => market.stock.add(goods),
            MarketMsg::Settle {
                person,
                goods,
                coins,
            } => {
                market.stock.add(goods);
                market.coins += coins;
                market.staff.pay(&self.router, &mut market.coins, person);
            }
        }
    }
}

#[derive(Clone, Debug, Deserialize, Serialize)]
pub(super) struct Cart {
    pub id: u64,
    pub x: usize,
    pub y: usize,
    pub owner: Option<u32>,
    /// Tiles travelled so far; the cart breaks once it has travelled its durability.
    pub wear: u32,
    pub broken: bool,
}

impl Cart {
    pub fn key(&self) -> EntityKey {
        EntityKey::new(Kind::Cart, self.id)
    }

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

#[derive(Clone, Debug, Deserialize, Serialize)]
pub(super) enum CartMsg {
    Observe(RequestId),
    /// Follows the owner to `to`, wearing out on the way.
    Follow {
        request: RequestId,
        to: (usize, usize),
        durability: u32,
    },
    SetOwner(Option<u32>),
    MoveTo((usize, usize)),
    Depart(RequestId, usize),
    Readmit(Cart),
}

pub(super) struct CartActor {
    cart: Cart,
    router: Arc<Router>,
    forward_to: Option<usize>,
}

impl ActorFactoryArgs<(Cart, Arc<Router>)> for CartActor {
    fn create_args((cart, router): (Cart, Arc<Router>)) -> Self {
        Self {
            cart,
            router,
            forward_to: None,
        }
    }
}

impl Actor for CartActor {
    type Msg = CartMsg;

    fn recv(&mut self, _ctx: &ActorContext<Self::Msg>, msg: Self::Msg, sender: Sender) {
        if let Some(shard) = self.forward_to {
            if let CartMsg::Readmit(cart) = msg {
                self.cart = cart;
                self.forward_to = None;
            } else {
                self.router.forward(shard, self.cart.key(), &msg);
            }
            return;
        }
        let cart = &mut self.cart;
        match msg {
            CartMsg::Observe(request) => reply(sender, request, ReplyBody::Cart(cart.clone())),
            CartMsg::Follow {
                request,
                to,
                durability,
            } => {
                let distance = cart.x.abs_diff(to.0).max(cart.y.abs_diff(to.1));
                cart.wear = cart
                    .wear
                    .saturating_add(u32::try_from(distance).unwrap_or(u32::MAX));
                (cart.x, cart.y) = to;
                cart.broken |= cart.wear >= durability;
                reply(sender, request, ReplyBody::Cart(cart.clone()));
            }
            CartMsg::SetOwner(owner) => cart.owner = owner,
            CartMsg::MoveTo(position) => (cart.x, cart.y) = position,
            CartMsg::Depart(request, shard) => {
                self.router.admit_at(shard, Admission::Cart(cart.clone()));
                self.forward_to = Some(shard);
                reply(sender, request, ReplyBody::Departed);
            }
            CartMsg::Readmit(readmitted) => *cart = readmitted,
        }
    }
}

#[cfg(test)]
mod tests {
    use super::{Good, Goods, Market, Pricing, Staff, Stance, haggle};

    fn pricing() -> Pricing {
        Pricing {
            base: [4, 2, 3, 14, 40],
            target: [60, 40, 30, 8, 2],
            rounds: 4,
        }
    }

    #[test]
    fn haggling_meets_between_the_limits_or_fails() {
        let seller = Stance { open: 12, limit: 4 };
        let buyer = Stance { open: 2, limit: 8 };
        let price = haggle(seller, buyer, 4).unwrap();
        assert!((4..=8).contains(&price));
        assert_eq!(haggle(Stance { open: 12, limit: 9 }, buyer, 4), None);
        // A stubborn seller pulls the price up.
        let stubborn = haggle(Stance { open: 30, limit: 4 }, buyer, 4).unwrap();
        assert!(stubborn >= price);
    }

    #[test]
    fn prices_follow_supply() {
        let pricing = pricing();
        assert_eq!(pricing.price(Good::Food, 0), 8);
        assert_eq!(pricing.price(Good::Food, 60), 4);
        assert!(pricing.price(Good::Food, 600) < 4);
        assert!(pricing.price(Good::Food, 1_000_000) >= 1);
    }

    #[test]
    fn markets_trade_within_their_coins_and_stock() {
        let pricing = pricing();
        let mut market = Market {
            id: 1,
            x: 0,
            y: 0,
            coins: 10,
            stock: Goods::default(),
            carts: Vec::new(),
            staff: Staff::default(),
        };
        let sold = market.buy(&pricing, Good::Food, 100, Stance::seller(8, 1));
        assert!(sold.quantity > 0 && sold.total() <= 10);
        assert_eq!(market.coins + sold.total(), 10);
        assert_eq!(market.stock.food, sold.quantity);
        let bought = market.sell(&pricing, Good::Food, 100, Stance::buyer(4, 100), 1000);
        assert_eq!(bought.quantity, sold.quantity);
        assert_eq!(market.stock.food, 0);
        assert!(
            bought.price >= sold.price,
            "the market buys low and sells high"
        );
    }
}