mod econ;

use std::{
    ops::Range,
    hash::Hash,
    fmt,
};
use hashbrown::{HashMap, HashSet};
use vek::*;
use rand::prelude::*;
use common::{
    terrain::TerrainChunkSize,
    vol::RectVolSize,
    store::{Id, Store},
    path::Path,
    astar::Astar,
};
use crate::sim::{WorldSim, SimChunk};

const CARDINALS: [Vec2<i32>; 4] = [
    Vec2::new(1, 0),
    Vec2::new(-1, 0),
    Vec2::new(0, 1),
    Vec2::new(0, -1),
];

const DIAGONALS: [Vec2<i32>; 8] = [
    Vec2::new(1, 0),
    Vec2::new(1, 1),
    Vec2::new(-1, 0),
    Vec2::new(-1, 1),
    Vec2::new(0, 1),
    Vec2::new(1, -1),
    Vec2::new(0, -1),
    Vec2::new(-1, -1),
];

fn attempt<T>(max_iters: usize, mut f: impl FnMut() -> Option<T>) -> Option<T> {
    (0..max_iters).find_map(|_| f())
}

const INITIAL_CIV_COUNT: usize = 16;

#[derive(Default)]
pub struct Civs {
    civs: Store<Civ>,
    places: Store<Place>,

    tracks: Store<Track>,
    track_map: HashMap<Id<Site>, HashMap<Id<Site>, Id<Track>>>,

    sites: Store<Site>,
}

pub struct GenCtx<'a, R: Rng> {
    sim: &'a mut WorldSim,
    rng: &'a mut R,
}

impl Civs {
    pub fn generate(seed: u32, sim: &mut WorldSim) -> Self {
        let mut this = Self::default();
        let mut rng = sim.rng.clone();
        let mut ctx = GenCtx { sim, rng: &mut rng };

        for _ in 0..INITIAL_CIV_COUNT {
            println!("Creating civilisation...");
            if let None = this.birth_civ(&mut ctx) {
                println!("Failed to find starting site for civilisation.");
            }
        }

        // Tick
        const SIM_YEARS: usize = 1000;
        for _ in 0..SIM_YEARS {
            this.tick(&mut ctx, 1.0);
        }

        // Temporary!
        for track in this.tracks.iter() {
            for loc in track.path.iter() {
                sim.get_mut(*loc).unwrap().place = Some(this.civs.iter().next().unwrap().homeland);
            }
        }

        this.display_info();

        this
    }

    pub fn place(&self, id: Id<Place>) -> &Place { self.places.get(id) }

    pub fn sites(&self) -> impl Iterator<Item=&Site> + '_ {
        self.sites.iter()
    }

    fn display_info(&self) {
        for (id, civ) in self.civs.iter_ids() {
            println!("# Civilisation {:?}", id);
            println!("Name: {}", "<unnamed>");
            println!("Homeland: {:#?}", self.places.get(civ.homeland));
        }

        for (id, site) in self.sites.iter_ids() {
            println!("# Site {:?}", id);
            println!("{:#?}", site);
        }
    }

    /// Return the direct track between two places
    fn track_between(&self, a: Id<Site>, b: Id<Site>) -> Option<Id<Track>> {
        self.track_map
            .get(&a)
            .and_then(|dests| dests.get(&b))
            .or_else(|| self.track_map
                .get(&b)
                .and_then(|dests| dests.get(&a)))
            .copied()
    }

    /// Return an iterator over a site's neighbors
    fn neighbors(&self, site: Id<Site>) -> impl Iterator<Item=Id<Site>> + '_ {
        let to = self.track_map.get(&site).map(|dests| dests.keys()).into_iter().flatten();
        let fro = self.track_map.iter().filter(move |(_, dests)| dests.contains_key(&site)).map(|(p, _)| p);
        to.chain(fro).filter(move |p| **p != site).copied()
    }

    /// Find the cheapest route between two places
    fn route_between(&self, a: Id<Site>, b: Id<Site>) -> Option<(Path<Id<Site>>, f32)> {
        let heuristic = move |p: &Id<Site>| (self.sites.get(*p).center.distance_squared(self.sites.get(b).center) as f32).sqrt();
        let neighbors = |p: &Id<Site>| self.neighbors(*p);
        let transition = |a: &Id<Site>, b: &Id<Site>| self.tracks.get(self.track_between(*a, *b).unwrap()).cost;
        let satisfied = |p: &Id<Site>| *p == b;
        let mut astar = Astar::new(100, a, heuristic);
        astar
            .poll(100, heuristic, neighbors, transition, satisfied)
            .into_path()
            .and_then(|path| astar.get_cheapest_cost().map(|cost| (path, cost)))
    }

    fn birth_civ(&mut self, ctx: &mut GenCtx<impl Rng>) -> Option<Id<Civ>> {
        let site = attempt(5, || {
            let loc = find_site_loc(ctx, None)?;
            self.establish_site(ctx, loc)
        })?;

        let civ = self.civs.insert(Civ {
            capital: site,
            homeland: self.sites.get(site).place,
        });

        Some(civ)
    }

    fn establish_place(&mut self, ctx: &mut GenCtx<impl Rng>, loc: Vec2<i32>, area: Range<usize>) -> Option<Id<Place>> {
        let mut dead = HashSet::new();
        let mut alive = HashSet::new();
        alive.insert(loc);

        // Fill the surrounding area
        while let Some(cloc) = alive.iter().choose(ctx.rng).copied() {
            for dir in CARDINALS.iter() {
                if site_in_dir(&ctx.sim, cloc, *dir) {
                    let rloc = cloc + *dir;
                    if !dead.contains(&rloc) && ctx.sim.get(rloc).map(|c| c.place.is_none()).unwrap_or(false) {
                        alive.insert(rloc);
                    }
                }
            }
            alive.remove(&cloc);
            dead.insert(cloc);

            if dead.len() + alive.len() >= area.end {
                break;
            }
        }
        // Make sure the place is large enough
        if dead.len() + alive.len() <= area.start {
            return None;
        }

        let place = self.places.insert(Place {
            center: loc,
            nat_res: NaturalResources::default(),
        });

        // Write place to map
        for cell in dead.union(&alive) {
            if let Some(chunk) = ctx.sim.get_mut(*cell) {
                chunk.place = Some(place);
                self.places.get_mut(place).nat_res.include_chunk(ctx, *cell);
            }
        }

        Some(place)
    }

    fn establish_site(&mut self, ctx: &mut GenCtx<impl Rng>, loc: Vec2<i32>) -> Option<Id<Site>> {
        const SITE_AREA: Range<usize> = 64..256;

        let place = match ctx.sim.get(loc).and_then(|site| site.place) {
            Some(place) => place,
            None => self.establish_place(ctx, loc, SITE_AREA)?,
        };

        let site = self.sites.insert(Site {
            kind: SiteKind::Settlement,
            center: loc,
            place: place,

            population: 24.0,

            stocks: Stocks::from_default(100.0),
            values: Stocks::from_default(None),

            labors: MapVec::from_default(0.01),
            yields: MapVec::from_default(1.0),

            //trade_states: Stocks::default(),
            //coin: 1000.0,
        });

        // Find neighbors
        const MAX_NEIGHBOR_DISTANCE: f32 = 250.0;
        let mut nearby = self.sites
            .iter_ids()
            .map(|(id, p)| (id, (p.center.distance_squared(loc) as f32).sqrt()))
            .filter(|(p, dist)| *dist < MAX_NEIGHBOR_DISTANCE)
            .collect::<Vec<_>>();
        nearby.sort_by_key(|(_, dist)| *dist as i32);

        for (nearby, _) in nearby.into_iter().take(5) {
            // Find a novel path
            if let Some((path, cost)) = find_path(ctx, loc, self.sites.get(nearby).center) {
                // Find a path using existing paths
                if self
                    .route_between(site, nearby)
                    // If the novel path isn't efficient compared to existing routes, don't use it
                    .filter(|(_, route_cost)| *route_cost < cost * 3.0)
                    .is_none()
                {
                    let track = self.tracks.insert(Track {
                        cost,
                        path,
                    });
                    self.track_map
                        .entry(site)
                        .or_default()
                        .insert(nearby, track);
                }
            }
        }

        Some(site)
    }

    fn tick(&mut self, ctx: &mut GenCtx<impl Rng>, years: f32) {
        println!("Tick!");
        for site in self.sites.iter_mut() {
            site.simulate(years, &self.places.get(site.place).nat_res);
        }

        // Trade stocks
        // let mut stocks = [FOOD, WOOD, ROCK];
        // stocks.shuffle(ctx.rng); // Give each stock a chance to be traded first
        // for stock in stocks.iter().copied() {
        //     let mut sell_orders = self.sites
        //         .iter_ids()
        //         .map(|(id, site)| (id, econ::SellOrder {
        //             quantity: site.trade_states[stock].surplus.min(site.stocks[stock]),
        //             price: site.trade_states[stock].sell_belief.choose_price(ctx) * 1.2, // Transport cost of 1.2x
        //             q_sold: 0.0,
        //         }))
        //         .filter(|(_, order)| order.quantity > 0.0)
        //         .collect::<Vec<_>>();

        //     let mut sites = self.sites
        //         .ids()
        //         .collect::<Vec<_>>();
        //     sites.shuffle(ctx.rng); // Give all sites a chance to buy first
        //     for site in sites {
        //         let (max_spend, max_price) = {
        //             let site = self.sites.get(site);
        //             let budget = site.coin * 0.5;
        //             (
        //                 (site.trade_states[stock].purchase_priority * budget).min(budget),
        //                 site.trade_states[stock].buy_belief.price,
        //             )
        //         };
        //         let (quantity, spent) = econ::buy_units(ctx, sell_orders
        //             .iter_mut()
        //             .filter(|(id, _)| site != *id && self.track_between(site, *id).is_some())
        //             .map(|(_, order)| order),
        //             1000000.0, // Max quantity TODO
        //             1000000.0, // Max price TODO
        //             max_spend,
        //         );
        //         let mut site = self.sites.get_mut(site);
        //         site.coin -= spent;
        //         if quantity > 0.0 {
        //             site.stocks[stock] += quantity;
        //             site.trade_states[stock].buy_belief.update_buyer(years, spent / quantity);
        //             println!("Belief: {:?}", site.trade_states[stock].buy_belief);
        //         }
        //     }

        //     for (site, order) in sell_orders {
        //         let mut site = self.sites.get_mut(site);
        //         site.coin += order.q_sold * order.price;
        //         if order.q_sold > 0.0 {
        //             site.stocks[stock] -= order.q_sold;
        //             site.trade_states[stock].sell_belief.update_seller(order.q_sold / order.quantity);
        //         }
        //     }
        // }

        // Consume stocks
        //for site in self.sites.iter_mut() {
        //    site.consume_stocks(years);
        //}
    }
}

/// Attempt to find a path between two locations
fn find_path(ctx: &mut GenCtx<impl Rng>, a: Vec2<i32>, b: Vec2<i32>) -> Option<(Path<Vec2<i32>>, f32)> {
    let sim = &ctx.sim;
    let heuristic = move |l: &Vec2<i32>| (l.distance_squared(b) as f32).sqrt();
    let neighbors = |l: &Vec2<i32>| {
        let l = *l;
        DIAGONALS.iter().filter(move |dir| walk_in_dir(sim, l, **dir).is_some()).map(move |dir| l + *dir)
    };
    let transition = |a: &Vec2<i32>, b: &Vec2<i32>| 1.0 + walk_in_dir(sim, *a, *b - *a).unwrap_or(10000.0);
    let satisfied = |l: &Vec2<i32>| *l == b;
    let mut astar = Astar::new(20000, a, heuristic);
    astar
        .poll(20000, heuristic, neighbors, transition, satisfied)
        .into_path()
        .and_then(|path| astar.get_cheapest_cost().map(|cost| (path, cost)))
}

/// Return true if travel between a location and a chunk next to it is permitted (TODO: by whom?)
fn walk_in_dir(sim: &WorldSim, a: Vec2<i32>, dir: Vec2<i32>) -> Option<f32> {
    if loc_suitable_for_walking(sim, a) &&
        loc_suitable_for_walking(sim, a + dir)
    {
        let a_alt = sim.get(a)?.alt;
        let b_alt = sim.get(a + dir)?.alt;
        Some((b_alt - a_alt).abs() / 2.5)
    } else {
        None
    }
}

/// Return true if a position is suitable for walking on
fn loc_suitable_for_walking(sim: &WorldSim, loc: Vec2<i32>) -> bool {
    if let Some(chunk) = sim.get(loc) {
        !chunk.river.is_ocean() && !chunk.river.is_lake()
    } else {
        false
    }
}

/// Return true if a site could be constructed between a location and a chunk next to it is permitted (TODO: by whom?)
fn site_in_dir(sim: &WorldSim, a: Vec2<i32>, dir: Vec2<i32>) -> bool {
    loc_suitable_for_site(sim, a) &&
    loc_suitable_for_site(sim, a + dir)
}

/// Return true if a position is suitable for site construction (TODO: criteria?)
fn loc_suitable_for_site(sim: &WorldSim, loc: Vec2<i32>) -> bool {
    if let Some(chunk) = sim.get(loc) {
        !chunk.river.is_ocean() &&
        !chunk.river.is_lake() &&
        sim.get_gradient_approx(loc).map(|grad| grad < 1.0).unwrap_or(false)
    } else {
        false
    }
}

/// Attempt to search for a location that's suitable for site construction
fn find_site_loc(ctx: &mut GenCtx<impl Rng>, near: Option<(Vec2<i32>, f32)>) -> Option<Vec2<i32>> {
    const MAX_ATTEMPTS: usize = 100;
    let mut loc = None;
    for _ in 0..MAX_ATTEMPTS {
        let test_loc = loc.unwrap_or_else(|| match near {
            Some((origin, dist)) => origin + (Vec2::new(
                ctx.rng.gen_range(-1.0, 1.0),
                ctx.rng.gen_range(-1.0, 1.0),
            ).try_normalized().unwrap_or(Vec2::zero()) * ctx.rng.gen::<f32>() * dist).map(|e| e as i32),
            None => Vec2::new(
                ctx.rng.gen_range(0, ctx.sim.get_size().x as i32),
                ctx.rng.gen_range(0, ctx.sim.get_size().y as i32),
            ),
        });

        if loc_suitable_for_site(&ctx.sim, test_loc) {
            return Some(test_loc);
        }

        loc = ctx.sim.get(test_loc).and_then(|c| Some(c.downhill?.map2(Vec2::from(TerrainChunkSize::RECT_SIZE), |e, sz: u32| {
            e / (sz as i32)
        })));
    }
    None
}

#[derive(Debug)]
pub struct Civ {
    capital: Id<Site>,
    homeland: Id<Place>,
}

#[derive(Debug)]
pub struct Place {
    center: Vec2<i32>,
    nat_res: NaturalResources,
}

// Productive capacity per year
#[derive(Default, Debug)]
pub struct NaturalResources {
    wood: f32,
    rock: f32,
    river: f32,
    farmland: f32,
}

impl NaturalResources {
    fn include_chunk(&mut self, ctx: &mut GenCtx<impl Rng>, loc: Vec2<i32>) {
        let chunk = if let Some(chunk) = ctx.sim.get(loc) { chunk } else { return };

        self.wood += chunk.tree_density;
        self.rock += chunk.rockiness;
        self.river += if chunk.river.is_river() { 5.0 } else { 0.0 };
        self.farmland += if
            chunk.humidity > 0.35 &&
            chunk.temp > -0.3 && chunk.temp < 0.75 &&
            chunk.chaos < 0.5 &&
            ctx.sim.get_gradient_approx(loc).map(|grad| grad < 0.7).unwrap_or(false)
        { 1.0 } else { 0.0 };
    }
}

pub struct Track {
    /// Cost of using this track relative to other paths. This cost is an arbitrary unit and
    /// doesn't make sense unless compared to other track costs.
    cost: f32,
    path: Path<Vec2<i32>>,
}

#[derive(Debug)]
pub struct Site {
    kind: SiteKind,
    center: Vec2<i32>,
    pub place: Id<Place>,

    population: f32,

    // Total amount of each stock
    stocks: Stocks<f32>,
    // For some goods, such a goods without any supply, it doesn't make sense to talk about value
    values: Stocks<Option<f32>>,

    // Proportion of individuals dedicated to an industry
    labors: MapVec<Occupation, f32>,
    // Per worker, per year, of their output good
    yields: MapVec<Occupation, f32>,

    //trade_states: Stocks<TradeState>,
    //coin: f32,
}

impl fmt::Display for Site {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        match self.kind {
            SiteKind::Settlement => writeln!(f, "Settlement")?,
        }
        writeln!(f, "- population: {}", self.population.floor() as u32)?;
        writeln!(f, "Stocks")?;
        for (stock, q) in self.stocks.iter() {
            writeln!(f, "- {}: {}", stock, q.floor())?;
        }
        writeln!(f, "Prices")?;
        for (stock, v) in self.values.iter() {
            writeln!(f, "- {}: {}", stock, v.map(|x| x.to_string()).unwrap_or_else(|| "N/A".to_string()))?;
        }
        writeln!(f, "Laborers")?;
        for (labor, n) in self.labors.iter() {
            writeln!(f, "- {}: {}", labor, (*n * self.population).floor() as u32)?;
        }

        Ok(())
    }
}

#[derive(Debug)]
pub enum SiteKind {
    Settlement,
}

impl Site {
    pub fn simulate(&mut self, years: f32, nat_res: &NaturalResources) {
        // Insert natural resources into the economy
        if self.stocks[FISH] < nat_res.river {
            self.stocks[FISH] = nat_res.river;
        }
        if self.stocks[WHEAT] < nat_res.farmland {
            self.stocks[WHEAT] = nat_res.farmland;
        }
        if self.stocks[LOGS] < nat_res.wood {
            self.stocks[LOGS] = nat_res.wood;
        }
        if self.stocks[GAME] < nat_res.wood {
            self.stocks[GAME] = nat_res.wood;
        }
        if self.stocks[ROCK] < nat_res.rock {
            self.stocks[ROCK] = nat_res.rock;
        }

        let orders = vec![
            (None, vec![(FOOD, 0.25)]),
            (Some(COOK), vec![(FLOUR, 6.5), (MEAT, 1.5)]),
            (Some(LUMBERJACK), vec![(LOGS, 4.5)]),
            (Some(MINER), vec![(ROCK, 7.5)]),
            (Some(FISHER), vec![(FISH, 4.0)]),
            (Some(HUNTER), vec![(GAME, 4.0)]),
            (Some(FARMER), vec![(WHEAT, 4.0)]),
        ]
            .into_iter()
            .collect::<HashMap<_, Vec<(Stock, f32)>>>();

        let mut demand = Stocks::from_default(0.0);
        for (labor, orders) in &orders {
            let scale = if let Some(labor) = labor { self.labors[*labor] } else { 1.0 } * self.population;
            for (stock, amount) in orders {
                debug_assert!(!amount.is_nan(), "{:?}, {}", labor, stock);
                debug_assert!(!scale.is_nan(), "{:?}, {}, {}", labor, stock, self.population);
                demand[*stock] += *amount * scale;
            }
        }

        let surplus = demand.clone().map(|stock, tgt| {
            debug_assert!(!self.stocks[stock].is_nan());
            debug_assert!(!demand[stock].is_nan());
            self.stocks[stock] - demand[stock]
        });

        // Update values according to the surplus of each stock
        surplus.iter().for_each(|(stock, surplus)| {
            let val = 2.5f32.powf(-*surplus / demand[stock]);
            self.values[stock] = if val > 0.01 && val < 10000.0 { Some(val) } else { None };
        });

        // Per labourer, per year
        let production = Stocks::from_list(&[
            (FARMER, (FLOUR, 2.0)),
            (LUMBERJACK, (WOOD, 1.5)),
            (MINER, (STONE, 0.6)),
            (FISHER, (MEAT, 3.0)),
            (HUNTER, (MEAT, 0.5)),
            (COOK, (FOOD, 8.0)),
        ]);

        let population = self.population;

        // Redistribute workforce according to relative good values
        let labor_ratios = production.clone().map(|labor, (output_stock, _)| {
            debug_assert!(self.values[output_stock].unwrap_or(0.0) < 1000000.0, "{:?}", self.values[output_stock]);
            debug_assert!(self.yields[labor] < 1000000.0, "{}", self.yields[labor]);
            self.values[output_stock].unwrap_or(0.0) * self.yields[labor]
        });
        let labor_ratio_sum = labor_ratios.iter().map(|(_, r)| *r).sum::<f32>().max(0.01);
        assert!(labor_ratio_sum > 0.0);
        production.iter().for_each(|(labor, _)| {
            debug_assert!(!labor_ratios[labor].is_nan() && !labor_ratios[labor].is_infinite(), "{:?}, {}", labor, labor_ratios[labor]);
            debug_assert!(!labor_ratio_sum.is_nan() && !labor_ratio_sum.is_infinite(), "{:?}, {}", labor, labor_ratio_sum);
            let smooth = 0.5;
            self.labors[labor] = smooth * self.labors[labor] + (1.0 - smooth) * (labor_ratios[labor].max(labor_ratio_sum / 1000.0) / labor_ratio_sum);
        });

        // Production
        let stocks_before = self.stocks.clone();
        self.stocks = Stocks::from_default(0.0);
        for (labor, orders) in orders.iter() {
            let scale = if let Some(labor) = labor { self.labors[*labor] } else { 1.0 } * population;

            // For each order, we try to find the minimum satisfaction rate - this limits how much
            // we can produce! For example, if we need 0.25 fish and 0.75 oats to make 1 unit of
            // food, but only 0.5 units of oats are available then we only need to consume 2/3rds
            // of other ingredients and leave the rest in stock
            let min_satisfaction = orders
                .iter()
                .map(|(stock, amount)| {
                    // What quantity is this order requesting?
                    let quantity = *amount * scale;
                    // What proportion of this order is the economy able to satisfy?
                    let satisfaction = (stocks_before[*stock] / demand[*stock]).min(1.0);
                    satisfaction
                })
                .min_by(|a, b| a.partial_cmp(b).unwrap())
                .unwrap_or_else(|| panic!("Industry {:?} requires at least one input order", labor));

            for (stock, amount) in orders {
                // What quantity is this order requesting?
                let quantity = *amount * scale;
                // What amount gets actually used in production?
                let used = quantity * min_satisfaction;

                // Deplete stocks accordingly
                //self.stocks[*stock] = (self.stocks[*stock] - used).max(0.0);
            }

            // Industries produce things
            if let Some(labor) = labor {
                let (stock, rate) = production[*labor];
                let yield_per_worker = min_satisfaction * rate;
                self.yields[*labor] = yield_per_worker;
                self.stocks[stock] += yield_per_worker * self.labors[*labor] * population;
            }
        }

        // Births/deaths
        const NATURAL_BIRTH_RATE: f32 = 0.15;
        const DEATH_RATE: f32 = 0.05;
        debug_assert!(!surplus[FOOD].is_nan());
        debug_assert!(!surplus[FOOD].is_infinite());
        let birth_rate = if surplus[FOOD] > 0.0 { NATURAL_BIRTH_RATE } else { 0.0 };
        self.population += years * self.population * (birth_rate - DEATH_RATE);
    }
}

type Occupation = &'static str;
const FARMER: Occupation = "farmer";
const LUMBERJACK: Occupation = "lumberjack";
const MINER: Occupation = "miner";
const FISHER: Occupation = "fisher";
const HUNTER: Occupation = "hunter";
const COOK: Occupation = "cook";

type Stock = &'static str;
const WHEAT: Stock = "wheat";
const FLOUR: Stock = "flour";
const MEAT: Stock = "meat";
const FISH: Stock = "fish";
const GAME: Stock = "game";
const FOOD: Stock = "food";
const LOGS: Stock = "logs";
const WOOD: Stock = "wood";
const ROCK: Stock = "rock";
const STONE: Stock = "stone";

#[derive(Debug, Clone)]
struct TradeState {
    buy_belief: econ::Belief,
    sell_belief: econ::Belief,
    /// The price/value assigned to the stock by the host settlement
    domestic_value: f32,
    surplus: f32,
    purchase_priority: f32,
}

impl Default for TradeState {
    fn default() -> Self {
        Self {
            buy_belief: econ::Belief {
                price: 1.0,
                confidence: 0.25,
            },
            sell_belief: econ::Belief {
                price: 1.0,
                confidence: 0.25,
            },
            domestic_value: 1.0,
            surplus: 0.0,
            purchase_priority: 1.0,
        }
    }
}

pub type Stocks<T> = MapVec<Stock, T>;

#[derive(Default, Clone, Debug)]
pub struct MapVec<K, T> {
    entries: HashMap<K, T>,
    default: T,
}

impl<K: Copy + Eq + Hash, T: Default + Clone> MapVec<K, T> {
    pub fn from_list<'a>(i: impl IntoIterator<Item=&'a (K, T)>) -> Self
        where K: 'a, T: 'a
    {
        Self {
            entries: i.into_iter().cloned().collect(),
            default: T::default(),
        }
    }

    pub fn from_default(default: T) -> Self {
        Self {
            entries: HashMap::default(),
            default,
        }
    }

    pub fn get_mut(&mut self, entry: K) -> &mut T {
        let default = &self.default;
        self
            .entries
            .entry(entry)
            .or_insert_with(|| default.clone())
    }

    pub fn get(&self, entry: K) -> &T {
        self.entries.get(&entry).unwrap_or(&self.default)
    }

    pub fn map<U: Default>(mut self, mut f: impl FnMut(K, T) -> U) -> MapVec<K, U> {
        MapVec {
            entries: self.entries.into_iter().map(|(s, v)| (s.clone(), f(s, v))).collect(),
            default: U::default(),
        }
    }

    pub fn iter(&self) -> impl Iterator<Item=(K, &T)> + '_ {
        self.entries.iter().map(|(s, v)| (*s, v))
    }

    pub fn iter_mut(&mut self) -> impl Iterator<Item=(K, &mut T)> + '_ {
        self.entries.iter_mut().map(|(s, v)| (*s, v))
    }
}

impl<K: Copy + Eq + Hash, T: Default + Clone> std::ops::Index<K> for MapVec<K, T> {
    type Output = T;
    fn index(&self, entry: K) -> &Self::Output { self.get(entry) }
}

impl<K: Copy + Eq + Hash, T: Default + Clone> std::ops::IndexMut<K> for MapVec<K, T> {
    fn index_mut(&mut self, entry: K) -> &mut Self::Output { self.get_mut(entry) }
}