import {
	AIR,
	CHUNK_HEIGHT,
	CHUNK_SIZE,
	FACE_OFFSETS,
	Faces,
	faces,
	PLAYER_EYE_HEIGHT,
	PLAYER_HEIGHT,
	PLAYER_WIDTH,
	TICK_DELTA,
	TICKS_PER_SECOND,
} from "$/common/constants.ts";
import { BlockRegistry, EverythingRegistry, ItemRegistry } from "$/common/everything_registry.ts";
import { click_slot, Container, ItemData, ItemStack, take_output } from "$/common/inventory.ts";
import {
	AIR_ID,
	BlockChange,
	ClientMessage,
	ContainerKey,
	CRAFTING_RESULT_SLOT,
	MAX_CHAT_LENGTH,
	MAX_NAME_LENGTH,
	PROTOCOL_VERSION,
	ServerMessage,
} from "$/common/protocol.ts";
import type { AtlasListing, ModListing, RecipeBook } from "$/common/mod_loader.ts";
import type { WorldgenSetup } from "$/common/generation.ts";
import { BLOCK_BEHAVIORS } from "./blocks.ts";
import { ModRuntime, run_guarded } from "./mod_runtime.ts";
import type { GameLoop } from "./game_loop.ts";
import { consume_recipe_items, update_crafting_result } from "./crafting.ts";
import { OpenScreen, SavedPlayer, ServerPlayer } from "./player.ts";
import { ServerWorld, Tile } from "./world.ts";
import {
	BLOCK_DROP_PICKUP_DELAY,
	DESPAWN_TICKS,
	ITEM_SIZE,
	ItemEntity,
	PLAYER_DROP_PICKUP_DELAY,
	SavedItemEntity,
} from "./item_entity.ts";

// how far from a player's eyes a block can be changed, a bit more than the client's reach
const MAX_REACH = 8;
// how far past a player's box items get picked up from, sideways and up or down, like minecraft
const PICKUP_REACH_XZ = 1;
const PICKUP_REACH_Y = 0.5;
// items this close (past their own size) merge into one stack
const MERGE_DISTANCE = 0.5;
// tiles further than this many chunks from every player don't tick
const SIMULATION_DISTANCE = 6;
const MAX_GIVE = 64 * 36;
// how long a client gets between welcome and ready to download and load everything
const READY_TIMEOUT_TICKS = 60 * TICKS_PER_SECOND;

// everything the game server needs from whatever runs it
export interface GameHost {
	send(conn: number, data: string): void;
	close(conn: number): void;
}

export interface SavedWorld {
	version: 2;
	seed: string;
	changes: BlockChange[];
	tiles: {
		id: string;
		x: number;
		y: number;
		z: number;
		data: Record<string, unknown>;
		containers: Record<string, (ItemData | null)[]>;
		// a mod block's data, what BlockRef.data holds
		mod_data?: unknown;
	}[];
	players: Record<string, SavedPlayer>;
	// items on the ground
	entities?: SavedItemEntity[];
	// ctx.storage of every mod, by mod id
	mod_storage?: Record<string, Record<string, unknown>>;
}

// the loaded mods, see load_mods.ts
export interface GameMods {
	listings: ModListing[];
	atlas: AtlasListing;
	recipes: RecipeBook;
	worldgen?: WorldgenSetup;
	runtime: ModRuntime;
}

export class GameServer {
	world: ServerWorld;
	#host: GameHost;
	#players = new Map<number, ServerPlayer>();
	// connections that got welcome and are downloading mods, by conn
	#pending = new Map<number, { name: unknown; since: number }>();
	#saved_players: Record<string, SavedPlayer> = {};
	#tick = 0;
	// items on the ground, by id
	#entities = new Map<string, ItemEntity>();
	#next_entity_id = 1;
	#mods: GameMods;
	mods: ModRuntime;
	recipes: RecipeBook;
	// what runs tick(), for /tps. tests call tick() themselves
	loop?: GameLoop;

	constructor(host: GameHost, save: string | undefined, default_seed: string, mods: GameMods) {
		this.#host = host;
		this.#mods = mods;
		this.mods = mods.runtime;
		this.recipes = mods.recipes;

		// version 1 saves only had the seed and block changes
		const saved: Partial<SavedWorld> | undefined = save ? JSON.parse(save) : undefined;
		this.world = new ServerWorld(saved?.seed ?? default_seed, mods.worldgen);
		this.mods.storage = saved?.mod_storage ?? {};
		this.world.load_changes(saved?.changes ?? []);
		for (const tile of saved?.tiles ?? []) {
			const containers: Record<string, Container> = {};
			for (const [name, items] of Object.entries(tile.containers)) {
				containers[name] = new Container(items.length);
				containers[name].load(items);
			}
			this.world.tiles.set(`${tile.x},${tile.y},${tile.z}`, { ...tile, containers });
		}
		for (const entity of saved?.entities ?? []) {
			const item = ItemEntity.load(this.#new_entity_id(), entity);
			this.#entities.set(item.id, item);
		}
		this.#saved_players = saved?.players ?? {};
		this.world.dirty = saved?.version !== 2;

		this.world.on_block_change = (x, y, z, id, state) =>
			this.#broadcast(state ? { type: "set_block", x, y, z, id, state } : { type: "set_block", x, y, z, id });
	}

	// connections

	on_connect(_conn: number) {}

	on_disconnect(conn: number) {
		this.#pending.delete(conn);
		const player = this.#players.get(conn);
		if (!player) {
			return;
		}
		this.#close_screen(player);
		this.#saved_players[player.name] = player.save();
		this.world.dirty = true;
		this.#players.delete(conn);
		this.mods.after.player_leave.emit({ player: this.mods.player(player) });
		this.#broadcast({ type: "player_leave", id: player.id });
		this.#broadcast({ type: "chat", text: `${player.name} left` });
		console.log(`${player.name} left (${this.#players.size} online)`);
	}

	on_message(conn: number, data: string) {
		let message: ClientMessage;
		try {
			message = JSON.parse(data);
		} catch {
			return;
		}
		if (typeof message !== "object" || message === null) {
			return;
		}

		const player = this.#players.get(conn);
		if (!player) {
			this.#handshake(conn, message);
			return;
		}

		this.#handle(player, message);
		this.#sync(player);
	}

	// game loop, the host calls this TICKS_PER_SECOND times a second

	tick() {
		this.#tick += 1;
		const second = this.#tick % TICKS_PER_SECOND === 0;

		for (const tile of [...this.world.tiles.values()]) {
			const behavior = BLOCK_BEHAVIORS[tile.id];
			const components = this.mods.components_of(tile.id)
				.filter(({ component }) => component.on_tick || component.on_second);
			if (!behavior?.on_tick && !behavior?.on_second && components.length === 0) {
				continue;
			}
			if (!this.#near_any_player(tile.x, tile.z)) {
				continue;
			}
			behavior?.on_tick?.(this, tile);
			if (second) {
				behavior?.on_second?.(this, tile);
			}
			if (components.length > 0 && tile.mod_data !== undefined) {
				const block = this.mods.block_ref(tile.x, tile.y, tile.z, tile.id);
				for (const { mod, id, component, params } of components) {
					if (component.on_tick) {
						run_guarded(mod, `${id} on_tick`, () => component.on_tick!(block, params, TICK_DELTA));
					}
					if (second && component.on_second) {
						run_guarded(mod, `${id} on_second`, () => component.on_second!(block, params, 1));
					}
				}
			}
			// tile data can change on any tick, saving is cheap enough to not track it exactly
			this.world.dirty = true;
		}

		this.#tick_items();

		this.mods.tick();
		this.mods.check_watched_containers();

		for (const [conn, pending] of this.#pending) {
			if (this.#tick - pending.since > READY_TIMEOUT_TICKS) {
				this.#pending.delete(conn);
				this.#reject(conn, "Took too long to load the server's mods");
			}
		}

		for (const player of this.#players.values()) {
			this.#sync(player);
		}
	}

	save(): string {
		for (const player of this.#players.values()) {
			this.#saved_players[player.name] = player.save();
		}
		const saved: SavedWorld = {
			version: 2,
			seed: this.world.seed,
			changes: this.world.all_changes(),
			tiles: [...this.world.tiles.values()].map((tile) => ({
				id: tile.id,
				x: tile.x,
				y: tile.y,
				z: tile.z,
				data: tile.data,
				containers: Object.fromEntries(
					Object.entries(tile.containers).map(([name, container]) => [name, container.to_data()]),
				),
				mod_data: tile.mod_data,
			})),
			players: this.#saved_players,
			entities: [...this.#entities.values()].map((entity) => entity.save()),
			mod_storage: this.mods.storage,
		};
		this.world.dirty = false;
		return JSON.stringify(saved);
	}

	// used by block behaviors and mods

	get current_tick() {
		return this.#tick;
	}

	players(): ServerPlayer[] {
		return [...this.#players.values()];
	}

	give(player: ServerPlayer, id: string, count: number, data?: unknown) {
		// split into stacks so max stack sizes are respected
		let left = count;
		while (left > 0) {
			const stack = new ItemStack(id);
			stack.amount = Math.min(left, stack.max_amount);
			if (data !== undefined) stack.data = structuredClone(data);
			left -= stack.amount;
			this.give_stack(player, stack);
		}
	}

	// into the inventory, whatever doesn't fit drops at the player's feet
	give_stack(player: ServerPlayer, stack: ItemStack) {
		if (player.inventory.add_item(stack) > 0) {
			this.spawn_item(player.x, player.y, player.z, stack, PLAYER_DROP_PICKUP_DELAY);
		}
	}

	spawn_item(x: number, y: number, z: number, stack: ItemStack, pickup_delay = 0): ItemEntity {
		const entity = new ItemEntity(this.#new_entity_id(), stack, x, y, z, pickup_delay);
		this.#entities.set(entity.id, entity);
		this.#broadcast({ type: "add_entity", entity: entity.info() });
		this.world.dirty = true;
		return entity;
	}

	// like minecraft's Block.popResource: from a bit off the block's middle, flying up and out a little
	pop_item(x: number, y: number, z: number, stack: ItemStack) {
		const spread = () => (Math.random() - 0.5) * 0.5;
		const entity = this.spawn_item(
			x + 0.5 + spread(),
			y + 0.5 - ITEM_SIZE / 2 + spread(),
			z + 0.5 + spread(),
			stack,
			BLOCK_DROP_PICKUP_DELAY,
		);
		entity.vx = (Math.random() - 0.5) * 4;
		entity.vy = 4;
		entity.vz = (Math.random() - 0.5) * 4;
		return entity;
	}

	// out of the player's face the way they're looking, like minecraft's Player.drop
	throw_item(player: ServerPlayer, stack: ItemStack) {
		const entity = this.spawn_item(
			player.x,
			player.y + PLAYER_EYE_HEIGHT - 0.3,
			player.z,
			stack,
			PLAYER_DROP_PICKUP_DELAY,
		);
		// minecraft's numbers are per tick
		const speed = 0.3 * TICKS_PER_SECOND;
		const cos_pitch = Math.cos(player.pitch);
		const spread_angle = Math.random() * Math.PI * 2;
		const spread = Math.random() * 0.02 * TICKS_PER_SECOND;
		entity.vx = -Math.sin(player.yaw) * cos_pitch * speed + Math.cos(spread_angle) * spread;
		entity.vy = Math.sin(player.pitch) * speed + (0.1 + (Math.random() - Math.random()) * 0.1) * TICKS_PER_SECOND;
		entity.vz = -Math.cos(player.yaw) * cos_pitch * speed + Math.sin(spread_angle) * spread;
		return entity;
	}

	item_entities(): ItemEntity[] {
		return [...this.#entities.values()];
	}

	send_chat(player: ServerPlayer, text: string) {
		this.#send(player, { type: "chat", text });
	}

	teleport(player: ServerPlayer, x: number, y: number, z: number) {
		Object.assign(player, { x, y, z });
		this.#send(player, { type: "teleport", x, y, z });
		this.#broadcast({ type: "player_move", id: player.id, x, y, z, yaw: player.yaw, pitch: player.pitch }, player);
	}

	// changes a block's state bits without anything else happening, see ServerWorld.get_block_value
	set_block_state(x: number, y: number, z: number, state: number) {
		const id = this.world.get_block_id(x, y, z);
		if (id !== AIR_ID) {
			this.world.set_block(x, y, z, id, state);
		}
	}

	set_block(x: number, y: number, z: number, id: string, player?: ServerPlayer, state?: number) {
		const old_id = this.world.get_block_id(x, y, z);
		if (old_id !== AIR_ID) {
			const components = this.mods.components_of(old_id).filter(({ component }) => component.on_break);
			if (components.length > 0) {
				const block = this.mods.block_ref(x, y, z, old_id);
				const player_api = player && this.mods.player(player);
				for (const { mod, id: component_id, component, params } of components) {
					run_guarded(mod, `${component_id} on_break`, () => component.on_break!(block, params, player_api));
				}
			}
		}

		const old_tile = this.world.get_tile(x, y, z);
		if (old_tile) {
			BLOCK_BEHAVIORS[old_tile.id]?.on_break?.(this, old_tile, player);
			this.world.remove_tile(x, y, z);
			for (const other of this.#players.values()) {
				if (other.screen?.tile === old_tile) {
					this.#close_screen(other);
					this.#send(other, { type: "close_screen" });
				}
			}
		}

		this.world.set_block(x, y, z, id, state);

		if (BLOCK_BEHAVIORS[id]?.create_tile) {
			this.get_or_create_tile(x, y, z);
		}

		if (id !== AIR_ID) {
			const block = this.mods.block_ref(x, y, z, id);
			for (const { mod, id: component_id, component, params } of this.mods.components_of(id)) {
				if (component.on_create) {
					run_guarded(mod, `${component_id} on_create`, () => component.on_create!(block, params));
				}
			}
		}
	}

	get_or_create_tile(x: number, y: number, z: number): Tile {
		let tile = this.world.get_tile(x, y, z);
		if (!tile) {
			// blocks placed before tiles were saved have none yet
			const id = this.world.get_block_id(x, y, z);
			tile = { id, x, y, z, data: {}, containers: {} };
			BLOCK_BEHAVIORS[id]?.create_tile?.(tile);
			this.world.add_tile(tile);
		}
		return tile;
	}

	open_screen(player: ServerPlayer, screen: OpenScreen) {
		this.#close_screen(player);
		player.screen = screen;
		this.#send(player, { type: "open_screen", layout: screen.layout, properties: screen.properties() });
		player.sent.set("properties", JSON.stringify(screen.properties()));
		player.sent.delete("screen");
	}

	// messages

	// hello -> welcome, then ready -> join. see "Delivery to clients" in MODS.md
	#handshake(conn: number, message: ClientMessage) {
		if (message.type === "hello") {
			if (this.#pending.has(conn)) {
				return;
			}
			if (message.protocol !== PROTOCOL_VERSION) {
				this.#reject(
					conn,
					`This server needs a game version with protocol ${PROTOCOL_VERSION}, yours has ${
						message.protocol ?? "none"
					}. Reload the page to update.`,
				);
				return;
			}
			this.#pending.set(conn, { name: message.name, since: this.#tick });
			this.#host.send(
				conn,
				JSON.stringify(
					{
						type: "welcome",
						protocol: PROTOCOL_VERSION,
						seed: this.world.seed,
						atlas: this.#mods.atlas,
						mods: this.#mods.listings,
					} satisfies ServerMessage,
				),
			);
			return;
		}

		if (message.type === "ready") {
			const pending = this.#pending.get(conn);
			if (!pending) {
				return;
			}
			this.#pending.delete(conn);
			this.#join(conn, pending.name);
		}
	}

	#reject(conn: number, reason: string) {
		this.#host.send(conn, JSON.stringify({ type: "rejected", reason } satisfies ServerMessage));
		this.#host.close(conn);
	}

	#join(conn: number, raw_name: unknown) {
		const player = new ServerPlayer(conn, this.#unique_name(raw_name));
		const saved = this.#saved_players[player.name];
		if (saved) {
			player.load(saved);
		} else {
			Object.assign(player, this.world.spawn_point());
		}

		this.#send(player, {
			type: "join",
			id: player.id,
			name: player.name,
			players: [...this.#players.values()].map((p) => p.info()),
			changes: this.world.all_changes(),
			entities: [...this.#entities.values()].map((entity) => entity.info()),
			spawn: { x: player.x, y: player.y, z: player.z, yaw: player.yaw, pitch: player.pitch },
			selected_slot: player.selected_slot,
		});
		this.#players.set(conn, player);
		this.#sync(player);

		this.#broadcast({ type: "player_join", player: player.info() }, player);
		this.#broadcast({ type: "chat", text: `${player.name} joined` });
		this.mods.after.player_join.emit({ player: this.mods.player(player) });
		console.log(`${player.name} joined (${this.#players.size} online)`);
	}

	#handle(player: ServerPlayer, message: ClientMessage) {
		switch (message.type) {
			case "move": {
				const { x, y, z, yaw, pitch } = message;
				if (![x, y, z, yaw, pitch].every(is_number)) {
					return;
				}
				Object.assign(player, { x, y, z, yaw, pitch });
				this.#broadcast({ type: "player_move", id: player.id, x, y, z, yaw, pitch }, player);
				break;
			}
			case "break_block":
				if (is_block_position(message.x, message.y, message.z)) {
					this.#break_block(player, message.x, message.y, message.z);
				}
				break;
			case "hit_block":
				if (is_block_position(message.x, message.y, message.z)) {
					this.#hit_block(player, message.x, message.y, message.z);
				}
				break;
			case "use_item":
				this.#use_item(player);
				break;
			case "use_block":
				if (is_block_position(message.x, message.y, message.z) && faces.includes(message.face)) {
					this.#use_block(player, message.x, message.y, message.z, message.face);
				}
				break;
			case "select_slot":
				if (is_int(message.slot) && message.slot >= 0 && message.slot < 9) {
					player.selected_slot = message.slot;
				}
				break;
			case "click":
				this.#click(player, message.container, message.index, message.button);
				break;
			case "drop_item":
				this.#drop_item(player, message.container, message.index, message.all === true);
				break;
			case "close_screen":
				this.#close_screen(player);
				break;
			case "chat":
				this.#chat(player, message.text);
				break;
		}
	}

	#break_block(player: ServerPlayer, x: number, y: number, z: number) {
		const info = this.world.get_block_info(x, y, z);
		// air, or something like water that can't be broken
		if (!info || info.toughness === undefined || !this.#in_reach(player, x, y, z)) {
			this.#correct(player, x, y, z);
			return;
		}

		const event = {
			player: this.mods.player(player),
			block: this.mods.block_ref(x, y, z, info.id),
			item: this.mods.player(player).held_item,
		};
		const before = { ...event, cancel: false };
		this.mods.before.block_break.emit(before);
		if (before.cancel) {
			this.#correct(player, x, y, z);
			return;
		}

		const held = EverythingRegistry.get<ItemRegistry>("items", player.held_item?.type_id ?? "");
		const drops = info.drop_table && (!info.requires_tool || held?.tool_type === info.tool_to_break);

		this.set_block(x, y, z, AIR_ID, player);
		if (drops) {
			this.pop_item(x, y, z, new ItemStack(info.drop_table!));
		}
		this.mods.after.block_break.emit(event);
	}

	#hit_block(player: ServerPlayer, x: number, y: number, z: number) {
		const info = this.world.get_block_info(x, y, z);
		if (!info || !this.#in_reach(player, x, y, z)) {
			return;
		}
		const components = this.mods.components_of(info.id).filter(({ component }) => component.on_click);
		if (components.length === 0) {
			return;
		}
		const block = this.mods.block_ref(x, y, z, info.id);
		const player_api = this.mods.player(player);
		for (const { mod, id, component, params } of components) {
			run_guarded(mod, `${id} on_click`, () => component.on_click!(block, params, player_api));
		}
	}

	// the held item's on_use handlers, returns whether it has any
	#use_item(player: ServerPlayer): boolean {
		const held = player.held_item;
		if (!held) {
			return false;
		}
		const components = this.mods.item_components_of(held.type_id).filter(({ component }) => component.on_use);
		if (components.length === 0) {
			return false;
		}
		const player_api = this.mods.player(player);
		const item = player_api.held_item!;
		for (const { mod, id, component, params } of components) {
			run_guarded(mod, `${id} on_use`, () => component.on_use!(item, params, player_api));
		}
		return true;
	}

	#use_block(player: ServerPlayer, x: number, y: number, z: number, face: Faces) {
		const offset = FACE_OFFSETS[face];
		const [tx, ty, tz] = [x + offset.x, y + offset.y, z + offset.z];

		const info = this.world.get_block_info(x, y, z);
		if (!info || !this.#in_reach(player, x, y, z)) {
			this.#correct(player, x, y, z);
			this.#correct(player, tx, ty, tz);
			return;
		}

		const player_api = this.mods.player(player);
		const block = this.mods.block_ref(x, y, z, info.id);
		const interact = { player: player_api, block, item: player_api.held_item };
		const before_interact = { ...interact, cancel: false };
		this.mods.before.block_interact.emit(before_interact);
		if (before_interact.cancel) {
			this.#correct(player, tx, ty, tz);
			return;
		}

		let handled = BLOCK_BEHAVIORS[info.id]?.on_interact?.(this, { x, y, z, id: info.id }, player) ?? false;
		for (const { mod, id: component_id, component, params } of this.mods.components_of(info.id)) {
			if (component.on_interact) {
				handled = run_guarded(mod, `${component_id} on_interact`, () =>
							component.on_interact!(block, params, player_api)) ===
						true || handled;
			}
		}
		if (handled) {
			this.mods.after.block_interact.emit(interact);
			// the client didn't guess a placement for interactive blocks, but it might have for others
			this.#correct(player, tx, ty, tz);
			return;
		}

		// an item that does something when used does that instead of being placed
		if (this.#use_item(player)) {
			this.#correct(player, tx, ty, tz);
			return;
		}

		// place the held block against the face
		const held_slot = player.inventory.get_slot(player.selected_slot);
		const held = EverythingRegistry.get<ItemRegistry>("items", held_slot.type_id ?? "");
		if (!held?.block_id || ty < 0 || ty >= CHUNK_HEIGHT || !this.#replaceable(tx, ty, tz)) {
			this.#correct(player, tx, ty, tz);
			return;
		}

		const place = {
			player: player_api,
			block: { id: held.block_id, x: tx, y: ty, z: tz },
			face,
			item: player_api.held_item,
		};
		const before_place = { ...place, cancel: false };
		this.mods.before.block_place.emit(before_place);
		if (before_place.cancel) {
			this.#correct(player, tx, ty, tz);
			return;
		}

		this.set_block(tx, ty, tz, held.block_id, player);
		held_slot.amount = held_slot.amount! - 1;
		this.mods.after.block_place.emit(place);
	}

	#drop_item(player: ServerPlayer, key: unknown, index: unknown, all: boolean) {
		if (!is_int(index)) {
			return;
		}
		const container = this.#get_container(player, key as ContainerKey);
		// the crafting result isn't an item yet, minecraft crafts it first but that's not supported
		if (
			!container || index < 0 || index >= container.size || (key === "crafting" && index === CRAFTING_RESULT_SLOT)
		) {
			return;
		}
		const item = container.get_item(index);
		if (!item) {
			return;
		}

		const thrown = item.clone();
		thrown.amount = all ? item.amount : 1;
		item.amount -= thrown.amount;
		container.set_item(index, item.amount > 0 ? item : undefined);
		if (key === "crafting") {
			update_crafting_result(container, this.recipes.shaped);
		}
		this.throw_item(player, thrown);
	}

	#click(player: ServerPlayer, key: unknown, index: unknown, button: unknown) {
		if (!is_int(index) || !is_int(button)) {
			return;
		}
		const container = this.#get_container(player, key as ContainerKey);
		if (!container || index < 0 || index >= container.size) {
			return;
		}

		if (key === "crafting" && index === CRAFTING_RESULT_SLOT) {
			const result = container.get_item(CRAFTING_RESULT_SLOT);
			if (result && take_output(result, player.cursor)) {
				consume_recipe_items(container);
			}
		} else if (key === "screen" && player.screen?.layout.slots.find((s) => s.index === index)?.output) {
			const item = container.get_item(index);
			if (item && take_output(item, player.cursor)) {
				container.set_item(index, undefined);
			}
		} else {
			click_slot(container, index, player.cursor, button);
		}

		if (key === "crafting") {
			update_crafting_result(container, this.recipes.shaped);
		}
	}

	#chat(player: ServerPlayer, raw: unknown) {
		if (typeof raw !== "string") {
			return;
		}
		const text = raw.trim().slice(0, MAX_CHAT_LENGTH);
		if (text.length === 0) {
			return;
		}
		if (text.startsWith("/")) {
			this.#command(player, text);
			return;
		}
		const before = { player: this.mods.player(player), message: text, cancel: false };
		this.mods.before.chat_send.emit(before);
		const message = String(before.message).slice(0, MAX_CHAT_LENGTH);
		if (before.cancel || message.length === 0) {
			return;
		}
		console.log(`<${player.name}> ${message}`);
		this.#broadcast({ type: "chat", from: player.name, text: message });
		this.mods.after.chat_send.emit({ player: before.player, message });
	}

	#command(player: ServerPlayer, text: string) {
		const [command, ...args] = text.slice(1).split(/\s+/);
		if (command === "give") {
			// TODO: only let operators do this
			let [item_id, count] = args;
			if (!item_id) {
				this.#send(player, { type: "chat", text: "Usage: /give <item> [count]" });
				return;
			}
			if (!item_id.includes(":")) {
				item_id = `bworld:${item_id}`;
			}
			const amount = count === undefined ? 1 : Number(count);
			if (!EverythingRegistry.get("items", item_id)) {
				this.#send(player, { type: "chat", text: `Unknown item ${item_id}` });
				return;
			}
			if (!Number.isInteger(amount) || amount < 1 || amount > MAX_GIVE) {
				this.#send(player, { type: "chat", text: `Count must be between 1 and ${MAX_GIVE}` });
				return;
			}
			this.give(player, item_id, amount);
			return;
		}
		if (command === "tps") {
			if (!this.loop) {
				this.#send(player, { type: "chat", text: "The game loop isn't running" });
				return;
			}
			const { tps, mspt, skipped } = this.loop.stats;
			this.#send(player, {
				type: "chat",
				text: `${tps} ticks per second (aiming for ${TICKS_PER_SECOND}), ${mspt.toFixed(1)} ms per tick, ` +
					`${skipped} skipped since starting`,
			});
			return;
		}
		const mod_command = this.mods.commands.get(command);
		if (mod_command) {
			run_guarded(mod_command.mod, `/${command}`, () => mod_command.command.run(args, this.mods.player(player)));
			return;
		}
		this.#send(player, { type: "chat", text: `Unknown command /${command}` });
	}

	#close_screen(player: ServerPlayer) {
		player.screen = undefined;
		player.sent.delete("screen");
		player.sent.delete("properties");

		// the crafting grid and whatever the cursor holds go back into the inventory
		for (let i = 0; i < 9; i++) {
			const item = player.crafting.get_item(i);
			if (item) {
				player.crafting.set_item(i, undefined);
				this.give_stack(player, item);
			}
		}
		update_crafting_result(player.crafting, this.recipes.shaped);
		if (player.cursor.item) {
			const item = player.cursor.item;
			player.cursor.item = undefined;
			this.give_stack(player, item);
		}
	}

	// items on the ground

	#new_entity_id() {
		return `item${this.#next_entity_id++}`;
	}

	#remove_item(entity: ItemEntity) {
		this.#entities.delete(entity.id);
		this.#broadcast({ type: "remove_entity", id: entity.id });
	}

	// like minecraft's ItemEntity.tick: only near players, falling, merging, despawning, and getting picked up
	#tick_items() {
		if (this.#entities.size === 0) {
			return;
		}
		const is_solid = (x: number, y: number, z: number) => this.world.has_collision(x, y, z);

		const active = [...this.#entities.values()].filter((entity) => this.#near_any_player(entity.x, entity.z));
		for (const entity of active) {
			entity.tick(is_solid);
			if (entity.age >= DESPAWN_TICKS) {
				this.#remove_item(entity);
			}
		}

		if (this.#tick % 2 === 0) {
			this.#merge_items(active.filter((entity) => this.#entities.has(entity.id)));
		}

		for (const player of this.#players.values()) {
			for (const entity of active) {
				if (this.#entities.has(entity.id) && entity.pickup_delay === 0 && this.#can_pick_up(player, entity)) {
					this.#pick_up(player, entity);
				}
			}
		}

		for (const entity of active) {
			const moved = entity.x !== entity.sent_x || entity.y !== entity.sent_y || entity.z !== entity.sent_z;
			if (moved && this.#entities.has(entity.id)) {
				entity.sent_x = entity.x;
				entity.sent_y = entity.y;
				entity.sent_z = entity.z;
				this.#broadcast({ type: "move_entity", id: entity.id, x: entity.x, y: entity.y, z: entity.z });
			}
		}

		this.world.dirty = true;
	}

	// the smaller stack goes into the bigger one, like minecraft's ItemEntity.tryToMerge
	#merge_items(entities: ItemEntity[]) {
		for (const a of entities) {
			for (const b of entities) {
				if (!this.#entities.has(a.id) || !this.#entities.has(b.id) || !a.can_merge_with(b)) {
					continue;
				}
				const reach = MERGE_DISTANCE + ITEM_SIZE;
				if (Math.abs(a.x - b.x) > reach || Math.abs(a.z - b.z) > reach || Math.abs(a.y - b.y) > ITEM_SIZE) {
					continue;
				}

				const [into, from] = a.item.amount >= b.item.amount ? [a, b] : [b, a];
				const moving = Math.min(from.item.amount, into.item.max_amount - into.item.amount);
				into.item.amount += moving;
				from.item.amount -= moving;
				into.pickup_delay = Math.max(into.pickup_delay, from.pickup_delay);
				into.age = Math.min(into.age, from.age);

				this.#broadcast({ type: "set_entity_item", id: into.id, item: into.item.to_data() });
				if (from.item.amount === 0) {
					this.#remove_item(from);
				} else {
					this.#broadcast({ type: "set_entity_item", id: from.id, item: from.item.to_data() });
				}
			}
		}
	}

	// the player's box grown by the pickup reach touches the item's box
	#can_pick_up(player: ServerPlayer, entity: ItemEntity) {
		const reach_xz = PLAYER_WIDTH / 2 + PICKUP_REACH_XZ + ITEM_SIZE / 2;
		return Math.abs(player.x - entity.x) <= reach_xz && Math.abs(player.z - entity.z) <= reach_xz &&
			entity.y + ITEM_SIZE >= player.y - PICKUP_REACH_Y &&
			entity.y <= player.y + PLAYER_HEIGHT + PICKUP_REACH_Y;
	}

	// as much as fits, the rest stays on the ground
	#pick_up(player: ServerPlayer, entity: ItemEntity) {
		const before = entity.item.amount;
		const left = player.inventory.add_item(entity.item);
		if (left === before) {
			return;
		}
		if (left === 0) {
			this.#entities.delete(entity.id);
			this.#broadcast({ type: "take_entity", id: entity.id, player: player.id });
		} else {
			this.#broadcast({ type: "set_entity_item", id: entity.id, item: entity.item.to_data() });
		}
	}

	// helpers

	#get_container(player: ServerPlayer, key: ContainerKey): Container | undefined {
		switch (key) {
			case "inventory":
				return player.inventory;
			case "crafting":
				return player.crafting;
			case "screen":
				return player.screen?.container;
		}
	}

	// sends whatever changed since last time
	#sync(player: ServerPlayer) {
		const sync = (key: string, value: unknown, message: () => ServerMessage) => {
			const json = JSON.stringify(value);
			if (player.sent.get(key) !== json) {
				player.sent.set(key, json);
				this.#send(player, message());
			}
		};

		const inventory = player.inventory.to_data();
		sync("inventory", inventory, () => ({ type: "container", container: "inventory", items: inventory }));
		const crafting = player.crafting.to_data();
		sync("crafting", crafting, () => ({ type: "container", container: "crafting", items: crafting }));
		const cursor = player.cursor.item?.to_data() ?? null;
		sync("cursor", cursor, () => ({ type: "cursor", item: cursor }));

		if (player.screen) {
			const items = player.screen.container.to_data();
			sync("screen", items, () => ({ type: "container", container: "screen", items }));
			const properties = player.screen.properties();
			sync("properties", properties, () => ({ type: "screen_properties", properties }));
		}
	}

	// tell the player what's really at a position, undoing anything their client guessed
	#correct(player: ServerPlayer, x: number, y: number, z: number) {
		if (y < 0 || y >= CHUNK_HEIGHT) {
			return;
		}
		this.#send(player, { type: "set_block", x, y, z, id: this.world.get_block_id(x, y, z) });
	}

	#replaceable(x: number, y: number, z: number) {
		const nid = this.world.get_block_nid(x, y, z);
		return nid === AIR || (EverythingRegistry.get_by_id<BlockRegistry>("blocks", nid)?.replaceable ?? false);
	}

	#in_reach(player: ServerPlayer, x: number, y: number, z: number) {
		const dx = x + 0.5 - player.x;
		const dy = y + 0.5 - (player.y + PLAYER_EYE_HEIGHT);
		const dz = z + 0.5 - player.z;
		return dx * dx + dy * dy + dz * dz <= MAX_REACH * MAX_REACH;
	}

	#near_any_player(x: number, z: number) {
		const chunk_x = Math.floor(x / CHUNK_SIZE);
		const chunk_z = Math.floor(z / CHUNK_SIZE);
		for (const player of this.#players.values()) {
			const px = Math.floor(player.x / CHUNK_SIZE);
			const pz = Math.floor(player.z / CHUNK_SIZE);
			if (Math.max(Math.abs(px - chunk_x), Math.abs(pz - chunk_z)) <= SIMULATION_DISTANCE) {
				return true;
			}
		}
		return false;
	}

	#unique_name(name: unknown): string {
		const cleaned = typeof name === "string" ? name.replace(/[^A-Za-z0-9_]/g, "").slice(0, MAX_NAME_LENGTH) : "";
		const base = cleaned || `player${Math.floor(Math.random() * 10000)}`;

		const taken = new Set([...this.#players.values()].map((p) => p.name));
		let final = base;
		let i = 2;
		while (taken.has(final)) {
			final = `${base}${i}`;
			i += 1;
		}
		return final;
	}

	#send(player: ServerPlayer, message: ServerMessage) {
		this.#host.send(player.conn, JSON.stringify(message));
	}

	#broadcast(message: ServerMessage, except?: ServerPlayer) {
		const data = JSON.stringify(message);
		for (const player of this.#players.values()) {
			if (player !== except) {
				this.#host.send(player.conn, data);
			}
		}
	}
}

function is_number(value: unknown): value is number {
	return typeof value === "number" && Number.isFinite(value);
}

function is_int(value: unknown): value is number {
	return Number.isInteger(value);
}

// messages are parsed json, so the types in ClientMessage are only what a well behaved client sends
function is_block_position(x: unknown, y: unknown, z: unknown): boolean {
	return is_int(x) && is_int(y) && is_int(z) && y >= 0 && y < CHUNK_HEIGHT &&
		Math.abs(x) < 30000 * CHUNK_SIZE && Math.abs(z) < 30000 * CHUNK_SIZE;
}