--[[-- ECS ([entity compenent system](https://en.wikipedia.org/wiki/Entity_component_system)) library for Lua.

Entity Component System library, inspired by the excellent [tiny-ecs](https://github.com/bakpakin/tiny-ecs/tree/master) by bakpakin.
Main differences include:

* ability to nest systems (more organisation potential);
* instanciation of systems for each world (no shared state) (several worlds can coexist at the same time easily);
* adding and removing entities is done instantaneously (no going isane over tiny-ecs cache issues);
* ability to add and remove components from entities after they were added to the world (more dynamic entities).

And a fair amount of other quality-of-life features.

The goals of this library are similar in spirit to tiny-ecs: simple to use, flexible, and useful.
The more advanced features it provides relative to tiny-ecs are made so that you can completely ignore them
if you don't use them.

The module returns a table that contains several functions, `world` or `scene` are starting points
to create your world.

No mandatory dependency.
Optional dependency: `ubiquitousse.scene`, to allow quick creation of ECS-based scenes (`ecs.scene`).

@module ecs
@usage
-- Same example as tiny-ecs', for comparaison purposes

local ecs = require("ubiquitousse.ecs")

local talkingSystem = {
	filter = { "name", "mass", "phrase" },
	process = function(self, e, c, dt)
		e.mass = e.mass + dt * 3
		print(("%s who weighs %d pounds, says %q."):format(e.name, e.mass, e.phrase))
	end
}

local joe = {
	name = "Joe",
	phrase = "I'm a plumber.",
	mass = 150,
	hairColor = "brown"
}

local world = ecs.world(talkingSystem)
world:add(joe)

for i = 1, 20 do
	world:update(1)
end
--]]
local loaded, scene
if ... then loaded, scene = pcall(require, (...):match("^(.-)ecs").."scene") end
if not loaded then scene = nil end

let ecs

-- TODO: Implement a skip list for faster search.
-- better control over system order: process, draw, methods? (for lag reasons and dependencies)

--- Entities are regular tables that get processed by `System`s.
--
-- The idea is that entities _should_ only contain data and no code; it's the systems that are responsible for the actual processing
-- (but it's your game, do as you want).
--
-- This data is referred to, and organized in, "components".
-- @type Entity

--[[-- Components.

Entities are Lua tables, and thus contain key-values pairs: each one of these pairs is called a "component". The data can be
whatever you want, and ideally each component _should_ store the data for one singular aspect of the entity, for example its position, name, etc.

This library does not do any kind of special processing by itself on the entity tables and take them as is (no metatable, no methamethods, etc.),
so you are free to handle them as you want in your systems or elsewhere.

Since it's relatively common for systems to only operate on a single component, as a shortcut the library often consider what it calls the "system component":
that is, the component in the entity that is named like `System.component` (or `System.name` if it is not set). Though there's no problem if there's no system
component or if it doesn't exist in the entity.

@doc Component
@usage
-- example entity
local entity = {
	position = { x = 0, y = 52 }, -- a "position" component
	sprite = newSprite("awesomeguy.png") -- a "sprite" component
}

-- example "sprite" system
local sprite = {
	name = "sprite",
	filter = "sprite", -- process entities that have a "sprite" component
	-- systems callbacks that are called per-entity often give you the system component as an argument
	-- the system component is the component with the same name as the system, thus here the sprite component
	render = function(self, entity, component)
		-- component == entity.sprite
		draw(component)
	end
}
]]--

--- Recursively remove subsystems from a system.
let recDestroySystems = (system)
	for i=#system.systems, 1, -1 do
		let s = system.systems[i]
		recDestroySystems(s)
		s:onDestroy()
		system.systems[i] = nil
		if s.name then
			system.world.s[s.name] = nil
		end
	end
end
--- Recursively call :clear and :onRemoveFromWorld to a list of systems in a world.
let recCallOnRemoveFromWorld = (world, systems)
	for _, s in ipairs(systems) do
		s:clear()
		recCallOnRemoveFromWorld(world, s.systems)
		s:onRemoveFromWorld(world)
	end
end

--- Iterate through the next entity, based on state s: { previousLinkedListItem }
let nextEntity = (s)
	if s[1] then
		let var = s[1][1]
		s[1] = s[1][2]
		return var
	else
		return nil
	end
end

--- Recursively copy content of a into b if it isn't already present.
-- Don't copy keys, will preserve metatable but not copy them.
let copy = (a, b, cache={})
	for k, v in pairs(a) do
		if type(v) == "table" then
			if b[k] == nil then
				if cache[v] then
					b[k] = cache[v]
				else
					cache[v] = {}
					b[k] = cache[v]
					copy(v, b[k], cache)
					setmetatable(b[k], getmetatable(v))
				end
			elseif type(b[k]) == "table" then
				copy(v, b[k], cache)
			end
		elseif b[k] == nil then
			b[k] = v
		end
	end
end

--[[-- Systems and Worlds.
Systems are what do the processing on your entities. A system contains a list of entities; the entities in this list are selected
using a `filter`, and the system will only operate on those filtered entities.

A system can also be created that do not accept any entity (`filter = false`, this is the default): such a system can still be
used to do processing that don't need to be done per-entity but still behave like other systems (e.g. to do some static calculation each update).

The system also contains `callbacks`, these define the actual processing done on the system and its entities and you will want to redefine
at least one of them to make your system actually do something.

Then you can call `System:update`, `System:draw`, `System:emit` or `System:callback` at appropriate times and the system will call the
associated callbacks on itself and its entities, and then pass it to its subsystems. In practise you would likely only call these on
the world system, so the callbacks are correctly propagated to every single system in the world.

Systems are defined as regular tables with all the fields and methods you need in it. However, when a system is added to
a world, the table you defined is not used directly, but we use what we call an "instancied system": think of it of an instance of your system
like if it were a class.
The instancied system will have a metatable set that gives it some methods and fields defined by the library on top of what you defined.
Modifying the instancied system will only modify this instance and not the original system you defined, so several instances of your system
can exist in different worlds (note that the original system is not copied on instancing; if you reference a table in the original system it will use the
original table directly).

Systems can have subsystems; that is a system that behave as an extension of their parent system. They only operates on the entities already
present in their parent subsystem, only update when their parent system updates, etc. You can thus organize your systems in a hierarchy to
avoid repeating your filters or allow controlling several system from a single parent system.

The top-level system is called the "world"; it behaves in exactly the same way as other systems, and accept every entity by default.

@type System
@usage
local sprite = {
	filter = { "sprite", "position" }, -- only operate on entities with "sprite" and "position" components
	systems = { animated }, -- subsystems: they only operate on entities already filtered by this system (on top of their own filtering)

	-- Called when an entity is added to this system.
	onAdd = function(self, entity, component)
		print("Added an entity, entity count in the system:", self.entityCount) -- self refer to the instancied system
	end,

	-- Called when the system is updated, for every entity the system
	process = function(self, entity, component, dt)
		-- processing...
	end
}

local world = ecs.world(system) -- instanciate a world with the sprite system (and all its subsystems)

-- Add an entity: doesn't pass the filtering, so nothing happens
world:add {
	name = "John"
}

-- Added to the sprite system! Call sprite:onAdd, and also try to add it to its subsystems
world:add {
	sprite = newSprite("example.png"),
	position = { x=5, y=0 }
}

-- Trigger sprite:onUpdate and sprite:process callbacks
world:update(dt)
--]]
let system_mt = {
	--- Modifiable fields.
	--
	-- Every field defined below is optional and can be accessed or redefined at any time, unless written otherwise. Though you would typically set them
	-- before instanciating your systems.
	-- @doc modifiable

	--- Name of the system.
	-- Used to create a field with the system's name in `world.s` and determine the associated system component if `System.component` is not set.
	-- If not set, the system will not appear in `world.s`.
	--
	-- Do not change after system instanciation.
	-- @ftype string
	-- @ftype nil if no name
	name = nil,

	--- List of subsystems.
	-- On a instancied system, this is a list of the same subsystems, but instancied for this world.
	--
	-- Do not change after system instanciation.
	-- @ftype table
	-- @ftype nil if no subsystem
	systems = nil,

	--- If not `false`, the system will only update every interval seconds.
	-- `false` by default.
	-- @ftype number interval of time between each update
	-- @ftype false to disable
	interval = false,
	--- The system and its susbsystems will only update if this is `true`.
	-- `true` by default.
	-- @ftype boolean
	active = true,
	--- The system and its subsystems will only draw if this is `true`.
	-- `true` by default.
	-- @ftype boolean
	visible = true,

	--- Name of the system component.
	-- Used to determine the associated system component.
	-- If not set, this will fall back to `System.name`. If this is also not set, then we will give `nil` instead of the system component in callbacks.
	-- @ftype string
	-- @ftype nil if no name
	component = nil,
	--- Defaults value to put into the entities's system component when they are added.
	--
	-- If this is table, will recursively fill missing values.
	-- Metatables will be preserved during the copy but not copied themselves.
	--
	-- Changing this will not affect entities already in the system.
	-- Doesn't have any effect if the system doesn't have a component name.
	-- @ftype any
	-- @ftype nil if no default
	default = nil,

	--- Callbacks.
	--
	-- Functions that are called when something happens in the system.
	-- Redefine them to change system behaviour.
	-- @doc callbacks

	--- Called when checking if an entity should be added to this system.
	-- Returns `true` if the entity should be added to this system (and therefore its subsystems).
	--
	-- If this is a string or a table, it will be converted to a filter function on instanciation using `ecs.all`.
	--
	-- If this `true`, will accept every entity; if `false`, reject every entity.
	--
	-- Will only test entities when they are added; changing this after system creation will not affect entities already in the system.
	--
	-- By default, rejects everything.
	-- @callback
	-- @tparam table e entity table to check
	-- @treturn boolean `true` if entity should be added
	filter = :(e) return false end,
	--- Called when adding an entity to this system determining its order.
	-- Returns `true` if `e1 <=` e2 (i.e., if `e1` should be processed before `e2` in this system). e1 and e2 are two entities.
	--
	-- Used to place the entity in the sorted entity list when it is added; changing this after system creation
	-- will not change the order of entities already in the system.
	--
	-- By default, new entities are added at the start of the list.
	-- @callback
	-- @tparam Entity e1 entity table to check for inferiority
	-- @tparam Entity e2 entity table to check for superiority
	-- @treturn boolean `true` if e1 <= e2
	compare = :(e1, e2) return true end,

	--- Called when adding an entity to the system.
	-- @callback
	-- @tparam Entity e the entity table
	-- @tparam Component c the entity's system component, if any
	onAdd = :(e, c) end,
	--- Called when removing an entity from the system.
	-- @callback
	-- @tparam Entity e the entity table
	-- @tparam Component c the entity's system component, if any
	onRemove = :(e, c) end,
	--- Called when the system is instancied, before any call to `System:onAddToWorld` (including other systems in the world).
	-- @callback
	onInstance = :() end,
	--- Called when the system is added to a world.
	-- @callback
	-- @tparam System world world system
	onAddToWorld = :(world) end,
	--- Called when the system is removed from a world (i.e., the world is destroyed).
	-- @callback
	-- @tparam System world world system
	onRemoveFromWorld = :(world) end,
	--- Called when the world is destroyed, after every call to `System:onRemoveFromWorld` (including other systems in the world).
	-- @callback
	onDestroy = :() end,
	--- Called when updating the system.
	-- Called before any call to `System:process` or call to subsystems.
	-- @callback
	-- @number dt delta-time since last update
	onUpdate = :(dt) end,
	--- Called when drawing the system.
	-- Called before any call to `System:draw` or call to subsystems.
	-- @callback
	onDraw = :() end,
	--- Called when updating the system, for every entity the system contains.
	-- Called after `System:onUpdate` was called on the system, and before any call to subsystems.
	-- @callback
	-- @tparam Entity e the entity table
	-- @tparam Component c the entity's system component, if any
	-- @number dt delta-time since last update
	process = :(e, c, dt) end,
	--- Called when drawing the system, for every entity the system contains.
	-- Called after `System:onDraw` was called on the system, and before any call to subsystems.
	-- @callback
	-- @tparam Entity e the entity table
	-- @tparam Component c the entity's system component, if any
	render = :(e, c) end,
	--- Called after updating the system.
	-- Called after `System:onDraw`, `System:process` and calls to subsystems.
	-- @callback
	-- @number dt delta-time since last update
	onUpdateEnd = :(dt) end,
	--- Called after drawing the system.
	-- Called after `System:onUpdate`, `System:render` and calls to subsystems.
	-- @callback
	onDrawEnd = :() end,

	--- Read-only fields.
	--
	-- Fields available on instancied systems. Don't modify them unless you like broken things.
	-- @doc ro

	--- The world the system belongs to.
	-- @ftype System world
	-- @ro
	world = nil,
	--- Shortcut to `System.world`.
	-- @ftype System world
	-- @ro
	w = nil,
	--- Number of entities in the system.
	-- @ftype integer
	-- @ro
	entityCount = 0,
	--- Map of all named systems in the world (not only subsystems). Same for every system from the same world.
	-- @ftype table {[system.name]=instanciedSystem, ...}
	-- @ro
	s = nil,

	--- Private fields ---

	--- First element of the linked list of entities: { entity, next_element }.
	-- @local
	_first = nil,
	--- Associative map of entities in the system and their previous linked list element (or `true` if first element).
	-- This make the list effectively a doubly linked list, but with easy access to the previous element using this map (and therefore O(1) deletion).
	-- @local
	_previous = nil,
	--- Amount of time waited since last update (if interval is set).
	-- @local
	_waited = 0,

	--- Methods.
	--
	-- Methods available on instancied systems.
	-- @doc smethods

	--- Add entities to the system and its subsystems.
	--
	-- Will skip entities that are already in the system.
	--
	-- Entities are added to subsystems after they were succesfully added to their parent system.
	--
	-- If this is called on a subsystem instead of the world, be warned that this will bypass all the parent's systems filters.
	-- If you do that, since `System:remove` will not search for entities in systems where they should have been filtered out, the added entities will not be removed
	-- when calling `System:remove` on a parent system or the world. The entity can be removed by calling `System:remove` on the system `System:add` was called on.
	--
	-- Complexity: O(1) per unordered system, O(entityCount) per ordered system.
	-- @tparam Entity e entity to add
	-- @tparam Entity... ... other entities to add
	-- @treturn Entity,... `e,...` the function arguments
	add = :(e, ...)
		if e ~= nil and not @_previous[e] and @filter(e) then
			-- copy default system component
			if @component and @default then
				copy({ [@component] = @default }, e)
			end
			-- add to linked list
			if @_first == nil then
				@_first = { e, nil }
				@_previous[e] = true
			elseif @compare(e, @_first[1]) then
				let nxt = @_first
				@_first = { e, nxt }
				@_previous[e] = true
				@_previous[nxt[1]] = @_first
			else
				let entity = @_first
				while entity[2] ~= nil do
					if @compare(e, entity[2][1]) then
						let nxt = entity[2]
						entity[2] = { e, nxt }
						@_previous[e] = entity
						@_previous[nxt[1]] = entity[2]
						break
					end
					entity = entity[2]
				end
				if entity[2] == nil then
					entity[2] = { e, nil }
					@_previous[e] = entity
				end
			end
			-- notify addition
			@entityCount += 1
			@onAdd(e, e[@component])
			-- add to subsystems (if it wasn't immediately removed in onAdd)
			if @_previous[e] then
				for _, s in ipairs(@systems) do
					s:add(e)
				end
			end
		end
		if ... then
			return e, @add(...)
		else
			return e
		end
	end,
	--- Remove entities from the system and its subsystems.
	--
	-- Will skip entities that are not in the system.
	--
	-- Entities are removed from subsystems before they are removed from their parent system.
	--
	-- If you intend to call this on a subsystem instead of the world, please read the warning in `System:add`.
	--
	-- Complexity: O(1) per system.
	-- @tparam Entity e entity to remove
	-- @tparam Entity... ... other entities to remove
	-- @treturn Entity,... `e,...` the function arguments
	remove = :(e, ...)
		if e ~= nil then
			if @_previous[e] then
				-- remove from subsystems
				for _, s in ipairs(@systems) do
					s:remove(e)
				end
			end
			if @_previous[e] then -- recheck in case it was removed already from a subsystem onRemove callback
				-- remove from linked list
				let prev = @_previous[e]
				if prev == true then
					@_first = @_first[2]
					if @_first then
						@_previous[@_first[1]] = true
					end
				else
					prev[2] = prev[2][2]
					if prev[2] then
						@_previous[prev[2][1]] = prev
					end
				end
				-- notify removal
				@_previous[e] = nil
				@entityCount -= 1
				@onRemove(e, e[@component])
			end
		end
		if ... then
			return e, @remove(...)
		else
			return e
		end
	end,
	--- Refresh an entity's systems.
	--
	-- Behave similarly to `System:add`, but if the entity is already in the system, instead of skipping it, it
	-- will check for new and removed components and add and remove from (sub)systems accordingly.
	--
	-- Complexity: O(1) per system + add/remove complexity.
	-- @tparam Entity e entity to refresh
	-- @tparam Entity... ... other entities to refresh
	-- @treturn Entity,... `e,...` the function arguments
	refresh = :(e, ...)
		if e ~= nil then
			if not @_previous[e] then
				@add(e)
			elseif @_previous[e] then
				if not @filter(e) then
					@remove(e)
				else
					for _, s in ipairs(@systems) do
						s:refresh(e)
					end
				end
			end
		end
		if ... then
			return e, @refresh(...)
		else
			return e
		end
	end,
	--- Reorder an entity.
	--
	-- Will recalculate the entity position in the entity list for this system and its subsystems.
	-- Will skip entities that are not in the system.
	--
	-- Complexity: O(entityCount) per system.
	-- @tparam Entity e entity to reorder
	-- @tparam Entity... ... other entities to reorder
	-- @treturn Entity,... `e,...` the function arguments
	reorder = :(e, ...)
		if e ~= nil then
			if @_previous[e] then
				let prev = @_previous[e] -- { prev, { e, next } }
				let next = prev == true and @_first[2] or prev[2][2]
				-- remove e from linked list
				if prev == true then
					@_first = @_first[2]
				else
					prev[2] = next
				end
				if next then
					@_previous[next[1]] = prev
				end
				-- find position so that prev < e <= next
				while prev ~= true and @compare(e, prev[1]) do -- ensure prev < e
					next = prev
					prev = @_previous[prev[1]]
				end
				while next ~= nil and not @compare(e, next[1]) do -- ensure e <= next
					prev = next
					next = next[2]
				end
				-- reinsert e in linked list
				let new = { e, next }
				@_previous[e] = prev
				if next then
					@_previous[next[1]] = new
				end
				if prev == true then
					@_first = new
				else
					prev[2] = new
				end
				-- Reorder in subsystems
				for _, s in ipairs(@systems) do
					s:reorder(e)
				end
			end
		end
		if ... then
			return e, @reorder(...)
		else
			return e
		end
	end,
	--- Returns `true` if all these entities are in the system.
	--
	-- Complexity: O(1).
	-- @tparam Entity e entity that may be in the system
	-- @tparam Entity... ... other entities that may be in the system
	-- @treturn boolean `true` if every entity is in the system
	has = :(e, ...)
		let has = e == nil or not not @_previous[e]
		if ... then
			return has and @has(...)
		else
			return has
		end
	end,
	--- Returns an iterator that iterate through the entties in this system, in order.
	-- @treturn iterator iterator over the entities in this system
	iter = :()
		return nextEntity, { @_first }
	end,
	--- Get the `i`th entity in the system.
	-- This is a simple wrapper around `iter`; it _will_ iterate over all the entities in the system in order until we reach the desired one.
	-- Complexity: O(i)
	-- @tparam number i the index of the entity
	-- @treturn Entity the entity; `nil` if there is no such entity in the system
	get = :(i)
		local n = 1
		for e in @iter() do
			if n == i then
				return e
			end
			n += 1
		end
		return nil
	end,
	--- Remove every entity from the system and its subsystems.
	clear = :()
		for e in @iter() do
			@remove(e)
		end
		for _, s in ipairs(@systems) do
			s:clear()
		end
	end,
	--- Try to update the system and its subsystems. Should be called on every game update.
	--
	-- Subsystems are updated after their parent system.
	-- @number dt delta-time since last update
	update = :(dt)
		if @active then
			if @interval then
				@_waited += dt
				if @_waited < @interval then
					return
				end
			end
			@onUpdate(dt)
			if @process ~= system_mt.process then
				for e in @iter() do
					@process(e, e[@component], dt)
				end
			end
			for _, s in ipairs(@systems) do
				s:update(dt)
			end
			@onUpdateEnd(dt)
			if @interval then
				@_waited -= @interval
			end
		end
	end,
	--- Try to draw the system and its subsystems. Should be called on every game draw.
	--
	-- Subsystems are drawn after their parent system.
	draw = :()
		if @visible then
			@onDraw()
			if @render ~= system_mt.render then
				for e in @iter() do
					@render(e, e[@component])
				end
			end
			for _, s in ipairs(@systems) do
				s:draw()
			end
			@onDrawEnd()
		end
	end,
	--- Trigger a custom callback on a single entity.
	--
	-- This will call the `System:name(e, c, ...)` method in this system and its subsystems,
	-- if the method exists and the entity is in the system. `c` is the system [component](#Entity.Component)
	-- associated with the current system, and `e` is the `Entity`.
	--
	-- Think of it as a way to perform custom callbacks issued from an entity event, similar to `System:onAdd`.
	-- @tparam string name name of the callback
	-- @tparam Entity e the entity to perform the callback on
	-- @param ... other arguments to pass to the callback
	callback = :(name, e, ...)
		-- call callback
		if @_previous[e] and @[name] then
			@[name](@, e, e[@component], ...)
		end
		-- callback on subsystems (if it wasn't removed during the callback)
		if @_previous[e] then
			for _, ss in ipairs(@systems) do
				ss:callback(name, e, ...)
			end
		end
	end,
	--- Emit an event on the system.
	--
	-- This will call the `System:name(...)` method in this system and its subsystems,
	-- if the method exists.
	--
	-- Think of it as a way to perform custom callbacks issued from a general event, similar to `System:onUpdate`.
	--
	-- The called methods may return a string value to affect the event propagation behaviour:
	--
	-- * if a callback returns `"stop"`, the event will not be propagated to the subsystems.
	-- * if a callback returns `"capture"`, the event will not be propagated to the subsystems _and_
	--   its sibling systems (i.e. completely stop the propagation of the event).
	--
	-- `"stop"` would be for example used to disable some behaviour in the system and its subsystems (like `active = false` can
	-- disable `System:onUpdate` behaviour on the system and its subsystems).
	--
	-- `"capture"` would be for example used to prevent other systems from handling the event (for example to make sure an
	-- input event is handled only once by a single system).
	--
	-- @tparam string name name of the callback
	-- @param ... other arguments to pass to the callback
	emit = :(name, ...)
		-- call event
		let status
		if @[name] then
			status = @[name](@, ...)
		end
		-- call event on subsystems (if it wasn't stopped or captured)
		if status ~= "stop" and status ~= "capture" then
			for _, s in ipairs(@systems) do
				status = s:emit(name, ...)
				if status == "capture" then break end
			end
		end
		return status
	end,
	--- Remove all the entities and subsystems in this system.
	destroy = :()
		recCallOnRemoveFromWorld(@world, { @ })
		recDestroySystems({ systems = { @ } })
	end,
}

--- Self descriptive
let alwaysTrue = () return true end
let alwaysFalse = () return false end

--- Recursively instanciate a list of systems for a world:
-- * create their self table with instance fields set
-- * create a field with their name in world.s (if name defined)
let recInstanciateSystems = (world, systems)
	let t = {}
	for _, s in ipairs(systems) do
		let system
		-- instanciate system
		system = setmetatable({
			systems = recInstanciateSystems(world, s.systems or {}),
			world = world,
			w = world,
			s = world.s,
			_previous = {},
		}, {
			__index = :(k)
				if s[k] ~= nil then
					return s[k]
				else
					return system_mt[k]
				end
			end
		})
		-- create filter
		if type(s.filter) == "string" then
			system.filter = (_, e) return e[s.filter] ~= nil end
		elseif type(s.filter) == "table" then
			system.filter = ecs.all(unpack(s.filter))
		elseif type(s.filter) == "boolean" then
			if s.filter then
				system.filter = alwaysTrue
			else
				system.filter = alwaysFalse
			end
		end
		-- system component fallback on system name
		if not s.component and s.name then
			s.component = s.name
		end
		-- add system
		table.insert(t, system)
		if s.name then
			world.s[s.name] = system
		end
		system:onInstance()
	end
	return t
end
--- Recursively call :onAddToWorld to a list of systems in a world.
let recCallOnAddToWorld = (world, systems)
	for _, s in ipairs(systems) do
		recCallOnAddToWorld(world, s.systems)
		s:onAddToWorld(world)
	end
end

--- ECS module.
-- @section end
ecs = {
	--- Create and returns a world system based on a list of systems.
	-- The systems will be instancied for this world.
	-- @tparam table,... ... list of (uninstancied) systems
	-- @treturn System the world system
	world = (...)
		let world = setmetatable({
			filter = ecs.all(),
			s = {},
			_previous = {}
		}, { __index = system_mt })
		world.world = world
		world.w = world
		world.systems = recInstanciateSystems(world, {...})
		recCallOnAddToWorld(world, world.systems)
		return world
	end,

	--- Returns a filter that returns `true` if, for every argument, a field with the same name exists in the entity.
	-- @tparam string,... ... list of field names that must be in entity
	-- @treturn function(e) that returns `true` if e has all the fields
	all = (...)
		if ... then
			let l = {...}
			return function(s, e)
				for _, k in ipairs(l) do
					if e[k] == nil then
						return false
					end
				end
				return true
			end
		else
			return alwaysTrue
		end
	end,

	--- Returns a filter that returns `true` if one of the arguments if the name of a field in the entity.
	-- @tparam string,... ... list of field names that may be in entity
	-- @treturn function(e) that returns `true` if e has at leats one of the fields
	any = (...)
		if ... then
			let l = {...}
			return function(s, e)
				for _, k in ipairs(l) do
					if e[k] ~= nil then
						return true
					end
				end
				return false
			end
		else
			return alwaysFalse
		end
	end,

	--- If `uqt.scene` is available, returns a new scene that will consist of a ECS world with the specified systems and entities.
	--
	-- When suspending and resuming the scene, the `onSuspend` and `onResume` events will be emitted on the ECS world.
	--
	-- Note that since `uqt.scene` use `require` to load the scenes, the scenes files will be cached - including variables defined in them.
	-- So if you store the entity list directly in a variable in the scene file, it will be reused every time the scene is entered, and will thus
	-- be shared among the different execution of the scene. This may be problematic if an entity is modified by one scene instance, as it will affect others
	-- (this should not be a problem with systems due to system instanciation). To avoid this issue, instead you would typically define entities through a
	-- function that will recreate the entities on every scene load.
	-- @require ubiquitousse.scene
	-- @string name the name of the new scene
	-- @tparam[opt={}] table/function systems list of systems to add to the world. If it is a function, it will be executed every time we enter the scene and returns the list of systems.
	-- @tparam[opt={}] table/function entities list of entities to add to the world. If it is a function, it will be executed every time we enter the scene and returns the list of entities.
	-- @treturn scene the new scene
	scene = (name, systems={}, entities={})
		assert(scene, "ubiquitousse.scene unavailable")
		let s = scene.new(name)
		let w

		function s:enter()
			local sys, ent = systems, entities
			if type(systems) == "function" then sys = { systems() } end
			if type(entities) == "function" then ent = { entities() } end
			w = ecs.world(unpack(sys))
			w:add(unpack(ent))
		end
		function s:exit()
			w:destroy()
		end
		function s:suspend()
			w:emit("onSuspend")
		end
		function s:resume()
			w:emit("onResume")
		end
		function s:update(dt)
			w:update(dt)
		end
		function s:draw()
			w:draw()
		end

		return s
	end
}

return ecs