--- ECS (entity compenent system) library
--
-- Entity Component System library, inspired by the excellent tiny-ecs. Main differences include:
--
-- * ability to nest systems;
-- * instanciation of systems for each world (no shared state);
-- * adding and removing entities is done instantaneously
-- * ability to add and remove components from entities after they were added to the world.
--
-- No mandatory dependency.
-- Optional dependency: `ubiquitousse.scene`, to allow quick creation of ECS-based scenes.
--
-- The module returns a table that contains several functions, `world` or `scene` are starting points
-- to create your world.
--
-- @module ecs
-- @usage TODO
local loaded, scene = pcall(require, (...):match("^(.-)ecs").."scene")
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)
-- more generic events?

-- TODO: :reorder (like refresh but update order)

-- TODO: clarify documentation on entity tables and instanciated system table

--- Entity table.
-- TODO
-- @section Entity

--- Entity system table.
-- @doc entity

--- @table entity
-- @field .. d

--- 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. No cycle detection.
let copy = (a, b)
	for k, v in pairs(a) do
		if type(v) == "table" then
			if b[k] == nil then
				b[k] = {}
				copy(v, b[k])
			elseif b[k] == "table" then
				copy(v, b[k])
			end
		elseif b[k] == nil then
			b[k] = v
		end
	end
end

--- System fields and methods.
--
-- When they are added to a world, a new, per-world self table is created and used for every method call (which we call "instancied system").
-- Instancied systems can be retrieved in `system.s` or `system.systems`.
--
-- The "world" is just the top-level system, behaving in exactly the same way as other systems.
--
-- Every field defined below is optional and can be accessed or redefined at any time, unless written otherwise. Though you would typically set them
-- when creating your system.
-- @type System
let system_mt = {
	--- Modifiable fields
	-- @doc modifiable

	--- Name of the system.
	-- Used to create a field with the system's name in `world.s` and into each entity (the "entity's system table") that's in this system.
	-- If not set, the entity will not have a system table.
	--
	-- 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,

	--- Defaults value to put into the entities's system table when they are added. Will recursively fill missing values.
	--
	-- When an entity is added to a system, a `.entity` field is always set in the system table, referring to the full entity table.
	--
	-- Changing this will not affect entities already in the system.
	-- @ftype table
	-- @ftype nil if no default
	default = nil,
	--- Defaults methods to assign to the entities's system table when they are added.
	--
	-- When calling the methods with `entity.systemName:method(...)`, the method will actually receive the
	-- arguments method(system, `system table, ...)`. Methamethods are accepted. New methods can be
	-- created anytime.
	-- @ftype table
	-- @ftype nil if no methods
	methods = 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.any.
	--
	-- 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. 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, entities are in the same order they were inserted.
	-- @callback
	-- @tparam table e1 entity table to check for inferiority
	-- @tparam table 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 table s the entity's system table
	onAdd = :(s) end,
	--- Called when removing an entity from the system.
	-- @callback
	-- @tparam table s the entity's system table
	onRemove = :(s) 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.
	-- @callback
	-- @number dt delta-time since last update
	onUpdate = :(dt) end,
	--- Called when drawing the system.
	-- @callback
	onDraw = :() end,
	--- Called when updating the system, for every entity the system contains. Called after `System:onUpdate` was called on the system.
	-- @callback
	-- @tparam table s the entity's system table
	-- @number dt delta-time since last update
	process = :(s, dt) end,
	--- Called when drawing the system, for every entity the system contains. Called after `System:onDraw` was called on the system.
	-- @callback
	-- @tparam table s the entity's system table
	render = :(s) end,

	--- Read-only fields
	-- @doc ro

	--- The world the system belongs to.
	-- @ftype System world
	-- @ro
	world = 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,
	--- Metatable of entities' system table.
	-- Contains the methods defined in the methods field, wrapped to be called with the correct arguments, as well as a
	-- __index field (if not redefined in methods).
	-- @local
	_methods_mt = nil,

	--- Methods.
	--
	-- Methods available on instancied system table.
	-- @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.
	--
	-- 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 :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 table e entity to add
	-- @tparam table... ... other entities to add
	-- @treturn e,... the function arguments
	add = :(e, ...)
		if e ~= nil and not @_previous[e] and @filter(e) then
			-- setup entity
			if @name then
				if not e[@name] e[@name] = {}
				if @default copy(@default, e[@name])
				if @methods setmetatable(e[@name], @_methods_mt)
				e[@name].entity = 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[@name])
			-- add to subsystems
			for _, s in ipairs(@systems) do
				s:add(e)
			end
		end
		if ... then
			return e, @add(...)
		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 table e entity to refresh
	-- @tparam table... ... other entities to refresh
	-- @treturn 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,
	--- Remove entities to 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`.
	--
	-- Returns all removed entities.
	--
	-- Complexity: O(1) per system.
	-- @tparam table e entity to remove
	-- @tparam table... ... other entities to remove
	-- @treturn 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[@name])
			end
		end
		if ... then
			return e, @remove(...)
		else
			return e
		end
	end,
	--- Returns true if every entity is in the system.
	--
	-- Complexity: O(1).
	-- @tparam table e entity that may be in the system
	-- @tparam table... ... 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.
	-- @treturn iterator iterator over the entities in this system, in order
	iter = :()
		return nextEntity, { @_first }
	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[@name], dt)
				end
			end
			for _, s in ipairs(@systems) do
				s:update(dt)
			end
			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[@name])
				end
			end
			for _, s in ipairs(@systems) do
				s:draw()
			end
		end
	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 true 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
		-- setup method table
		let methods_mt = {}
		if s.methods then
			methods_mt.__index = methods_mt
			for k, v in pairs(s.methods) do
				methods_mt[k] = :(...)
					return v(system, @, ...)
				end
			end
			setmetatable(s.methods, {
				__newindex = :(k, v)
					rawset(@, k, v)
					methods_mt[k] = :(...)
						return v(system, @, ...)
					end
				end
			})
		end
		-- instanciate system
		system = setmetatable({
			systems = recInstanciateSystems(world, s.systems or {}),
			world = world,
			s = world.s,
			_previous = {},
			_methods_mt = methods_mt
		}, {
			__index = :(k)
				if s[k] ~= nil then
					return s[k]
				else
					return system_mt[k]
				end
			end
		})
		if type(s.filter) == "string" then
			system.filter = (_, e) return e[s.filter] ~= nil end
		elseif type(s.filter) == "table" then
			system.filter = ecs.any(unpack(s.filter))
		elseif type(s.filter) == "boolean" then
			if s.filter then
				system.filter = alwaysTrue
			else
				system.filter = alwaysFalse
			end
		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) system tables
	-- @treturn System the world system
	world = (...)
		let world = setmetatable({
			filter = ecs.all(),
			s = {},
			_previous = {}
		}, { __index = system_mt })
		world.world = 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.
	-- @require ubiquitousse.scene
	-- @string name the name of the new scene
	-- @tparam[opt={}] table systems list of systems to add to the world
	-- @tparam[opt={}] table entities list of entities to add to the world
	-- @treturn scene the new scene
	scene = (name, systems={}, entities={})
		assert(scene, "ubiquitousse.scene unavailable")
		let s = scene.new(name)
		let w

		function s:enter()
			w = ecs.world(unpack(systems))
			w:add(unpack(entities))
		end
		function s:exit()
			w:destroy()
		end
		function s:update(dt)
			w:update(dt)
		end
		function s:draw()
			w:draw()
		end

		return s
	end
}

return ecs