jecs/src/init.luau

--!optimize 2
--!native
--!strict
--draft 4

type i53 = number
type i24 = number

type Ty = { i53 }
type ArchetypeId = number

type Column = { any }

type ArchetypeEdge = {
	add: Archetype,
	remove: Archetype,
}

type Archetype = {
	id: number,
	edges: { [i53]: ArchetypeEdge },
	types: Ty,
	type: string | number,
	entities: { number },
	columns: { Column },
	records: { [number]: number },
}

type Record = {
	archetype: Archetype,
	row: number,
	dense: i24,
	componentRecord: ArchetypeMap,
}

type EntityIndex = { dense: { [i24]: i53 }, sparse: { [i53]: Record } }

type ArchetypeRecord = number
--[[
TODO:
{
	index: number,
	count: number,
	column: number
}

]]

type ArchetypeMap = {
	cache: { ArchetypeRecord },
	first: ArchetypeMap,
	second: ArchetypeMap,
	parent: ArchetypeMap,
	size: number,
}

type ComponentIndex = { [i24]: ArchetypeMap }

type Archetypes = { [ArchetypeId]: Archetype }

type ArchetypeDiff = {
	added: Ty,
	removed: Ty,
}

local HI_COMPONENT_ID 		= 256

local EcsOnAdd 				= HI_COMPONENT_ID + 1
local EcsOnRemove 			= HI_COMPONENT_ID + 2
local EcsOnSet 				= HI_COMPONENT_ID + 3
local EcsWildcard 			= HI_COMPONENT_ID + 4
local EcsChildOf 			= HI_COMPONENT_ID + 5
local EcsComponent  		= HI_COMPONENT_ID + 6
local EcsRest 				= HI_COMPONENT_ID + 7

local ECS_PAIR_FLAG 		= 0x8
local ECS_ID_FLAGS_MASK 	= 0x10
local ECS_ENTITY_MASK 		= bit32.lshift(1, 24)
local ECS_GENERATION_MASK 	= bit32.lshift(1, 16)

local function addFlags(isPair: boolean): number
	local typeFlags = 0x0

	if isPair then
		typeFlags = bit32.bor(typeFlags, ECS_PAIR_FLAG) -- HIGHEST bit in the ID.
	end
	if false then
		typeFlags = bit32.bor(typeFlags, 0x4) -- Set the second flag to true
	end
	if false then
		typeFlags = bit32.bor(typeFlags, 0x2) -- Set the third flag to true
	end
	if false then
		typeFlags = bit32.bor(typeFlags, 0x1) -- LAST BIT in the ID.
	end

	return typeFlags
end

local function ECS_COMBINE(source: number, target: number): i53
	return (source * 268435456) + (target * ECS_ID_FLAGS_MASK)
end

local function ECS_IS_PAIR(e: number): boolean
	return if e > ECS_ENTITY_MASK then (e % ECS_ID_FLAGS_MASK) // ECS_PAIR_FLAG ~= 0 else false
end

-- HIGH 24 bits LOW 24 bits
local function ECS_GENERATION(e: i53): i24
	return if e > ECS_ENTITY_MASK then (e // ECS_ID_FLAGS_MASK) % ECS_GENERATION_MASK else 0
end

local function ECS_GENERATION_INC(e: i53)
	if e > ECS_ENTITY_MASK then
		local flags = e // ECS_ID_FLAGS_MASK
		local id = flags // ECS_ENTITY_MASK
		local generation = flags % ECS_GENERATION_MASK

		return ECS_COMBINE(id, generation + 1) + flags
	end
	return ECS_COMBINE(e, 1)
end

-- FIRST gets the high ID
local function ECS_ENTITY_T_HI(e: i53): i24
	return if e > ECS_ENTITY_MASK then (e // ECS_ID_FLAGS_MASK) % ECS_ENTITY_MASK else e
end

-- SECOND
local function ECS_ENTITY_T_LO(e: i53): i24
	return if e > ECS_ENTITY_MASK then (e // ECS_ID_FLAGS_MASK) // ECS_ENTITY_MASK else e
end

local function STRIP_GENERATION(e: i53): i24
	return ECS_ENTITY_T_LO(e)
end

local function ECS_PAIR(pred: i53, obj: i53): i53
	return ECS_COMBINE(ECS_ENTITY_T_LO(obj), ECS_ENTITY_T_LO(pred)) + addFlags(--[[isPair]] true) :: i53
end

local ERROR_ENTITY_NOT_ALIVE = "Entity is not alive"
local ERROR_GENERATION_INVALID = "INVALID GENERATION"

local function getAlive(index: EntityIndex, e: i24): i53
	local denseArray = index.dense
    local id = denseArray[ECS_ENTITY_T_LO(e)]

	if id then
		local currentGeneration = ECS_GENERATION(id)
		local gen = ECS_GENERATION(e)
		if gen == currentGeneration then
			return id
		end

		error(ERROR_GENERATION_INVALID)
	end

	error(ERROR_ENTITY_NOT_ALIVE)
end

local function sparseGet(entityIndex, id)
	return entityIndex.sparse[getAlive(entityIndex, id)]
end

-- ECS_PAIR_FIRST, gets the relationship target / obj / HIGH bits
local function ECS_PAIR_RELATION(entityIndex, e)
	return getAlive(entityIndex, ECS_ENTITY_T_HI(e))
end

-- ECS_PAIR_SECOND gets the relationship / pred / LOW bits
local function ECS_PAIR_OBJECT(entityIndex, e)
	return getAlive(entityIndex, ECS_ENTITY_T_LO(e))
end

local function nextEntityId(entityIndex: EntityIndex, index: i24): i53
	--local id = ECS_COMBINE(index, 0)
	local id = index
	entityIndex.sparse[id] = {
		dense = index,
	} :: Record
	entityIndex.dense[index] = id

	return id
end

local function transitionArchetype(entityIndex: EntityIndex, to: Archetype,
	destinationRow: i24, from: Archetype, sourceRow: i24)

	local columns = from.columns
	local sourceEntities = from.entities
	local destinationEntities = to.entities
	local destinationColumns = to.columns
	local tr = to.records
	local types = from.types

	for i, column in columns do
		-- Retrieves the new column index from the source archetype's record from each component
		-- We have to do this because the columns are tightly packed and indexes may not correspond to each other.
		local targetColumn = destinationColumns[tr[types[i]]]

		-- Sometimes target column may not exist, e.g. when you remove a component.
		if targetColumn then
			targetColumn[destinationRow] = column[sourceRow]
		end
		-- If the entity is the last row in the archetype then swapping it would be meaningless.
		local last = #column
		if sourceRow ~= last then
			-- Swap rempves columns to ensure there are no holes in the archetype.
			column[sourceRow] = column[last]
		end
		column[last] = nil
	end

	local sparse = entityIndex.sparse
	local movedAway = #sourceEntities

	-- Move the entity from the source to the destination archetype.
	-- Because we have swapped columns we now have to update the records
	-- corresponding to the entities' rows that were swapped.
	local e1 = sourceEntities[sourceRow]
	local e2 = sourceEntities[movedAway]

	if sourceRow ~= movedAway then
		sourceEntities[sourceRow] = e2
	end

	sourceEntities[movedAway] = nil :: any
	destinationEntities[destinationRow] = e1

	local record1 = sparse[e1]
	local record2 = sparse[e2]

	record1.row = destinationRow
	record2.row = sourceRow
end

local function archetypeAppend(entity: number, archetype: Archetype): number
	local entities = archetype.entities
	local length = #entities + 1
	entities[length] = entity
	return length
end

local function newEntity(entityId: i53, record: Record, archetype: Archetype): Record
	local row = archetypeAppend(entityId, archetype)
	record.archetype = archetype
	record.row = row
	return record
end

local function moveEntity(entityIndex: EntityIndex, entityId: i53, record: Record, to: Archetype)
	local sourceRow = record.row
	local from = record.archetype
	local destinationRow = archetypeAppend(entityId, to)
	transitionArchetype(entityIndex, to, destinationRow, from, sourceRow)
	record.archetype = to
	record.row = destinationRow
end

local function hash(arr: { number }): string
	return table.concat(arr, "_")
end

local function ensureComponentRecord(
	componentIndex: ComponentIndex,
	componentId: number
): ArchetypeMap
	local archetypesMap = componentIndex[componentId]

	if not archetypesMap then
		archetypesMap = ({ size = 0, cache = {} } :: any) :: ArchetypeMap
		componentIndex[componentId] = archetypesMap
	end

	return archetypesMap
end

local function ECS_ID_IS_WILDCARD(e: i53): boolean
	assert(ECS_IS_PAIR(e))
	local first = ECS_ENTITY_T_HI(e)
	local second = ECS_ENTITY_T_LO(e)
	return first == EcsWildcard or second == EcsWildcard
end

local function archetypeOf(world: any, types: { i24 }, prev: Archetype?): Archetype
	local ty = hash(types)

	local id = world.nextArchetypeId + 1
	world.nextArchetypeId = id

	local length = #types
	local columns = (table.create(length) :: any) :: { Column }
	local componentIndex = world.componentIndex

	local records = {}
	for i, componentId in types do
		local idr = ensureComponentRecord(componentIndex, componentId)
		idr.cache[id] = i
		records[componentId] = i
		if ECS_IS_PAIR(componentId) then
			local relation = ECS_PAIR_RELATION(world.entityIndex, componentId)
			local object = ECS_PAIR_OBJECT(world.entityIndex, componentId)

			local r = ECS_PAIR(relation, EcsWildcard)
			local idr_r = ensureComponentRecord(componentIndex, r)

			local o = ECS_PAIR(EcsWildcard, object)
			local idr_o = ensureComponentRecord(componentIndex, o)

			records[r] = i
			records[o] = i

			idr_r.cache[id] = i
			idr_o.cache[id] = i

			idr_r.size += 1
			idr_o.size += 1
		end
		columns[i] = {}
	end

	local archetype: Archetype = {
		columns = columns,
		edges = {},
		entities = {},
		id = id,
		records = records,
		type = ty,
		types = types,
	}

	world.archetypeIndex[ty] = archetype
	world.archetypes[id] = archetype

	return archetype
end

export type World = {
	archetypeIndex: { [string]: Archetype },
	archetypes: Archetypes,
	componentIndex: ComponentIndex,
	entityIndex: EntityIndex,
	nextArchetypeId: number,
	nextComponentId: number,
	nextEntityId: number,
	ROOT_ARCHETYPE: Archetype
}

local function entity(world: World): i53
	local entityId = world.nextEntityId + 1
	world.nextEntityId = entityId
	return nextEntityId(world.entityIndex, entityId + EcsRest)
end

-- TODO:
-- should have an additional `nth` parameter which selects the nth target
-- this is important when an entity can have multiple relationships with the same target
local function target(world: World, entity: i53, relation: i24--[[, nth: number]]): i24?
	local entityIndex = world.entityIndex
	local record = entityIndex.sparse[entity]
	local archetype = record.archetype
	if not archetype then
		return nil
	end

	local componentRecord = world.componentIndex[ECS_PAIR(relation, EcsWildcard)]
	if not componentRecord then
		return nil
	end

	local archetypeRecord = componentRecord.cache[archetype.id]
	if not archetypeRecord then
		return nil
	end

	return ECS_PAIR_OBJECT(entityIndex, archetype.types[archetypeRecord])
end

local function parent(world: World, entity: i53)
	return target(world, entity, EcsChildOf)
end

local function ensureArchetype(world: World, types, prev): Archetype
	if #types < 1 then
		return world.ROOT_ARCHETYPE
	end

	local ty = hash(types)
	local archetype = world.archetypeIndex[ty]
	if archetype then
		return archetype
	end

	return archetypeOf(world, types, prev)
end

local function findInsert(types: { i53 }, toAdd: i53): number
	for i, id in types do
		if id == toAdd then
			return -1
		end
		if id > toAdd then
			return i
		end
	end
	return #types + 1
end

local function findArchetypeWith(world: World, node: Archetype, componentId: i53): Archetype
	local types = node.types
	-- Component IDs are added incrementally, so inserting and sorting
	-- them each time would be expensive. Instead this insertion sort can find the insertion
	-- point in the types array.

	local destinationType = table.clone(node.types) :: { i53 }
	local at = findInsert(types, componentId)
	if at == -1 then
		-- If it finds a duplicate, it just means it is the same archetype so it can return it
		-- directly instead of needing to hash types for a lookup to the archetype.
		return node
	end
	table.insert(destinationType, at, componentId)

	return ensureArchetype(world, destinationType, node)
end

local function ensureEdge(archetype: Archetype, componentId: i53): ArchetypeEdge
	local edges = archetype.edges
	local edge = edges[componentId]
	if not edge then
		edge = {} :: any
		edges[componentId] = edge
	end
	return edge
end

local function archetypeTraverseAdd(world: World, componentId: i53, from: Archetype): Archetype
	from = from or world.ROOT_ARCHETYPE

	local edge = ensureEdge(from, componentId)
	local add = edge.add
	if not add then
		-- Save an edge using the component ID to the archetype to allow
		-- faster traversals to adjacent archetypes.
		add = findArchetypeWith(world, from, componentId)
		edge.add = add :: never
	end

	return add
end

local function add(world: World, entityId: i53, componentId: i53)
	local entityIndex = world.entityIndex
	local record = entityIndex.sparse[entityId]
	local from = record.archetype
	local to = archetypeTraverseAdd(world, componentId, from)
	if from == to then
		return
	end
	if from then
		moveEntity(entityIndex, entityId, record, to)
	else
		if #to.types > 0 then
			newEntity(entityId, record, to)
		end
	end
end

-- Symmetric like `World.add` but idempotent
local function set(world: World, entityId: i53, componentId: i53, data: unknown)
	local record = world.entityIndex.sparse[entityId]
	local from = record.archetype
	local to = archetypeTraverseAdd(world, componentId, from)

	if from == to then
		-- If the archetypes are the same it can avoid moving the entity
		-- and just set the data directly.
		local archetypeRecord = to.records[componentId]
		from.columns[archetypeRecord][record.row] = data
		-- Should fire an OnSet event here.
		return
	end

	if from then
		-- If there was a previous archetype, then the entity needs to move the archetype
		moveEntity(world.entityIndex, entityId, record, to)
	else
		if #to.types > 0 then
			-- When there is no previous archetype it should create the archetype
			newEntity(entityId, record, to)
		end
	end

	local archetypeRecord = to.records[componentId]
	to.columns[archetypeRecord][record.row] = data
end

local function newComponent(world: World): i53
	local componentId = world.nextComponentId + 1
	if componentId > HI_COMPONENT_ID then
		-- IDs are partitioned into ranges because component IDs are not nominal,
		-- so it needs to error when IDs intersect into the entity range.
		error("Too many components, consider using world:entity() instead to create components.")
	end
	world.nextComponentId = componentId
	local id = nextEntityId(world.entityIndex, componentId)
	add(world, id, EcsComponent)
	return id
end


local function archetypeTraverseRemove(world: World, componentId: i53, from: Archetype): Archetype
	local edge = ensureEdge(from, componentId)

	local remove = edge.remove
	if not remove then
		local to = table.clone(from.types) :: { i53 }
		local at = table.find(to, componentId)
		if not at then
			return from
		end
		table.remove(to, at)
		remove = ensureArchetype(world, to, from)
		edge.remove = remove :: never
	end

	return remove
end

local function remove(world: World, entityId: i53, componentId: i53)
	local entityIndex = world.entityIndex
	local record = entityIndex.sparse[entityId]
	local sourceArchetype = record.archetype
	local destinationArchetype = archetypeTraverseRemove(world, componentId, sourceArchetype)

	if sourceArchetype and not (sourceArchetype == destinationArchetype) then
		moveEntity(entityIndex, entityId, record, destinationArchetype)
	end
end

-- should reuse this logic in World.set instead of swap removing in transition archetype
local function destructColumns(columns: { Column }, count: number, row: number)
	if row == count then
		for _, column in columns do
			column[count] = nil
		end
	else
		for _, column in columns do
			column[row] = column[count]
			column[count] = nil
		end
	end
end

local function archetypeDelete(world: World, id: i53)
	local componentIndex = world.componentIndex
	local archetypesMap = componentIndex[id]
	local archetypes = world.archetypes

	if archetypesMap then
		for archetypeId in archetypesMap.cache do
			for _, entity in archetypes[archetypeId].entities do
				remove(world, entity, id)
			end
		end

		componentIndex[id] = nil :: any
	end
end

local function delete(world: World, entityId: i53)
	local record = world.entityIndex.sparse[entityId]
	if not record then
		return
	end
	local entityIndex = world.entityIndex
	local sparse, dense = entityIndex.sparse, entityIndex.dense
	local archetype = record.archetype
	local row = record.row

	archetypeDelete(world, entityId)
	-- TODO: should traverse linked )component records to pairs including entityId
	archetypeDelete(world, ECS_PAIR(entityId, EcsWildcard))
	archetypeDelete(world, ECS_PAIR(EcsWildcard, entityId))

	if archetype then
		local entities = archetype.entities
		local last = #entities

		if row ~= last then
			local entityToMove = entities[last]
			dense[record.dense] = entityToMove
			sparse[entityToMove] = record
		end

		entities[row], entities[last] = entities[last], nil :: any

		local columns = archetype.columns

		destructColumns(columns, last, row)
	end

	sparse[entityId] = nil :: any
	dense[#dense] = nil :: any

end

local function clear(world: World, entityId: i53)
	--TODO: use sparse_get (stashed)
	local record = world.entityIndex.sparse[entityId]
	if not record then
		return
	end

	local ROOT_ARCHETYPE = world.ROOT_ARCHETYPE
	local archetype = record.archetype

	if archetype == nil or archetype == ROOT_ARCHETYPE then
		return
	end

	moveEntity(world.entityIndex, entityId, record, ROOT_ARCHETYPE)
end

-- Keeping the function as small as possible to enable inlining
local function fetch(record: Record, componentId: i24): any
	local archetype = record.archetype
	if not archetype then
		return nil
	end

	local archetypeRecord = archetype.records[componentId]

	if not archetypeRecord then
		return nil
	end

	return archetype.columns[archetypeRecord][record.row]
end

local function get(world: World, entityId: i53, a: i53, b: i53?, c: i53?, d: i53?, e: i53?): ...any
	local id = entityId
	local record = world.entityIndex.sparse[id]
	if not record then
		return nil
	end

	local va = fetch(record, a)

	if b == nil then
		return va
	elseif c == nil then
		return va, fetch(record, b)
	elseif d == nil then
		return va, fetch(record, b), fetch(record, c)
	elseif e == nil then
		return va, fetch(record, b), fetch(record, c), fetch(record, d)
	else
		error("args exceeded")
	end
end

local function noop()
	return nil
end

local EmptyQuery = {
	__iter = function()
		return noop
	end,
	next = noop,
	replace = noop,
	without = function(self)
		return self
	end
}

setmetatable(EmptyQuery, EmptyQuery)

export type Query = typeof(EmptyQuery)

type CompatibleArchetype = { archetype: Archetype, indices: { number } }

local function replaceMult(row, columns, ...)
	for i, column in columns do
		column[row] = select(i, ...)
	end
end

local function preparedQuery(compatibleArchetypes: { Archetype },
	components: { i53? }, indices: { { number } })

	local queryLength = #components

	local lastArchetype = 1
	local archetype: Archetype = compatibleArchetypes[lastArchetype]

	if not archetype then
		return EmptyQuery
	end

	local queryOutput = {}

	local entities = archetype.entities
	local i = #entities

	local function queryNext(): ...any
		local entityId = entities[i]
		while entityId == nil do
			lastArchetype += 1
			archetype = compatibleArchetypes[lastArchetype]

			if not archetype then
				return
			end

			entities = archetype.entities
			i = #entities
			entityId = entities[i]
		end

		local row = i
		i-=1

		local columns = archetype.columns
		local tr = indices[lastArchetype]

		if queryLength == 1 then
			return entityId, columns[tr[1]][row]
		elseif queryLength == 2 then
			return entityId, columns[tr[1]][row], columns[tr[2]][row]
		elseif queryLength == 3 then
			return entityId, columns[tr[1]][row], columns[tr[2]][row], columns[tr[3]][row]
		elseif queryLength == 4 then
			return entityId, columns[tr[1]][row], columns[tr[2]][row], columns[tr[3]][row], columns[tr[4]][row]
		elseif queryLength == 5 then
			return entityId,
				columns[tr[1]][row],
				columns[tr[2]][row],
				columns[tr[3]][row],
				columns[tr[4]][row],
				columns[tr[5]][row]
		elseif queryLength == 6 then
			return entityId,
				columns[tr[1]][row],
				columns[tr[2]][row],
				columns[tr[3]][row],
				columns[tr[4]][row],
				columns[tr[5]][row],
				columns[tr[6]][row]
		elseif queryLength == 7 then
			return entityId,
				columns[tr[1]][row],
				columns[tr[2]][row],
				columns[tr[3]][row],
				columns[tr[4]][row],
				columns[tr[5]][row],
				columns[tr[6]][row],
				columns[tr[7]][row]
		elseif queryLength == 8 then
			return entityId,
				columns[tr[1]][row],
				columns[tr[2]][row],
				columns[tr[3]][row],
				columns[tr[4]][row],
				columns[tr[5]][row],
				columns[tr[6]][row],
				columns[tr[7]][row],
				columns[tr[8]][row]
		end

		for i in components do
			queryOutput[i] = columns[tr[i]][row]
		end

		return entityId, unpack(queryOutput, 1, queryLength)
	end

	local function without(self, ...): Query
		local withoutComponents = { ... }
		for i = #compatibleArchetypes, 1, -1 do
			local archetype = compatibleArchetypes[i]
			local records = archetype.records
			local shouldRemove = false

			for _, componentId in withoutComponents do
				if records[componentId] then
					shouldRemove = true
					break
				end
			end

			if shouldRemove then
				table.remove(compatibleArchetypes, i)
			end
		end

		if #compatibleArchetypes == 0 then
		    return EmptyQuery
		end

		return self
	end

	local function iter()
        lastArchetype = 1
		archetype = compatibleArchetypes[1]
		entities = archetype.entities
		i = #entities

		return queryNext
	end

	local function replace(_, fn: any)
        for i, archetype in compatibleArchetypes do
			local tr = indices[i]
			local columns = archetype.columns

			for row in archetype.entities do
				if queryLength == 1 then
					local a = columns[tr[1]]
					local pa = fn(a[row])

					a[row] = pa
				elseif queryLength == 2 then
					local a = columns[tr[1]]
					local b = columns[tr[2]]

					a[row], b[row] = fn(a[row], b[row])
				elseif queryLength == 3 then
					local a = columns[tr[1]]
					local b = columns[tr[2]]
					local c = columns[tr[3]]

					a[row], b[row], c[row] = fn(a[row], b[row], c[row])
				elseif queryLength == 4 then
					local a = columns[tr[1]]
					local b = columns[tr[2]]
					local c = columns[tr[3]]
					local d = columns[tr[4]]

					a[row], b[row], c[row], d[row] = fn(
						a[row], b[row], c[row], d[row])
				else
					for i = 1, queryLength do
						queryOutput[i] = columns[tr[i]][row]
					end
					replaceMult(row, columns, fn(unpack(queryOutput)))
				end
			end
		end
	end

	local it = {
		__iter = iter,
		next = queryNext,
		without = without,
		replace = replace
	}

	return setmetatable(it, it) :: any
end

local function query(world: World, ...: number): Query
	-- breaking?
	if (...) == nil then
		error("Missing components")
	end

	local indices: { { number } } = {}
	local compatibleArchetypes: { Archetype } = {}
	local length = 0

	local components: { number } = { ... }
	local archetypes: { Archetype } = world.archetypes :: any

	local firstArchetypeMap: ArchetypeMap
	local componentIndex = world.componentIndex

	for _, componentId in components do
		local map: ArchetypeMap = componentIndex[componentId] :: any
		if not map then
			return EmptyQuery
		end

		if (firstArchetypeMap :: any) == nil or firstArchetypeMap.size < map.size then
			firstArchetypeMap = map
		end
	end

	for id in firstArchetypeMap.cache do
		local archetype = archetypes[id]
		local archetypeRecords = archetype.records

		local records: { number } = {}
		local skip = false

		for i, componentId in components do
			local index = archetypeRecords[componentId]
			if not index then
				skip = true
				break
			end
			-- index should be index.offset
			records[i] = index
		end

		if skip then
			continue
		end

		length += 1
		compatibleArchetypes[length] = archetype
		indices[length] = records
	end

	return preparedQuery(compatibleArchetypes, components, indices)
end

type WorldIterator = (() -> (i53, { [unknown]: unknown? })) & (() -> ()) & (() -> i53)
-- __nominal_type_dont_use could not be any or T as it causes a type error
-- or produces a union
export type Entity<T = any> = number & { __nominal_type_dont_use: T }
export type Pair = number

export type QueryShim<T...> = typeof(setmetatable({
	without = function(...): QueryShim<T...>
		return nil :: any
	end,
}, {
	__iter = function(): () -> (number, T...)
		return nil :: any
	end,
}))

export type WorldShim = typeof(setmetatable(
	{} :: {

		--- Creates a new entity
		entity: (WorldShim) -> Entity,
		--- Creates a new entity located in the first 256 ids.
		--- These should be used for static components for fast access.
		component: <T>(WorldShim) -> Entity<T>,
		--- Gets the target of an relationship. For example, when a user calls
		--- `world:target(id, ChildOf(parent))`, you will obtain the parent entity.
		target: (WorldShim, id: Entity, relation: Entity) -> Entity?,
		--- Deletes an entity and all it's related components and relationships.
		delete: (WorldShim, id: Entity) -> (),

		--- Adds a component to the entity with no value
		add: <T>(WorldShim, id: Entity, component: Entity<T>) -> (),
		--- Assigns a value to a component on the given entity
		set: <T>(WorldShim, id: Entity, component: Entity<T>, data: T) -> (),

		-- Clears an entity from the world
		clear: (WorldShim, id: Entity) -> (),
		--- Removes a component from the given entity
		remove: (WorldShim, id: Entity, component: Entity) -> (),
		--- Retrieves the value of up to 4 components. These values may be nil.
		get: (<A>(WorldShim, id: any, Entity<A>) -> A)
			& (<A, B>(WorldShim, id: Entity, Entity<A>, Entity<B>) -> (A, B))
			& (<A, B, C>(WorldShim, id: Entity, Entity<A>, Entity<B>, Entity<C>) -> (A, B, C))
			& <A, B, C, D>(WorldShim, id: Entity, Entity<A>, Entity<B>, Entity<C>, Entity<D>) -> (A, B, C, D),

		--- Searches the world for entities that match a given query
		query: (<A>(WorldShim, Entity<A>) -> QueryShim<A>)
			& (<A, B>(WorldShim, Entity<A>, Entity<B>) -> QueryShim<A, B>)
			& (<A, B, C>(WorldShim, Entity<A>, Entity<B>, Entity<C>) -> QueryShim<A, B, C>)
			& (<A, B, C, D>(WorldShim, Entity<A>, Entity<B>, Entity<C>, Entity<D>) -> QueryShim<A, B, C, D>)
			& (<A, B, C, D, E>(
				WorldShim,
				Entity<A>,
				Entity<B>,
				Entity<C>,
				Entity<D>,
				Entity<E>
			) -> QueryShim<A, B, C, D, E>)
			& (<A, B, C, D, E, F>(
				WorldShim,
				Entity<A>,
				Entity<B>,
				Entity<C>,
				Entity<D>,
				Entity<E>,
				Entity<F>
			) -> QueryShim<A, B, C, D, E, F>)
			& (<A, B, C, D, E, F, G>(
				WorldShim,
				Entity<A>,
				Entity<B>,
				Entity<C>,
				Entity<D>,
				Entity<E>,
				Entity<F>,
				Entity<G>
			) -> QueryShim<A, B, C, D, E, F, G>)
			& (<A, B, C, D, E, F, G, H>(
				WorldShim,
				Entity<A>,
				Entity<B>,
				Entity<C>,
				Entity<D>,
				Entity<E>,
				Entity<F>,
				Entity<G>,
				Entity<H>
			) -> QueryShim<A, B, C, D, E, F, G, H>)
			& (<A, B, C, D, E, F, G, H, I>(
				WorldShim,
				Entity<A>,
				Entity<B>,
				Entity<C>,
				Entity<D>,
				Entity<E>,
				Entity<F>,
				Entity<G>,
				Entity<H>,
				Entity<I>
			) -> QueryShim<A, B, C, D, E, F, G, H, I>)
			& (<A, B, C, D, E, F, G, H, I, J>(
				WorldShim,
				Entity<A>,
				Entity<B>,
				Entity<C>,
				Entity<D>,
				Entity<E>,
				Entity<F>,
				Entity<G>,
				Entity<H>,
				Entity<I>,
				Entity<J>
			) -> QueryShim<A, B, C, D, E, F, G, H, I, J>)
			& (<A, B, C, D, E, F, G, H, I, J, K>(
				WorldShim,
				Entity<A>,
				Entity<B>,
				Entity<C>,
				Entity<D>,
				Entity<E>,
				Entity<F>,
				Entity<G>,
				Entity<H>,
				Entity<I>,
				Entity<J>,
				Entity<K>,
				...Entity<any>
			) -> QueryShim<A, B, C, D, E, F, G, H, I, J, K>),
	},
	{} :: {
		__iter: (world: WorldShim) -> () -> (number, { [unknown]: unknown? }),
	}
))

local World = {}
World.__index = World

function World.new()
	local self = setmetatable({
		archetypeIndex = {} :: { [string]: Archetype },
		archetypes = {} :: Archetypes,
		componentIndex = {} :: ComponentIndex,
		entityIndex = {
			dense = {} :: { [i24]: i53 },
			sparse = {} :: { [i53]: Record },
		} :: EntityIndex,
		hooks = {
			[EcsOnAdd] = {},
		},
		nextArchetypeId = 0,
		nextComponentId = 0,
		nextEntityId = 0,
		ROOT_ARCHETYPE = (nil :: any) :: Archetype,
	}, World)
	self.ROOT_ARCHETYPE = archetypeOf(self, {})

	-- Initialize built-in components
	nextEntityId(self.entityIndex, EcsChildOf)

	return self
end

World.entity = entity
World.query = query
World.remove = remove
World.clear = clear
World.delete = delete
World.component = newComponent
World.add = add
World.set = set
World.get = get
World.target = target
World.parent = parent

return {
	World = World :: { new: () -> WorldShim },

	OnAdd = EcsOnAdd :: Entity,
	OnRemove = EcsOnRemove :: Entity,
	OnSet = EcsOnSet :: Entity,

	Wildcard = EcsWildcard :: Entity,
	w = EcsWildcard :: Entity,
	ChildOf = EcsChildOf,
	Component = EcsComponent,

	Rest = EcsRest,

	IS_PAIR = ECS_IS_PAIR,
	ECS_ID = ECS_ENTITY_T_LO,
	ECS_PAIR = ECS_PAIR,
	ECS_GENERATION_INC = ECS_GENERATION_INC,
	ECS_GENERATION = ECS_GENERATION,
	ECS_PAIR_RELATION = ECS_PAIR_RELATION,
	ECS_PAIR_OBJECT = ECS_PAIR_OBJECT,

	pair = (ECS_PAIR :: any) :: <R, T>(pred: Entity, obj: Entity) -> number,
	getAlive = getAlive,
}