local module = {}
setmetatable(module, {__index=_G})
setfenv(1, module)


--===== Components =====--

-- Components provide a kind-of-type-safe way of storing values, since they are
-- guaranteed to have a specific set of keys and no other values in them. They also serialize
-- nicely for storage.

Component = {}

function Component.newFactory(name, params, serialize)
	-- create the metatable for this component type
	local metatable = {}
	metatable.__tostring = serialize or Component.serialize -- nice serialization
	metatable.__newindex = function(tbl, index, value)
		if params[index] ~= nil then
			tbl[index] = value
		else
			-- throw an error if we try to set a key that is not in the valid set
			error(string.format("%q is not a valid key for a component of type %q", index, name))
		end
	end

	-- the factory function for creating components
	return function(tbl)
		local tbl = tbl or {}
		local self = { __type=name }

		-- ensure that all keys are present
		for k, v in pairs(params) do
			self[k] = v
		end
		setmetatable(self, metatable)

		-- set the keys from the factory call
		for k, v in pairs(tbl) do
			self[k] = v
		end

		return self
	end
end


function Component.serialize(self)
	local str = "{"
	for k, v in pairs(self) do
		if type(v) == "string" then
			str = str .. string.format("%s=%q,", k, v)
		else
			str = str .. string.format("%s=%s,", k, tostring(v))
		end
	end
	str = string.sub(str, 1, -2) .. "}"
	return str
end


--===== EntityDb =====--

EntityDb = {}
EntityDb.__index = EntityDb


function EntityDb.new(_)
	local self = {
		entities = {},
		components = {},
	}
	setmetatable(self, EntityDb)
	return self
end
setmetatable(EntityDb, {__call=EntityDb.new})


function EntityDb.checkIsValid(self, id)
	if not self.entities[id] then
		error(string.format("invalid entity id: 0x%x", id))
	end
end


local guid = (function()
	local id = 0
	return function() id=id+1 return id end
end)()
function EntityDb.createEntity(self)
	local id = guid()
	self.entities[id] = true
	return id
end


function EntityDb.addComponent(self, id, name, value)
	self:checkIsValid(id)
	if not self.components[name] then
		self.components[name] = { count=0 }
	end

	local component = self.components[name]
	component[id] = value
	component.count = component.count + 1
end


local function shallowCopy(tbl)
	local copy = {}
	for k, v in pairs(tbl) do copy[k] = v end
	return copy
end


function EntityDb.queryComponent(self, name)
	local query = shallowCopy(self.components[name])
	query.count = nil
	return query
end


function EntityDb.queryEntity(self, id)
	self:checkIsValid(id)
	local query = {}
	for name, component in pairs(self.components) do
		query[name] = component[id]
	end
	return query
end


function EntityDb.removeComponent(self, id, name)
	self:checkIsValid(id)
	local component = self.components[name]
	if component[id] ~= nil then
		component[id] = nil
		component.count = component.count - 1
		if component.count == 0 then
			self.components[name] = nil
		end
	end
end


function EntityDb.deleteEntity(self, id)
	self:checkIsValid(id)
	for name in pairs(self.components) do
		self:removeComponent(id, name)
	end
	self.entities[id] = nil
end

return module