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Lua ECS

A simple Entity-Componet library for Lua.

-- Create a world
local w = ecs.world()

-- Register a new type "vector( x:float, y:float )" into world.
-- It's a C component (The data is in C memory).
w:register {
	name = "vector",
	"x:float",
	"y:float",
}

-- Register a new type "name" into world.
-- It's a Lua component (The value can be any lua object)
w:register {
	name = "name",
	type = "lua",
}

-- Create a new entity with components vector and name.
local eid = w:new {
	vector = { 1.0, 2.0 },
	name = "point",
}

-- Iterate every entity with component vector and name
for v in w:select "vector:in name:in" do
	-- v is the iterator, we can read compionent from it.
	-- DO NOT store v outside of the iteration
	print(v.name)	-- output : point
	print(v.vector.x, v.vector.y)	-- output: 1.0, 2.0
end

Component Types

C Component

The component is stored in C side, it's compat and can be accessed from C side easily. The supported buildin types are:

  • int (32bits)
  • float
  • bool (8bits)
  • int64
  • dword (unsigned 32bits)
  • word (unsigned 16bits)
  • byte (unsigned 8bits)
  • double
  • userdata (void *)
w:register {
	name = "vector",
	"x:float",
	"y:float",
}

The equivalent C struct is

struct vector {
	float x;
	float y;
};

A single buildin C type can be register as

w:register {
	name = "visible",
	type = "bool",
}

Lua component

The component is stored in Lua side , and can be any lua types, such as string, table, userdata, etc.

w:register {
	name = "name",
	type = "lua",
}

Tag

The tag is a special component , it can be read as a boolean type, and the value must be true. It used for selection.

w:register {
	name = "visible"
}

Entity ID

Each entity has a build-in readonly component eid , it's a 64bits unique monotonic ID. The newest entity always has the biggest eid.

Select Pattern

Use for v in world:select(pattern) to manage components. It iterate all the entities in the world and filter out the entities matching the pattern. The order of iteration is the creation order of the entities.

-- print all the entities' eid
for v in w:select "eid:in" do
	print(v.eid)
end

The pattern is a space-separated combination of componentname[:?]action, and the action can be

  • in : read the component
  • out : write the component
  • update : read / write
  • absent : check if the component is not exist
  • exist (default) : check if the component is exist
  • new : create the component
  • ? means it's an optional action if the component is not exist
-- clear temp component from all entities
w:clear "temp"

-- Iterate the entity with visible/value/output components and without readonly component
-- Create temp component for each.
for v in w:select "visible readonly:absent value:in output:out temp:new" do
	v.output = v.value + 1
	v.temp = v.value
end

NOTICE: If you use action new , you must guarantee the component is clear (None entity has this component) before iteration.

Create Entity

-- create an empty entity
local eid = w:new()
-- Import component "name" into entity (eid)
w:import(eid, { name = "foobar" })
-- Or you can use
local eid = w:new { name = "foobar" }

You can also create an entity from a template :

-- Create a template first
local t = w:template {
	name = "foobar"
}
-- instance the template into an entity, and add visible tag.
-- The additional components ( { visible = true } ) is optional.
local eid = w:template_instance(w:new(), t, { visible = true })

You can offer init / marshal / unmarshal functions for the component type in w:register().

marshal and unmarshal is for lua component of template, and init is the constructor of the component, the common use is for C component initialization.

NOTICE: C component can be initialize by a lua string, you can use string.pack() to generate the C structure data.

Remove Entity

-- Remove an entity with eid
w:remove(eid)
-- Or remove an entity with an iterator
for v in w:select "value:in" do
	if v.value == 42 then
		w:remove(v)
	end
end

for v in w:select "REMOVED value:in do
	assert(v.value == 42)
end

-- delete all the entities removed
w:update()

The entities removed are not going to disppear immediately, they are tagged with REMOVED only, and deleted after you call w:update().

Group

You can add an entity into a group after creation. Each entity can belongs one or more groups (or no group).

-- Add entity (eid) into a group with groupid (32bit integer)
w:group_add(groupid, eid)

You can tags entities in groups with w:group_enable(groupid1, groupid2,...)

Persistance

Only C components can be persistance.

-- Save
-- Create a writer to file "saves.bin"
local writer = ecs.writer "saves.bin"
writer:write(w, w:component_id "eid") -- write component eid
writer:write(w, w:component_id "value") -- write component value
writer:write(w, w:component_id "tag") -- write component tag
local meta = writer:close()
local function print_section(s)
	print("offset =", s.offset)
	print("stride =", s.stride)
	print("n = ", s.n)
end
print_section(meta[1])	-- meta information of eid
print_section(meta[2])	-- meta information of value
print_section(meta[3])	-- meta information of tag
-- Load
local reader = ecs.reader "saves.bin"
local maxid = w:read_component(reader, "eid", meta[1].offset, meta[1].stride, meta[1].n)
local value_n = w:read_component(reader, "value", meta[2].offset, meta[2].stride, meta[2].n)
local tag_n = w:read_component(reader, "tag", meta[3].offset, meta[3].stride, meta[3].n)
reader:close()

You can also use w:generate_eid() instead of reading eid from file

Other APIs

w:exist(eid) -- Check if the entity with eid exist

w:clear(typename) -- delete component (typename) from all the entities

w:clearall() -- delete all the entities

w:component_id(typename) -- returns the component id of the component (typename) . It's for C systems.

w:type(typename) -- returns "tag", "lua" or "c"

w:first(pattern) -- Read the first component with the pattern.

w:filter(tagname, pattern) -- Enable tags marching the pattern

Access Components from C side

  1. Create a context by w:context { component1, component2, ... } for C. Export the components C concerned.
  2. Define C structs of components in C.
  3. Use APIs in luaecs.h

void * entity_iter(struct ecs_context *ctx, cid_t cid, int index) void * entity_sibling(struct ecs_context *ctx, cid_t cid, int index, cid_t sibling_id)

struct component *v;
int i;
// iterate all the components with COMPONENT_ID. COMPONENT_ID is the index of context (base 0) or component id from lua MAKE_COMPONENT_ID(cid)
for (i = 0; (v = (struct component *)entity_iter(ctx, COMPONENT_ID, i)); i++) {
	// Read the component2 associate with component (the same entity)
	struct component2 * c2 = (struct component2 *)entity_sibling(ctx, COMPONENT_ID, i, COMPONENT_ID2);
}

int entity_sibling_id(struct ecs_context *ctx, cid_t cid, int index, cid_t sibling_id)

The same with entity_sibling, but returns an id (base 1). 0 : not exist.

void * entity_add_sibling(struct ecs_context *ctx, cid_t cid, int index, cid_t sibling_id, const void *buffer)

int entity_new(struct ecs_context *ctx, cid_t cid, const void *buffer)

Create an entity with one component

void entity_remove(struct ecs_context *ctx, cid_t cid, int index)

void entity_enable_tag(struct ecs_context *ctx, cid_t cid, int index, cid_t tag_id)

void entity_disable_tag(struct ecs_context *ctx, cid_t cid, int index, cid_t tag_id)

int entity_get_lua(struct ecs_context *ctx, cid_t cid, int index, void *L) int entity_sibling_lua(struct ecs_context *ctx, cid_t cid, int index, cid_t sibling_id, void *L)

void entity_group_enable(struct ecs_context *ctx, int tagid, int n, int groupid[])