Update to version 1.1-6.

Remove useless code in world metatable
Make tiny.world(...) return newworld, ...

README.md

@@ -1,13 +1,18 @@
 # tiny-ecs #
+
+[![License](http://img.shields.io/badge/Licence-MIT-brightgreen.svg)](LICENSE)
+
 Tiny-ecs is an Entity Component System for lua that's simple, flexible, and useful.
 Because of lua's tabular nature, Entity Component Systems are a natural choice
 for simulating large and complex systems. For more explanation on Entity
 Component Systems, here is some
 [basic info](http://en.wikipedia.org/wiki/Entity_component_system "Wikipedia").
 
-Tiny-ecs also works well with Objected Oriented programming in lua because
+Tiny-ecs also works well with objected oriented programming in lua because
 Systems and Entities do not use metatables. This means you can subclass your
 Systems and Entities, and use existing lua class frameworks with tiny-ecs, no problem.
+For an example on how to use tiny-ecs with object-oriented lua, take a look at the
+demo branch, specifically the systems and entities sub-directories.
 
 ## Overview ##
 Tiny-ecs has four important types: Worlds, Filters, Systems, and Entities.
@@ -29,7 +34,7 @@ Systems in tiny-ecs describe how to update Entities. Systems select certain Enti
 using a Filter, and then only update those select Entities. Some Systems don't
 update Entities, and instead just act as function callbacks every update. Tiny-ecs
 provides functions for creating Systems easily, as well as creating Systems that
-can be used in an Object Orientented fashion.
+can be used in an object oriented fashion.
 
 ### Filters ###
 Filters are used to select Entities. Filters can be any lua function, but
@@ -64,7 +69,7 @@ end
 ## Use It ##
 Copy paste tiny.lua into your source folder. For stability and consistent API,
 please use a tagged release or use luarocks. Tagged releases will have a version
-number in `tiny._VERSION`, while other commits will just have the string 'dev'.
+number in `tiny._VERSION`, while other commits will just have the string 'scm'.
 
 ## Luarocks ##
 Tiny-ecs is also on [Luarocks](https://luarocks.org/) and can be installed with
@@ -73,7 +78,7 @@ Tiny-ecs is also on [Luarocks](https://luarocks.org/) and can be installed with
 ## Demo ##
 Check out the [demo](https://github.com/bakpakin/tiny-ecs/tree/demo-commandokibbles), a game
 originally written for Ludum Dare 32 with the theme 'An Unconventional Weapon'. The demo uses
-[LOVE](https://love2d.org/), an amazing game framework for lua.
+[LÖVE](https://love2d.org/), an amazing game framework for lua.
 
 ## Testing ##
 Tiny-ecs uses [busted](http://olivinelabs.com/busted/) for testing. Install and run
@@ -81,11 +86,6 @@ Tiny-ecs uses [busted](http://olivinelabs.com/busted/) for testing. Install and
 
 ## Documentation ##
 See API [here](http://bakpakin.github.io/tiny-ecs/doc/).
-Documentation can be generated locally with [LDoc](http://stevedonovan.github.io/ldoc/).
+For the most up-to-date documentation, read the source code, or generate the HTML
-
+locally with [LDoc](http://stevedonovan.github.io/ldoc/).
-## TODO ##
+See the original forum thread [here](https://love2d.org/forums/viewtopic.php?f=5&t=79937&p=182589).
-
-* More testing
-* Performance testing / optimization
-* Add more System types
-* Improve Documentation

config.ld

@@ -10,3 +10,4 @@ style = '!fixed'
 package = 'tiny-ecs'
 not_luadoc = true
 boilerplate = true
+no_return_or_parms = true

rockspecs/tiny-ecs-1.1-2.rockspec

@@ -0,0 +1,24 @@
+package = "tiny-ecs"
+version = "1.1-2"
+source = {
+    url = "git://github.com/bakpakin/tiny-ecs",
+    tag = "1.1-2"
+}
+description = {
+    summary = "Entity Component System for Lua.",
+    detailed = [[
+        Pure Lua implementation of an easy to use, compact, fast, and flexible
+        Entity Component System. Works well with Object Orientation.
+    ]],
+    homepage = "https://github.com/bakpakin/tiny-ecs",
+    license = "MIT"
+}
+dependencies = {
+    "lua >= 5.1"
+}
+build = {
+    type = "builtin",
+    modules = {
+        tiny = "tiny.lua"
+    }
+}

rockspecs/tiny-ecs-1.1-3.rockspec

@@ -0,0 +1,24 @@
+package = "tiny-ecs"
+version = "1.1-3"
+source = {
+    url = "git://github.com/bakpakin/tiny-ecs",
+    tag = "1.1-3"
+}
+description = {
+    summary = "Entity Component System for Lua.",
+    detailed = [[
+        Pure Lua implementation of an easy to use, compact, fast, and flexible
+        Entity Component System. Works well with Object Orientation.
+    ]],
+    homepage = "https://github.com/bakpakin/tiny-ecs",
+    license = "MIT"
+}
+dependencies = {
+    "lua >= 5.1"
+}
+build = {
+    type = "builtin",
+    modules = {
+        tiny = "tiny.lua"
+    }
+}

rockspecs/tiny-ecs-1.1-4.rockspec

@@ -0,0 +1,24 @@
+package = "tiny-ecs"
+version = "1.1-4"
+source = {
+    url = "git://github.com/bakpakin/tiny-ecs",
+    tag = "1.1-4"
+}
+description = {
+    summary = "Entity Component System for Lua.",
+    detailed = [[
+        Pure Lua implementation of an easy to use, compact, fast, and flexible
+        Entity Component System. Works well with Object Orientation.
+    ]],
+    homepage = "https://github.com/bakpakin/tiny-ecs",
+    license = "MIT"
+}
+dependencies = {
+    "lua >= 5.1"
+}
+build = {
+    type = "builtin",
+    modules = {
+        tiny = "tiny.lua"
+    }
+}

rockspecs/tiny-ecs-1.1-5.rockspec

@@ -0,0 +1,24 @@
+package = "tiny-ecs"
+version = "1.1-5"
+source = {
+    url = "git://github.com/bakpakin/tiny-ecs",
+    tag = "1.1-5"
+}
+description = {
+    summary = "Entity Component System for Lua.",
+    detailed = [[
+        Pure Lua implementation of an easy to use, compact, fast, and flexible
+        Entity Component System. Works well with Object Orientation.
+    ]],
+    homepage = "https://github.com/bakpakin/tiny-ecs",
+    license = "MIT"
+}
+dependencies = {
+    "lua >= 5.1"
+}
+build = {
+    type = "builtin",
+    modules = {
+        tiny = "tiny.lua"
+    }
+}

rockspecs/tiny-ecs-1.1-6.rockspec

@@ -0,0 +1,24 @@
+package = "tiny-ecs"
+version = "1.1-6"
+source = {
+    url = "git://github.com/bakpakin/tiny-ecs",
+    tag = "1.1-6"
+}
+description = {
+    summary = "Entity Component System for Lua.",
+    detailed = [[
+        Pure Lua implementation of an easy to use, compact, fast, and flexible
+        Entity Component System. Works well with Object Orientation.
+    ]],
+    homepage = "https://github.com/bakpakin/tiny-ecs",
+    license = "MIT"
+}
+dependencies = {
+    "lua >= 5.1"
+}
+build = {
+    type = "builtin",
+    modules = {
+        tiny = "tiny.lua"
+    }
+}

spec/tiny_spec.lua

@@ -56,7 +56,14 @@ describe('tiny-ecs:', function()
             local ftap = tiny.requireAll("spinalTap")
             local fvel = tiny.requireAll("vel")
             local fxform = tiny.requireAll("xform")
-            local fall = tiny.requireOne("spinalTap", "onlyTen", "littleMan")
+            local fall = tiny.requireAny("spinalTap", "onlyTen", "littleMan")
+
+            -- Only select Entities without "spinalTap"
+            local frtap = tiny.rejectAny("spinalTap")
+
+            -- Select Entities without all three: "spinalTap", "onlyTen", and
+            -- "littleMan"
+            local frall = tiny.rejectAll("spinalTap", "onlyTen", "littleMan")
 
             assert.truthy(fall(nil, entity1))
             assert.truthy(ftap(nil, entity1))
@@ -68,6 +75,14 @@ describe('tiny-ecs:', function()
             assert.truthy(fall(nil, entity2))
             assert.truthy(fall(nil, entity3))
 
+            assert.falsy(frtap(nil, entity1))
+            assert.truthy(frtap(nil, entity2))
+            assert.truthy(frtap(nil, entity3))
+
+            assert.truthy(frall(nil, entity1))
+            assert.truthy(frall(nil, entity2))
+            assert.truthy(frall(nil, entity3))
+
         end)
 
     end)
@@ -196,8 +211,21 @@ describe('tiny-ecs:', function()
             world:update(1)
             world:setSystemIndex(moveSystem, 2)
             world:update(1)
-            assert.equals(2, world:getSystemIndex(moveSystem))
+            assert.equals(2, moveSystem.index)
-            assert.equals(1, world:getSystemIndex(oneTimeSystem))
+            assert.equals(1, oneTimeSystem.index)
+        end)
+
+        it("Sorts Entities in Sorting Systems", function()
+            local sortsys = tiny.sortedProcessingSystem()
+            sortsys.filter = tiny.requireAll("vel")
+            function sortsys:compare(e1, e2)
+                return e1.vel.x < e2.vel.x
+            end
+            world:add(sortsys)
+            world:update(0)
+            assert.equals(sortsys.entities[1], entity2)
+            assert.equals(sortsys.entities[2], entity3)
+            assert.equals(sortsys.entities[3], entity1)
         end)
 
     end)

tiny-ecs-scm-0.rockspec

@@ -0,0 +1,23 @@
+package = "tiny-ecs"
+version = "scm-0"
+source = {
+    url = "git://github.com/bakpakin/tiny-ecs",
+}
+description = {
+    summary = "Entity Component System for Lua.",
+    detailed = [[
+        Pure Lua implementation of an easy to use, compact, fast, and flexible
+        Entity Component System. Works well with Object Orientation.
+    ]],
+    homepage = "https://github.com/bakpakin/tiny-ecs",
+    license = "MIT"
+}
+dependencies = {
+    "lua >= 5.1"
+}
+build = {
+    type = "builtin",
+    modules = {
+        tiny = "tiny.lua"
+    }
+}

tiny.lua

@@ -23,16 +23,16 @@ CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
 -- @author Calvin Rose
 -- @license MIT
 -- @copyright 2015
-local tiny = { _VERSION = "1.1-1" }
+local tiny = { _VERSION = "1.1-6" }
 
 -- Local versions of standard lua functions
 local tinsert = table.insert
 local tremove = table.remove
 local tsort = table.sort
 local pairs = pairs
-local ipairs = ipairs
 local setmetatable = setmetatable
 local type = type
+local select = select
 
 -- Local versions of the library functions
 local tiny_manageEntities
@@ -50,11 +50,11 @@ local tiny_remove
 -- value indicating if the Entity should be processed by the System.
 --
 -- Filters must be added to Systems by setting the `filter` field of the System.
--- Filter's returned by `tiny.requireAll` and `tiny.requireOne` are immutable
+-- Filter's returned by tiny-ecs's Filter functions are immutable and can be
--- and can be used by multiple Systems.
+-- used by multiple Systems.
 --
 --    local f1 = tiny.requireAll("position", "velocity", "size")
---    local f2 = tiny.requireOne("position", "velocity", "size")
+--    local f2 = tiny.requireAny("position", "velocity", "size")
 --
 --    local e1 = {
 --        position = {2, 3},
@@ -75,11 +75,18 @@ local tiny_remove
 --    print(f1(nil, e1), f1(nil, e2), f1(nil, e3)) -- prints true, false, false
 --    print(f2(nil, e1), f2(nil, e2), f2(nil, e3)) -- prints true, true, true
 --
+-- Filters can also be passed as arguments to other Filter constructors. This is
+-- a powerful way to create complex, custom Filters that select a very specific
+-- set of Entities.
+--
+--    -- Selects Entities with an "image" Component, but not Entities with a
+--    -- "Player" or "Enemy" Component.
+--    filter = tiny.requireAll("image", tiny.rejectAny("Player", "Enemy"))
+--
 -- @section Filter
 
 --- Makes a Filter that selects Entities with all specified Components and
 -- Filters.
--- @param ... List of required Components and other Filters.
 function tiny.requireAll(...)
     local components = {...}
     local len = #components
@@ -101,8 +108,7 @@ end
 
 --- Makes a Filter that selects Entities with at least one of the specified
 -- Components and Filters.
--- @param ... List of required Components and other Filters.
+function tiny.requireAny(...)
-function tiny.requireOne(...)
     local components = {...}
     local len = #components
     return function(system, e)
@@ -121,45 +127,110 @@ function tiny.requireOne(...)
     end
 end
 
---- System functions.
+--- Makes a Filter that rejects Entities with all specified Components and
--- A System is a wrapper around function callbacks for manipulating Entities.
+-- Filters, and selects all other Entities.
--- @section System
+function tiny.rejectAll(...)
-
+    local components = {...}
--- Use an empty table as a key for identifying Systems. Any table that contains
+    local len = #components
--- this key is considered a System rather than an Entity.
+    return function(system, e)
-local systemTableKey = { "SYSTEM_TABLE_KEY" }
+        local c
-
+        for i = 1, len do
--- Check if tables are systems.
+            c = components[i]
-local function isSystem(table)
+            if type(c) == 'function' then
-    return table[systemTableKey]
+                if not c(system, e) then
+                    return true
+                end
+            elseif e[c] == nil then
+                return true
+            end
+        end
+        return false
+    end
 end
 
---- Creates a System. Systems are tables that contain at least one method;
+--- Makes a Filter that rejects Entities with at least one of the specified
+-- Components and Filters, and selects all other Entities.
+function tiny.rejectAny(...)
+    local components = {...}
+    local len = #components
+    return function(system, e)
+        local c
+        for i = 1, len do
+            c = components[i]
+            if type(c) == 'function' then
+                if c(system, e) then
+                    return false
+                end
+            elseif e[c] ~= nil then
+                return false
+            end
+        end
+        return true
+    end
+end
+
+--- System functions.
+-- A System is a wrapper around function callbacks for manipulating Entities.
+-- Systems are implemented as tables that contain at least one method;
 -- an update function that takes parameters like so:
 --
 --   * `function system:update(dt)`.
 --
 -- There are also a few other optional callbacks:
 --
---   * `function system:filter(entity)`
+--   * `function system:filter(entity)` - Returns true if this System should
---   * `function system:onAdd(entity)`
+-- include this Entity, otherwise should return false. If this isn't specified,
---   * `function system:onRemove(entity)`
+-- no Entities are included in the System.
---   * `function system:onModify(dt)`
+--   * `function system:onAdd(entity)` - Called when an Entity is added to the
+-- System.
+--   * `function system:onRemove(entity)` - Called when an Entity is removed
+-- from the System.
+--   * `function system:onModify(dt)` - Called when the System is modified by
+-- adding or removing Entities from the System.
 --
--- For Filters, it is conveient to use `tiny.requireAll` or `tiny.requireOne`,
+-- For Filters, it is convenient to use `tiny.requireAll` or `tiny.requireAny`,
--- but one can write their own filters as well. Set the Filter of your System
+-- but one can write their own filters as well. Set the Filter of a System like
--- like so:
+-- so:
 --    system.filter = tiny.requireAll("a", "b", "c")
 -- or
 --    function system:filter(entity)
 --        return entity.myRequiredComponentName ~= nil
 --    end
 --
--- @param table A table to be used as a System, or `nil` to create a new System.
+-- All Systems also have a few important fields that are initialized when the
-function tiny.system(table)
+-- system is added to the World. A few are important, and few should be less
-    table = table or {}
+-- commonly used.
-    table[systemTableKey] = true
+--
-    return table
+--   * The `world` field points to the World that the System belongs to. Useful
+-- for adding and removing Entities from the world dynamically via the System.
+--   * The `active` flag is whether or not the System is updated automatically.
+-- Inactive Systems should be updated manually or not at all via
+-- `system:update(dt)`. Defaults to true.
+--   * The `entities` field is an ordered list of Entities in the System. This
+-- list can be used to quickly iterate through all Entities in a System.
+--   * The `interval` field is an optional field that makes Systems update at
+-- certain intervals using buffered time, regardless of World update frequency.
+-- For example, to make a System update once a second, set the System's interval
+-- to 1.
+--   * The `index` field is the System's index in the World. Lower indexed
+-- Systems are processed before higher indices. The `index` is a read only
+-- field; to set the `index`, use `tiny.setSystemIndex(world, system)`.
+--   * The `indices` field is a table of Entity keys to their indices in the
+-- `entities` list. Most Systems can ignore this.
+--   * The `modified` flag is an indicator if the System has been modified in
+-- the last update. If so, the `onModify` callback will be called on the System
+-- in the next update, if it has one. This is usually managed by tiny-ecs, so
+-- users should mostly ignore this, too.
+--
+-- @section System
+
+-- Use an empty table as a key for identifying Systems. Any table that contains
+-- this key is considered a System rather than an Entity.
+local systemTableKey = { "SYSTEM_TABLE_KEY" }
+
+-- Checks if a table is a System.
+local function isSystem(table)
+    return table[systemTableKey]
 end
 
 -- Update function for all Processing Systems.
@@ -187,22 +258,44 @@ local function processingSystemUpdate(system, dt)
     end
 end
 
---- Creates a Processing System.
+-- Sorts Systems by a function system.sort(entity1, entity2) on modify.
+local function sortedSystemOnModify(system, dt)
+    local entities = system.entities
+    local indices = system.indices
+    local sortDelegate = system.sortDelegate
+    if not sortDelegate then
+        local compare = system.compare
+        sortDelegate = function(e1, e2)
+            return compare(system, e1, e2)
+        end
+        system.sortDelegate = sortDelegate
+    end
+    tsort(entities, sortDelegate)
+    for i = 1, #entities do
+        local entity = entities[i]
+        indices[entity] = i
+    end
+end
+
+--- Creates a new System or System class from the supplied table. If `table` is
+-- nil, creates a new table.
+function tiny.system(table)
+    table = table or {}
+    table[systemTableKey] = true
+    return table
+end
+
+--- Creates a new Processing System or Processing System class. Processing
+-- Systems process each entity individual, and are usually what is needed.
+-- Processing Systems have three extra callbacks besides those inheritted from
+-- vanilla Systems.
 --
--- A Processing System iterates through its Entities in no particluar order, and
+--     function system:preProcess(entities, dt) -- Called before iteration.
--- updates them individually. It has two important fields:
+--     function system:process(entities, dt) -- Process each entity.
+--     function system:postProcess(entity, dt) -- Called after iteration.
 --
---   * `function system:process(entity, dt)`
+-- Processing Systems have their own `update` method, so don't implement a
---   * `function system:filter(entity)`
+-- a custom `update` callback for Processing Systems.
---
--- There are also a few other optional callbacks, including the optional
--- callbacks in `tiny.system`:
---
---   * `function system:preProcess(entities, dt)`
---   * `function system:postProcess(entities, dt)`
---
--- @param table A table to be used as a System, or `nil` to create a new
--- Processing System.
 -- @see system
 function tiny.processingSystem(table)
     table = table or {}
@@ -211,44 +304,31 @@ function tiny.processingSystem(table)
     return table
 end
 
--- Sorts Systems by a function system.sort(entity1, entity2) on modify.
+--- Creates a new Sorted System or Sorted System class. Sorted Systems sort
-local function sortedSystemOnModify(system, dt)
+-- their Entities according to a user-defined method, `system:compare(e1, e2)`,
-    local entities = system.entities
+-- which should return true if `e1` should come before `e2` and false otherwise.
-    local entityIndices = system.entityIndices
+-- Sorted Systems also override the default System's `onModify` callback, so be
-    local sortDelegate = system.sortDelegate
+-- careful if defining a custom callback. However, for processing the sorted
-    if not sortDelegate then
+-- entities, consider `tiny.sortedProcessingSystem(table)`.
-        local compare = system.compare
+-- @see system
-        sortDelegate = function(e1, e2)
+function tiny.sortedSystem(table)
-            compare(system, e1, e2)
+    table = table or {}
-        end
+    table[systemTableKey] = true
-        system.sortDelegate = sortDelegate
+    table.onModify = sortedSystemOnModify
-    end
+    return table
-    tsort(entities, sortDelegate)
-    for i = 1, #entities do
-        local entity = entities[i]
-        entityIndices[entity] = i
-    end
 end
 
---- Creates a Sorted Processing System. A Sorted System iterates through its
+--- Creates a new Sorted Processing System or Sorted Processing System class.
--- Entities in a specific order, and updates them individually. It has three
+-- Sorted Processing Systems have both the aspects of Processing Systems and
--- important methods:
+-- Sorted Systems.
---
---   * `function system:process(entity, dt)`
---   * `function system:compare(entity1, entity2)`
---   * `function system:filter(entity)`
---
--- Sorted Systems have the same optitonal callbacks as ProcessingSystems.
--- @param table A table to be used as a System, or `nil` to create a new
--- Sorted System.
 -- @see system
 -- @see processingSystem
-function tiny.sortedSystem(table)
+-- @see sortedSystem
+function tiny.sortedProcessingSystem(table)
     table = table or {}
     table[systemTableKey] = true
     table.update = processingSystemUpdate
     table.onModify = sortedSystemOnModify
-    table.sort = sortedSystemOnModify
     return table
 end
 
@@ -256,20 +336,18 @@ end
 -- A World is a container that manages Entities and Systems. Typically, a
 -- program uses one World at a time.
 --
--- The tiny-ecs module is set to be the `__index` of all World tables, so the
+-- For all World functions except `tiny.world(...)`, object-oriented syntax can
--- often clearer syntax of `world:method()` can be used for any function in the
+-- be used instead of the documented syntax. For example,
--- library. For example, `tiny.add(world, e1, e2, e3)` is the same as
+-- `tiny.add(world, e1, e2, e3)` is the same as `world:add(e1, e2, e3)`.
--- `world:add(e1, e2, e3).`
 -- @section World
 
-local worldMetaTable = { __index = tiny }
+-- Forward declaration
+local worldMetaTable
 
 --- Creates a new World.
--- Can optionally add default Systems and Entities.
+-- Can optionally add default Systems and Entities. Returns the new World along
--- @param ... Systems and Entities to add to the World
+-- with default Entities and Systems.
--- @return A new World
 function tiny.world(...)
-
     local ret = {
 
         -- List of Entities to add
@@ -287,103 +365,95 @@ function tiny.world(...)
         -- Set of Entities
         entities = {},
 
-        -- Number of Entities in World.
+        -- Number of Entities in World
         entityCount = 0,
 
-        -- List of System Data. A data element is a table with 4
+        -- List of Systems
-        -- keys: system, indices, entities, and active.
+        systems = {}
-        systems = {},
-
-        -- Table of Systems to System Indices
-        systemIndices = {}
     }
 
     tiny_add(ret, ...)
     tiny_manageSystems(ret)
     tiny_manageEntities(ret)
 
-    return setmetatable(ret, worldMetaTable)
+    return setmetatable(ret, worldMetaTable), ...
-
 end
 
 --- Adds an Entity to the world.
 -- Also call this on Entities that have changed Components such that they
--- match different Filters.
+-- match different Filters. Returns the Entity.
--- @param world
--- @param entity
 function tiny.addEntity(world, entity)
     local e2a = world.entitiesToAdd
     e2a[#e2a + 1] = entity
     if world.entities[entity] then
         tiny_removeEntity(world, entity)
     end
+    return entity
 end
 tiny_addEntity = tiny.addEntity
 
---- Adds a System to the world.
+--- Adds a System to the world. Returns the System.
--- @param world
--- @param system
 function tiny.addSystem(world, system)
     local s2a = world.systemsToAdd
     s2a[#s2a + 1] = system
+    return system
 end
 tiny_addSystem = tiny.addSystem
 
---- Shortcut for adding multiple Entities and Systems to the World.
+--- Shortcut for adding multiple Entities and Systems to the World. Returns all
--- @param world
+-- added Entities and Systems.
--- @param ... Systems and Entities
--- @see addEntity
--- @see addSystem
 function tiny.add(world, ...)
-    local args = {...}
+    local obj
-    for _, obj in ipairs(args) do
+    for i = 1, select("#", ...) do
-        if isSystem(obj) then
+        obj = select(i, ...)
-            tiny_addSystem(world, obj)
+        if obj then
-        else -- Assume obj is an Entity
+            if isSystem(obj) then
-            tiny_addEntity(world, obj)
+                tiny_addSystem(world, obj)
+            else -- Assume obj is an Entity
+                tiny_addEntity(world, obj)
+            end
         end
     end
+    return ...
 end
 tiny_add = tiny.add
 
---- Removes an Entity to the World.
+--- Removes an Entity to the World. Returns the Entity.
--- @param world
--- @param entity
 function tiny.removeEntity(world, entity)
     local e2r = world.entitiesToRemove
     e2r[#e2r + 1] = entity
+    return entity
 end
 tiny_removeEntity = tiny.removeEntity
 
---- Removes a System from the world.
+--- Removes a System from the world. Returns the System.
--- @param world
--- @param system
 function tiny.removeSystem(world, system)
     local s2r = world.systemsToRemove
     s2r[#s2r + 1] = system
+    return system
 end
 tiny_removeSystem = tiny.removeSystem
 
---- Shortcut for removing multiple Entities and Systems from the World.
+--- Shortcut for removing multiple Entities and Systems from the World. Returns
--- @param world
+-- all rmeove Systems and Entities
--- @param ... Systems and Entities
--- @see removeEntity
--- @see removeSystem
 function tiny.remove(world, ...)
-    local args = {...}
+    local obj
-    for _, obj in ipairs(args) do
+    for i = 1, select("#", ...) do
-        if isSystem(obj) then
+        obj = select(i, ...)
-            tiny_removeSystem(world, obj)
+        if obj then
-        else -- Assume obj is an Entity
+            if isSystem(obj) then
-           tiny_removeEntity(world, obj)
+                tiny_removeSystem(world, obj)
+            else -- Assume obj is an Entity
+               tiny_removeEntity(world, obj)
+            end
         end
     end
+    return ...
 end
 tiny_remove = tiny.remove
 
 -- Adds and removes Systems that have been marked from the World.
 function tiny_manageSystems(world)
-
         local s2a, s2r = world.systemsToAdd, world.systemsToRemove
 
         -- Early exit
@@ -391,7 +461,6 @@ function tiny_manageSystems(world)
             return
         end
 
-        local systemIndices = world.systemIndices
         local entities = world.entities
         local systems = world.systems
         local system, index, filter
@@ -400,8 +469,8 @@ function tiny_manageSystems(world)
         -- Remove Systems
         for i = 1, #s2r do
             system = s2r[i]
-            index = systemIndices[system]
+            index = system.index
-            if index then
+            if system.world == world then
                 onRemove = system.onRemove
                 if onRemove then
                     entityList = system.entities
@@ -409,27 +478,35 @@ function tiny_manageSystems(world)
                         onRemove(system, entityList[j])
                     end
                 end
-                systemIndices[system] = nil
                 tremove(systems, index)
                 for j = index, #systems do
-                    systemIndices[systems[j]] = j
+                    systems[j].index = j
                 end
             end
             s2r[i] = nil
+
+            -- Clean up System
+            system.world = nil
+            system.entities = nil
+            system.indices = nil
+            system.index = nil
         end
 
         -- Add Systems
         for i = 1, #s2a do
             system = s2a[i]
-            if not systemIndices[system] then
+            if systems[system.index] ~= system then
                 entityList = {}
                 entityIndices = {}
                 system.entities = entityList
                 system.indices = entityIndices
-                system.active = true
+                if system.active == nil then
+                    system.active = true
+                end
                 system.modified = true
+                system.world = world
                 index = #systems + 1
-                systemIndices[system] = index
+                system.index = index
                 systems[index] = system
 
                 -- Try to add Entities
@@ -450,12 +527,10 @@ function tiny_manageSystems(world)
             end
             s2a[i] = nil
         end
-
 end
 
 -- Adds and removes Entities that have been marked.
 function tiny_manageEntities(world)
-
     local e2a, e2r = world.entitiesToAdd, world.entitiesToRemove
 
     -- Early exit
@@ -474,6 +549,7 @@ function tiny_manageEntities(world)
         entity = e2r[i]
         if entities[entity] then
             entities[entity] = nil
+            entityCount = entityCount - 1
 
             for j = 1, #systems do
                 system = systems[j]
@@ -488,7 +564,6 @@ function tiny_manageEntities(world)
                     seis[tmpEntity] = index
                     seis[entity] = nil
                     ses[#ses] = nil
-                    entityCount = entityCount - 1
                     onRemove = system.onRemove
                     if onRemove then
                         onRemove(system, entity)
@@ -501,12 +576,12 @@ function tiny_manageEntities(world)
         e2r[i] = nil
     end
 
-
     -- Add Entities
     for i = 1, #e2a do
         entity = e2a[i]
         if not entities[entity] then
             entities[entity] = true
+            entityCount = entityCount + 1
 
             for j = 1, #systems do
                 system = systems[j]
@@ -518,7 +593,6 @@ function tiny_manageEntities(world)
                     index = #ses + 1
                     ses[index] = entity
                     seis[entity] = index
-                    entityCount = entityCount + 1
                     onAdd = system.onAdd
                     if onAdd then
                         onAdd(system, entity)
@@ -533,25 +607,24 @@ function tiny_manageEntities(world)
 
     -- Update Entity count
     world.entityCount = entityCount
-
 end
 
---- Updates the World.
+--- Updates the World by dt (delta time). Takes an optional parameter, `filter`,
--- Put this in your main loop.
+-- which is a Filter that selects Systems from the World, and updates only those
--- @param world
+-- Systems. If `filter` is not supplied, all Systems are updated. Put this
--- @param dt Delta time
+-- function in your main loop.
-function tiny.update(world, dt)
+function tiny.update(world, dt, filter)
 
     tiny_manageSystems(world)
     tiny_manageEntities(world)
 
     local systems = world.systems
-    local system, update, onModify, entities
+    local system, update, interval, onModify
 
     --  Iterate through Systems IN ORDER
     for i = 1, #systems do
         system = systems[i]
-        if system.active then
+        if system.active and ((not filter) or filter(world, system)) then
 
             -- Call the modify callback on Systems that have been modified.
             onModify = system.onModify
@@ -559,10 +632,22 @@ function tiny.update(world, dt)
                 onModify(system, dt)
             end
 
-            --Update Systems that have an update method (most Systems)
+            -- Update Systems that have an update method (most Systems)
             update = system.update
             if update then
-                update(system, dt)
+                interval = system.interval
+                if interval then
+                    local bufferedTime = (system.bufferedTime or 0) + dt
+                    while bufferedTime >= interval do
+                        bufferedTime = bufferedTime - interval
+                        if update then
+                            update(system, interval)
+                        end
+                    end
+                    system.bufferedTime = bufferedTime
+                else
+                    update(system, dt)
+                end
             end
 
             system.modified = false
@@ -571,7 +656,6 @@ function tiny.update(world, dt)
 end
 
 --- Removes all Entities from the World.
--- @param world
 function tiny.clearEntities(world)
     for e in pairs(world.entities) do
         tiny_removeEntity(world, e)
@@ -579,7 +663,6 @@ function tiny.clearEntities(world)
 end
 
 --- Removes all Systems from the World.
--- @param world
 function tiny.clearSystems(world)
     local systems = world.systems
     for i = #systems, 1, -1 do
@@ -587,44 +670,56 @@ function tiny.clearSystems(world)
     end
 end
 
---- Gets count of Entities in World.
+--- Gets number of Entities in the World.
--- @param world
 function tiny.getEntityCount(world)
     return world.entityCount
 end
 
---- Gets count of Systems in World.
+--- Gets number of Systems in World.
--- @param world
 function tiny.getSystemCount(world)
     return #(world.systems)
 end
 
---- Gets the index of a System in the World. Lower indexed Systems are processed
+--- Gets the index of the System in the World.
--- before higher indexed systems.
+-- A simpler alternative is `system.index`.
--- @param world
--- @param system
 function tiny.getSystemIndex(world, system)
-    return world.systemIndices[system]
+    return system.index
 end
 
---- Sets the index of a System in the World. Changes the order in
+--- Sets the index of a System in the World, and returns the old index. Changes
--- which they Systems processed, because lower indexed Systems are processed
+-- the order in which they Systems processed, because lower indexed Systems are
--- first.
+-- processed first. Returns the old system.index.
--- @param world
--- @param system
--- @param index
 function tiny.setSystemIndex(world, system, index)
-    local systemIndices = world.systemIndices
+    local oldIndex = system.index
-    local oldIndex = systemIndices[system]
     local systems = world.systems
-    local system = systems[oldIndex]
 
     tremove(systems, oldIndex)
     tinsert(systems, index, system)
 
     for i = oldIndex, index, index >= oldIndex and 1 or -1 do
-        systemIndices[systems[i]] = i
+        systems[i].index = i
     end
+
+    return oldIndex
 end
 
+-- Construct world metatable.
+worldMetaTable = {
+    __index = {
+        add = tiny.add,
+        addEntity = tiny.addEntity,
+        addSystem = tiny.addSystem,
+        remove = tiny.remove,
+        removeEntity = tiny.removeEntity,
+        removeSystem = tiny.removeSystem,
+        update = tiny.update,
+        clearEntities = tiny.clearEntities,
+        clearSystems = tiny.clearSystems,
+        getEntityCount = tiny.getEntityCount,
+        getSystemCount = tiny.getSystemCount,
+        getSystemIndex = tiny.getSystemIndex,
+        setSystemIndex = tiny.setSystemIndex
+    }
+}
+
 return tiny