ecs

--- Just a template for component
function Component_Empty()
    local self = {
        name = "put_unique_name"
        -- properties ...
        }
    return self
end

function Component_Position(_x, _y)
    local self = {
        name = "position",
        x = _x or 0,
        y = _y or 0 }
    return self
end

function Component_Velocity(_vx, _vy)
    local s = {
        name = "velocity",
        vx = _vx or 0,
        vy = _vy or 0 }
    return s
end

-- Systems --

Systems = {}

-- This is not a template! Custom systems must be inherited from this class
function Systems.Base()
    local self = {}
    self.name = "base"
    local subscribers = {}
    local entities = {}

    function self.subscribe(entity)
        table.insert(subscribers, entity)
    end

    function self.set_entities(_entities)
        entities = _entities
    end

    function self.get_entities()
        return {entities, subscribers}
    end

    return self
end

function Systems.Moving()
    local self = Systems.Base()
    self.name = "moving"

    function self.update(dt)
        local entities = self.get_entities()
        for i, e in iter_select(entities) do
            local p = e.position
            local v = e.velocity
            p.x = p.x + v.vx * dt
            p.y = p.y + v.vy * dt
        end
    end

    return self
end

function Systems.Handling()
    local self = Systems.Base()
    self.name = "handling"

    function self.update(dt)
        local entities = self.get_entities()
        for i, e in iter_select(entities) do
            local v = e.velocity
            v.vx, v.vy = 0, 0
            if      love.keyboard.isDown("up")      then v.vy = -100
            elseif  love.keyboard.isDown("down")    then v.vy = 100
            end
            if      love.keyboard.isDown("right")   then v.vx = 100
            elseif  love.keyboard.isDown("left")    then v.vx = -100
            end
        end
    end

    return self
end

function Systems.Drawing()
    local self = Systems.Base()
    self.name = "drawing"

    function self.draw()
        local entities = self.get_entities()
        for i, e in iter_select(entities) do
            local p = e.position
            love.graphics.circle("fill", p.x, p.y, 10)
        end
    end

    return self
end

function SystemManager()
    local self = {}
    local systems = {}
    local entities = {}

    -- registering systems
    for i, s in pairs(Systems) do
        local sys = s()
        sys.set_entities(entities)
        systems[sys.name] = sys
    end

    function self.newEntity()
        table.insert(entities, {})
        return #entities
    end

    function self.addComponent(entity, component)
        entities[entity][component.name] = component
    end

    function self.subscribe(entity, system)
        local sys = systems[system]
        sys.subscribe(entity)
    end

    function self.update(dt)
        for k, system in pairs(systems) do
            local upd = system.update
            if upd then upd(dt) end
        end
    end

    function self.draw()
        for k, system in pairs(systems) do
            local drw = system.draw
            if drw then drw(dt) end
        end
    end

    return self
end

--- Assembling custom entity
function create_player(x, y, vx, vy)
    local entity = ecs.newEntity()
    ecs.addComponent(entity, Component_Position(x, y))
    ecs.addComponent(entity, Component_Velocity(vx, vy))
    ecs.subscribe(entity, "moving")
    ecs.subscribe(entity, "handling")
    ecs.subscribe(entity, "drawing")
    return entity
end

function create_moving_dot(x, y, vx, vy)
    local entity = ecs.newEntity()
    ecs.addComponent(entity, Component_Position(x, y))
    ecs.addComponent(entity, Component_Velocity(vx, vy))
    ecs.subscribe(entity, "moving")
    ecs.subscribe(entity, "drawing")
    return entity
end

function create_static_dot(x, y)
    local entity = ecs.newEntity()
    ecs.addComponent(entity, Component_Position(x, y))
    ecs.subscribe(entity, "drawing")
    return entity
end

-- Main --
-- testing ECS framework

--- Iterator for iterating _objects_ only with selected _indexes_
-- Function takes two tables: objects and indexes
-- Tables can be passed in one table
function iter_select(...)
    local i = 0
    local arg = {...}
    local objects, indexes

    if #arg == 2 then
        objects, indexes = arg[1], arg[2]
    elseif #arg[1] == 2 then
        objects, indexes = arg[1][1], arg[1][2]
    end

    -- "next" element function
    return function ()
        local v, index = {}, nil
        i, index = next(indexes, i)
        while index do
            v = objects[index]
            if v then
                return index, v, nil
            end
            i, index = next(indexes, i)
        end
        return index, v, nil
    end
end

function love.load()
    ecs = SystemManager()
    e1 = create_player(400, 300)
    create_player(100, 100)
    create_moving_dot(200, 100, 50, 0)
    create_static_dot(500, 400)
end

function love.update(dt)
    ecs.update(dt)
end

function love.draw()
    ecs.draw()
end