-- ENTITY COMPONENT SYSTEM
-- Created by Koldinium
-- Licensed under 0BSD
local M = {}

-- Maintain one consistent global state across instances
_G.__kold_ecs = _G.__kold_ecs or {}
local globals = _G.__kold_ecs

globals.entities = globals.entities or 0
globals.component_arrays = globals.component_arrays or {}
globals.components = globals.components or {}
globals.systems = globals.systems or {}


function M.create_entity(components)
    local id = globals.entities + 1
    for k,v in pairs(components) do
        globals.component_arrays[k][id] = globals.components[k](v)
    end
    return id
end

function M.delete_entity(id)
    for _,v in pairs(globals.component_arrays) do
        v[id] = nil
    end
end

function M.add_component_to_entity(entity, component_id, component_val)
    globals.component_arrays[component_id][entity] = globals.components[component_id](component_val)
end

function M.remove_component_from_entity(entity, component_id)
    globals.component_arrays[component_id][entity] = nil
end

function M.declare_system(query, system)
    table.insert(globals.systems, {query = query, system=system})
end

function M.declare_component(component, initializer)
    globals.components[component] = initializer
end


function M.run_systems()
    for _,system in pairs(globals.systems) do
        local first_component_id = system.query[1]
        for id, component in globals.component_arrays[first_component_id] do
            if #system.query == 1 then
                system.system({component})
            else
                local arg = {}
                for k,v in pairs(system.query) do
                    if component_arrays[v][id] then
                        table.insert(arg, component_arrays[v][id])
                    else
                        break
                    end
                end
                if #arg == #system.query then
                    system.system(arg)
                end
            end
        end
    end
end

return M