--- @module tiny-ecs
-- @author Calvin Rose
local tiny = {}

--- Tiny-ecs Version, a period-separated three number string like "1.2.3"
tiny._VERSION = "1.0.0"

-- Local versions of standard lua functions
local tinsert = table.insert
local tremove = table.remove
local pairs = pairs
local ipairs = ipairs
local setmetatable = setmetatable
local type = type

-- Local versions of the library functions
local tiny_manageEntities
local tiny_manageSystems
local tiny_updateSystem
local tiny_addEntity
local tiny_addSystem
local tiny_add
local tiny_removeEntity
local tiny_removeSystem
local tiny_remove

--- Filter functions.
-- A Filter is a function that selects which Entities apply to a System.
-- @section Filter

--- Makes a Filter that filters Entities with specified Components.
-- An Entity must have all Components to match the filter.
-- @param ... List of Components
function tiny.requireAll(...)
    local components = {...}
    local len = #components
    return function(_, e)
        local c
        for i = 1, len do
            c = components[i]
            if type(c) == 'function' then
                if not c(_, e) then
                    return false
                end
            elseif e[c] == nil then
                return false
            end
        end
        return true
    end
end

--- Makes a Filter that filters Entities with specified Components.
-- An Entity must have at least one specified Component to match the filter.
-- @param ... List of Components
function tiny.requireOne(...)
    local components = {...}
    local len = #components
    return function(_, e)
        local c
        for i = 1, len do
            c = components[i]
            if type(c) == 'function' then
                if c(_, e) then
                    return true
                end
            elseif e[c] ~= nil then
                return true
            end
        end
        return false
    end
end

--- System functions.
-- A System is a wrapper around function callbacks for manipulating Entities.
-- @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 = {}

-- Check if tables are systems.
local function isSystem(table)
    return table[systemTableKey]
end

--- Creates a System. Systems are tables that contain at least one field; 
-- an update function that takes parameters like so: 
-- `function system:update(entities, dt)`. `entities` is an unordered table of 
-- Entities with Entities as KEYS, and `dt` is the delta time. There are also a 
-- few other optional callbacks:
-- `function system:filter(entity)` - returns a boolean,
-- `function system:onAdd(entity)` - returns nil,
-- `function system:onRemove(entity)` - returns nil.
-- For Filters, it is conveient to use `tiny.requireAll` or `tiny.requireOne`,
-- but one can write their own filters as well.
-- @param table A table to be used as a System, or `nil` to create a new System.
function tiny.system(table)
    if table == nil then
        table = {}
    end
    table[systemTableKey] = true
    return table
end

-- Update function for all Processing Systems.
local function processingSystemUpdate(system, entities, dt)
    local preProcess = system.preProcess
    local process = system.process
    local postProcess = system.postProcess
    if preProcess then
        preProcess(system, entities, dt)
    end
    if process then
        for entity in pairs(entities) do
            process(system, entity, dt)
        end
    end
    if postProcess then
        postProcess(system, entities, dt)
    end
end

--- Creates a Processing System. A Processing System iterates through its 
-- Entities in no particluar order, and updates them individually. It has two 
-- important fields, `function system:process(entity, dt)`, and `function 
-- system:filter(entity)`. `entities` is an unordered table of Entities with 
-- Entities as KEYS, and `dt` is the delta time. There are also a  few other 
-- optional callbacks:
-- `function system:preProcess(entities, dt)` - returns nil,
-- `function system:postProcess(entities, dt)` - returns nil,
-- `function system:onAdd(entity)` - returns nil,
-- `function system:onRemove(entity)` - returns nil.
-- For Filters, it is conveient to use `tiny.requireAll` or `tiny.requireOne`,
-- but one can write their own filters as well.
-- @param table A table to be used as a System, or `nil` to create a new 
-- Processing System.
function tiny.processingSystem(table)
    if table == nil then
        table = {}
    end
    table[systemTableKey] = true
    table.update = processingSystemUpdate
    return table
end

--- World functions.
-- A World is a container that manages Entities and Systems. The tiny-ecs module
-- is set to be the `__index` of all World tables, so the often clearer syntax of
-- World:method can be used for any function in the library. For example,
-- `tiny.add(world, e1, e2, e3)` is the same as `world:add(e1, e2, e3).`
-- @section World

local worldMetaTable = { __index = tiny }

--- Creates a new World.
-- Can optionally add default Systems and Entities.
-- @param ... Systems and Entities to add to the World
-- @return A new World
function tiny.world(...)

    local ret = {

        -- Table of Entities to status
        -- Statuses: remove, add
        entityStatus = {},

        -- Table of Systems to status
        -- Statuses: remove, add
        systemStatus = {},

        -- Set of Entities
        entities = {},

        -- Number of Entities in World.
        entityCount = 0,

        -- Number of Systems in World.
        systemCount = 0,

        -- List of Systems
        systems = {},

        -- Table of Systems to whether or not they are active.
        activeSystems = {},

        -- Table of Systems to System Indices
        systemIndices = {},

        -- Table of Systems to Sets of matching Entities
        systemEntities = {}
    }

    tiny_add(ret, ...)
    tiny_manageSystems(ret)
    tiny_manageEntities(ret)

    return setmetatable(ret, worldMetaTable)

end

--- Adds an Entity to the world.
-- The new Entity will enter the world next time World:update is called.
-- Also call this on Entities that have changed Components such that it 
-- matches different systems.
-- @param world
-- @param entity
function tiny.addEntity(world, entity)
    world.entityStatus[entity] = "add"
end
tiny_addEntity = tiny.addEntity

--- Adds a System to the world.
-- The new System will enter the world next time World:update is called.
-- @param world
-- @param system
function tiny.addSystem(world, system)
    world.systemStatus[system] = "add"
end
tiny_addSystem = tiny.addSystem

--- Shortcut for adding multiple Entities and Systems to the World.
-- New objects will enter the World the next time World:update(dt) is called.
-- Also call this method when an Entity has had its Components changed, such
-- that it matches different Filters.
-- @param world
-- @param ... Systems and Entities
function tiny.add(world, ...)
    local args = {...}
    for _, obj in ipairs(args) do
        if isSystem(obj) then
            tiny_addSystem(world, obj)
        else -- Assume obj is an Entity
            tiny_addEntity(world, obj)
        end
    end
end
tiny_add = tiny.add

--- Removes an Entity to the World.
-- The Entity will exit the World next time World:update is called.
-- Also call this on Entities that have changed Components such that it 
-- matches different systems.
-- @param world
-- @param entity
function tiny.removeEntity(world, entity)
    world.entityStatus[entity] = "remove"
end
tiny_removeEntity = tiny.removeEntity

--- Removes a System from the world.
-- The System will exit the World next time World:update is called.
-- @param world
-- @param system
function tiny.removeSystem(world, system)
    world.systemStatus[system] = "remove"
end
tiny_removeSystem = tiny.removeSystem

--- Shortcut for removing multiple Entities and Systems from the World. 
-- Objects will exit the World the next time World:update(dt) is called.
-- @param world
-- @param ... Systems and Entities
function tiny.remove(world, ...)
    local args = {...}
    for _, obj in ipairs(args) do
        if isSystem(obj) then
            tiny_removeSystem(world, obj)
        else -- Assume obj is an Entity
           tiny_removeEntity(world, obj)
        end
    end
end
tiny_remove = tiny.remove

--- Updates a System.
-- @param world
-- @param system A System in the World to update
-- @param dt Delta time
function tiny.updateSystem(world, system, dt)
    local es = world.systemEntities[system]
    system:update(es, dt)
end
tiny_updateSystem = tiny.updateSystem

--- Adds and removes Systems that have been marked from the World.
-- The user of this library should seldom if ever call this.
-- @param world
function tiny.manageSystems(world)

        local systemEntities = world.systemEntities
        local systemIndices = world.systemIndices
        local entities = world.entities
        local systems = world.systems
        local systemStatus = world.systemStatus
        local activeSystems = world.activeSystems
        local systemCount = world.systemCount

        for system, status in pairs(systemStatus) do

            if status == "add" then
                local es = {}
                systemEntities[system] = es
                systemIndices[system] = systemCount + 1
                systemCount =  systemCount + 1
                systems[systemIndices[system]] = system
                activeSystems[system] = true
                local filter = system.filter
                local onAdd = system.onAdd
                if filter then
                    for e in pairs(entities) do
                        local added = filter(system, e) and true or nil
                        es[e] = added
                        if added and onAdd then
                            onAdd(system, e)
                        end
                    end
                end
            elseif status == "remove" then
                local index = systemIndices[system]
                systemCount =  systemCount - 1
                tremove(systems, index)
                local onRemove = system.onRemove
                if onRemove then
                    for e in pairs(systemEntities[sys]) do
                        onRemove(e)
                    end
                end
                systemEntities[system] = nil
                activeSystems[system] = nil
            end

            systemStatus[system] = nil
        end

        -- Update the number of Systems in the World
        world.systemCount = systemCount
end
tiny_manageSystems = tiny.manageSystems

--- Adds and removes Entities that have been marked.
-- The user of this library should seldom if ever call this.
-- @param world
function tiny.manageEntities(world)

    local entityStatus = world.entityStatus
    local systemEntities = world.systemEntities
    local entities = world.entities
    local systems = world.systems
    local entityCount = world.entityCount

    -- Add, remove, or change Entities
    for e, s in pairs(entityStatus) do
        if s == "add" then
            if not entities[e] then
                entityCount = entityCount + 1
            end
            entities[e] = true
            for system, es in pairs(systemEntities) do
                local filter = system.filter
                if filter then
                    local matches = filter(system, e) and true or nil
                    local onRemove = system.onRemove
                    if not matches and es[e] and onRemove then
                        onRemove(system, e)
                    end
                    local onAdd = system.onAdd
                    if onAdd and matches and not es[e] then
                        onAdd(system, e)
                    end
                    es[e] = matches
                end
            end
        elseif s == "remove" then
            if entities[e] then
                entityCount = entityCount - 1
            end
            entities[e] = nil
            for system, es in pairs(systemEntities) do
                local onRemove = system.onRemove
                if es[e] and onRemove then
                    onRemove(system, e)
                end
                es[e] = nil
            end
        end
        entityStatus[e] = nil
    end

    -- Update Entity count
    world.entityCount = entityCount

end
tiny_manageEntities = tiny.manageEntities

--- Updates the World.
-- Frees Entities that have been marked for freeing, adds
-- entities that have been marked for adding, etc.
-- @param world
-- @param dt Delta time
function tiny.update(world, dt)

    tiny_manageSystems(world)
    tiny_manageEntities(world)

    --  Iterate through Systems IN ORDER
    for _, s in ipairs(world.systems) do
        if world.activeSystems[s] then
            tiny_updateSystem(world, s, dt)
        end
    end
end

--- Removes all Entities from the World.
-- When World:update(dt) is next called,
-- all Entities will be removed.
-- @param world
function tiny.clearEntities(world)
    local entityStatus = world.entityStatus
    for e in pairs(world.entities) do
        entityStatus[e] = "remove"
    end
end

--- Removes all Systems from the World.
-- When World:update(dt) is next called,
-- all Systems will be removed.
-- @param world
function tiny.clearSystems(world)
    local systemStatus = world.systemStatus
    for _, s in ipairs(world.systems) do
        systemStatus[s] = "remove"
    end
end

--- Gets count of Entities in World.
-- @param world
function tiny.getEntityCount(world)
    return world.entityCount
end

--- Gets count of Systems in World.
-- @param world
function tiny.getSystemCount(world)
    return world.systemCount
end

--- Gets the index of a System in the world. Lower indexed Systems are processed
-- before higher indexed systems.
-- @param world
-- @param system
function tiny.getSystemIndex(world, system)
    return world.systemIndices[system]
end

--- Sets the index of a System in the world. Changes the order in
-- which they Systems processed, because lower indexed Systems are processed 
-- first.
-- @param world
-- @param system
-- @param index
function tiny.setSystemIndex(world, system, index)
    local oldIndex = world.systemIndices[system]
    local systems = world.systems
    tremove(systems, oldIndex)
    tinsert(systems, index, system)
    world.systemIndices[system] = index
end

--- Activates Systems in the World.
-- Activated Systems will be update whenever tiny.update(world, dt) is called.
-- @param world
-- @param ... Systems to activate. The Systems must already be added to the
-- World.
function tiny.activate(world, ...)
    local args = {...}
    for _, obj in ipairs(args) do
        world.activeSystems[obj] = true
    end
end

--- Deactivates Systems in the World.
-- Deactivated Systems must be update manually, and will not update when the
-- rest of World updates. They will, however, process new Entities added while
-- the System is deactivated.
-- @param world
-- @param ... Systems to deactivate. The Systems must already be added to the
-- World.
function tiny.deactivate(world, ...)
    local args = {...}
    for _, obj in ipairs(args) do
        world.activeSystems[obj] = nil
    end
end

return tiny