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

--- Tiny-ecs Version, a period-separated three number string like "1.2.3" with
-- no leading zeros.
tiny._VERSION = "0.3.0"

local tinsert = table.insert
local tremove = table.remove
local tconcat = table.concat
local pairs = pairs
local ipairs = ipairs
local setmetatable = setmetatable
local getmetatable = getmetatable

-- Local versions of the library functions
local tiny_system
local tiny_manageEntities
local tiny_manageSystems
local tiny_updateSystem
local tiny_add
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 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 e[c] ~= nil then
                return true
            end
        end
        return false
    end
end

--- System functions.
-- A System a wrapper around function callbacks for manipulating Entities.
-- @section System

local systemMetaTable = {}

--- Creates a System.
-- @param callback Function of one argument, delta time, that is called once
-- per world update
-- @param filter Function of one argument, an Entity, that returns a boolean
-- @param entityCallback Function of two arguments, an Entity and delta time
-- @param onAdd Optional callback for when Enities are added to the System that
-- takes one argument, an Entity
-- @param onRemove Similar to onAdd, but is instead called when an Entity is
-- removed from the System
function tiny.system(callback, filter, entityCallback, onAdd, onRemove)
    local ret = {
        callback = callback,
        filter = filter,
        entityCallback = entityCallback,
        onAdd = onAdd,
        onRemove = onRemove
    }
    setmetatable(ret, systemMetaTable)
    return ret
end
tiny_system = tiny.system

--- Creates a System that processes Entities every update. Also provides
-- optional callbacks for when Entities are added or removed from the System.
-- @param filter Function of one argument, an Entity, that returns a boolean
-- @param entityCallback Function of two arguments, an Entity and delta time
-- @param onAdd Optional callback for when Entities are added to the System that
-- takes one argument, an Entity
-- @param onRemove Similar to onAdd, but is instead called when an Entity is
-- removed from the System
function tiny.processingSystem(filter, entityCallback, onAdd, onRemove)
    return tiny_system(nil, filter, entityCallback, onAdd, onRemove)
end

local worldMetaTable = { __index = tiny }

--- 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

--- 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
        status = {},

        -- 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 = {},

        -- List of Systems to add next update
        systemsToAdd = {},

        -- List of Systems to remove next update
        systemsToRemove = {}

    }

    tiny.add(ret, ...)
    tiny.manageSystems(ret)
    tiny.manageEntities(ret)

    setmetatable(ret, worldMetaTable)
    return ret

end

--- Adds 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 = {...}
    local status = world.status
    local entities = world.entities
    local systemsToAdd = world.systemsToAdd
    for _, obj in ipairs(args) do
        if getmetatable(obj) == systemMetaTable then
            tinsert(systemsToAdd, obj)
        else -- Assume obj is an Entity
            entities[obj] = true
            status[obj] = "add"
        end
    end
end
tiny_add = tiny.add

--- Removes 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 = {...}
    local status = world.status
    local entities = world.entities
    local systemsToRemove = world.systemsToRemove
    for _, obj in ipairs(args) do
        if getmetatable(obj) == systemMetaTable then
            tinsert(systemsToRemove, obj)
        elseif entities[obj] then -- Assume obj is an Entity
            status[obj] = "remove"
        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 callback = system.callback
    local entityCallback = system.entityCallback

    if callback then
        callback(dt)
    end

    if entityCallback then
        local entities = world.entities
        local es = world.systemEntities[system]
        if es then
            for e in pairs(es) do
                entityCallback(e, dt)
            end
        end
    end
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 systemsToAdd = world.systemsToAdd
        local systemsToRemove = world.systemsToRemove
        local activeSystems = world.activeSystems

        -- Keep track of the number of Systems in the world
        local deltaSystemCount = 0

        -- Remove all Systems queued for removal
        for i = 1, #systemsToRemove do
            -- Pop system off the remove queue
            local sys = systemsToRemove[i]
            systemsToRemove[i] = nil

            local sysID = systemIndices[sys]
            if sysID then
                tremove(systems, sysID)

                local onRemove = sys.onRemove
                if onRemove then
                    for e in pairs(systemEntities[sys]) do
                        onRemove(e)
                    end
                end

                systemEntities[sys] = nil
                activeSystems[sys] = nil
                deltaSystemCount = deltaSystemCount - 1
            end
        end

        -- Add Systems queued for addition
        for i = 1, #systemsToAdd do
            -- Pop system off the add queue
            local sys = systemsToAdd[i]
            systemsToAdd[i] = nil

            -- Add system to world
            local es = {}
            systemEntities[sys] = es
            tinsert(systems, sys)
            systemIndices[sys] = #systems
            activeSystems[sys] = true

            local filter = sys.filter
            if filter then
                for e in pairs(entities) do
                    es[e] = filter(e) and true or nil
                end
            end

            deltaSystemCount = deltaSystemCount + 1
        end

        -- Update the number of Systems in the World
        world.systemCount = world.systemCount + deltaSystemCount
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 statuses = world.status
    local systemEntities = world.systemEntities
    local entities = world.entities
    local systems = world.systems

    -- Keep track of the number of Entities in the World
    local deltaEntityCount = 0

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

    -- Update Entity count
    world.entityCount = world.entityCount + deltaEntityCount

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 status = world.status
    for e in pairs(world.entities) do
        status[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 newSystemsToRemove = {}
    local systems = world.systems
    for i = 1, #systems do
        newSystemsToRemove[i] = systems[i]
    end
    world.systemsToRemove = newSystemsToRemove
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.getSystemsCount(world)
    return world.systemCount
end

--- Sets if a System is active in a world. If the system is active, it will
-- update automatically when World:update(dt) is called. Otherwise, the user
-- must call World:updateSystem(system, dt) to update the unactivated system.
-- @param world
-- @param system A System in the World activate/deactivate
-- @param active Boolean new state of the System
function tiny.setSystemActive(world, system, active)
    world.activeSystems[system] = active and true or nil
end

return tiny