--[[
Copyright (c) 2015 Calvin Rose

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]]

--- @module tiny-ecs
-- @author Calvin Rose
-- @license MIT
-- @copyright 2015
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 setmetatable = setmetatable
local type = type
local select = select

-- Local versions of the library functions
local tiny_manageEntities
local tiny_manageSystems
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.
-- Filters take two parameters, the System and the Entity, and return a boolean
-- 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-ecs's Filter functions are immutable and can be
-- used by multiple Systems.
--
--    local f1 = tiny.requireAll("position", "velocity", "size")
--    local f2 = tiny.requireAny("position", "velocity", "size")
--
--    local e1 = {
--        position = {2, 3},
--        velocity = {3, 3},
--        size = {4, 4}
--    }
--
--    local entity2 = {
--        position = {4, 5},
--        size = {4, 4}
--    }
--
--    local e3 = {
--        position = {2, 3},
--        velocity = {3, 3}
--    }
--
--    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.
function tiny.requireAll(...)
    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 not c(system, e) then
                    return false
                end
            elseif e[c] == nil then
                return false
            end
        end
        return true
    end
end

--- Makes a Filter that selects Entities with at least one of the specified
-- Components and Filters.
function tiny.requireAny(...)
    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 true
                end
            elseif e[c] ~= nil then
                return true
            end
        end
        return false
    end
end

--- Makes a Filter that rejects Entities with all specified Components and
-- Filters, and selects all other Entities.
function tiny.rejectAll(...)
    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 not c(system, e) then
                    return true
                end
            elseif e[c] == nil then
                return true
            end
        end
        return false
    end
end

--- 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)` - Returns true if this System should
-- include this Entity, otherwise should return false. If this isn't specified,
-- no Entities are included in the System.
--   * `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 convenient to use `tiny.requireAll` or `tiny.requireAny`,
-- but one can write their own filters as well. Set the Filter of a System like
-- so:
--    system.filter = tiny.requireAll("a", "b", "c")
-- or
--    function system:filter(entity)
--        return entity.myRequiredComponentName ~= nil
--    end
--
-- All Systems also have a few important fields that are initialized when the
-- system is added to the World. A few are important, and few should be less
-- commonly used.
--
--   * 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.
local function processingSystemUpdate(system, dt)
    local entities = system.entities
    local preProcess = system.preProcess
    local process = system.process
    local postProcess = system.postProcess
    local entity

    if preProcess then
        preProcess(system, dt)
    end

    if process then
        local len = #entities
        for i = 1, len do
            entity = entities[i]
            process(system, entity, dt)
        end
    end

    if postProcess then
        postProcess(system, dt)
    end
end

-- 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.
--
--     function system:preProcess(entities, dt) -- Called before iteration.
--     function system:process(entities, dt) -- Process each entity.
--     function system:postProcess(entity, dt) -- Called after iteration.
--
-- Processing Systems have their own `update` method, so don't implement a
-- a custom `update` callback for Processing Systems.
-- @see system
function tiny.processingSystem(table)
    table = table or {}
    table[systemTableKey] = true
    table.update = processingSystemUpdate
    return table
end

--- Creates a new Sorted System or Sorted System class. Sorted Systems sort
-- their Entities according to a user-defined method, `system:compare(e1, e2)`,
-- which should return true if `e1` should come before `e2` and false otherwise.
-- Sorted Systems also override the default System's `onModify` callback, so be
-- careful if defining a custom callback. However, for processing the sorted
-- entities, consider `tiny.sortedProcessingSystem(table)`.
-- @see system
function tiny.sortedSystem(table)
    table = table or {}
    table[systemTableKey] = true
    table.onModify = sortedSystemOnModify
    return table
end

--- Creates a new Sorted Processing System or Sorted Processing System class.
-- Sorted Processing Systems have both the aspects of Processing Systems and
-- Sorted Systems.
-- @see system
-- @see processingSystem
-- @see sortedSystem
function tiny.sortedProcessingSystem(table)
    table = table or {}
    table[systemTableKey] = true
    table.update = processingSystemUpdate
    table.onModify = sortedSystemOnModify
    return table
end

--- World functions.
-- A World is a container that manages Entities and Systems. Typically, a
-- program uses one World at a time.
--
-- For all World functions except `tiny.world(...)`, object-oriented syntax can
-- be used instead of the documented syntax. For example,
-- `tiny.add(world, e1, e2, e3)` is the same as `world:add(e1, e2, e3)`.
-- @section World

-- Forward declaration
local worldMetaTable

--- Creates a new World.
-- Can optionally add default Systems and Entities. Returns the new World along
-- with default Entities and Systems.
function tiny.world(...)
    local ret = {

        -- List of Entities to add
        entitiesToAdd = {},

        -- List of Entities to remove
        entitiesToRemove = {},

        -- List of Entities to add
        systemsToAdd = {},

        -- List of Entities to remove
        systemsToRemove = {},

        -- Set of Entities
        entities = {},

        -- Number of Entities in World
        entityCount = 0,

        -- List of Systems
        systems = {}
    }

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

    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. Returns the 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. Returns the 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. Returns all
-- added Entities and Systems.
function tiny.add(world, ...)
    local obj
    for i = 1, select("#", ...) do
        obj = select(i, ...)
        if obj then
            if isSystem(obj) then
                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. Returns the 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. Returns the 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. Returns
-- all rmeove Systems and Entities
function tiny.remove(world, ...)
    local obj
    for i = 1, select("#", ...) do
        obj = select(i, ...)
        if obj then
            if isSystem(obj) then
                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
        if #s2a == 0 and #s2r == 0 then
            return
        end

        local entities = world.entities
        local systems = world.systems
        local system, index, filter
        local entityList, entityIndices, entityIndex, onRemove, onAdd

        -- Remove Systems
        for i = 1, #s2r do
            system = s2r[i]
            index = system.index
            if system.world == world then
                onRemove = system.onRemove
                if onRemove then
                    entityList = system.entities
                    for j = 1, #entityList do
                        onRemove(system, entityList[j])
                    end
                end
                tremove(systems, index)
                for j = index, #systems do
                    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 systems[system.index] ~= system then
                entityList = {}
                entityIndices = {}
                system.entities = entityList
                system.indices = entityIndices
                if system.active == nil then
                    system.active = true
                end
                system.modified = true
                system.world = world
                index = #systems + 1
                system.index = index
                systems[index] = system

                -- Try to add Entities
                onAdd = system.onAdd
                filter = system.filter
                if filter then
                    for entity in pairs(entities) do
                        if filter(system, entity) then
                            entityIndex = #entityList + 1
                            entityList[entityIndex] = entity
                            entityIndices[entity] = entityIndex
                            if onAdd then
                                onAdd(system, entity)
                            end
                        end
                    end
                end
            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
    if #e2a == 0 and #e2r == 0 then
        return
    end

    local entities = world.entities
    local systems = world.systems
    local entityCount = world.entityCount
    local entity, system, index
    local onRemove, onAdd, ses, seis, filter, tmpEntity

    -- Remove Entities
    for i = 1, #e2r do
        entity = e2r[i]
        if entities[entity] then
            entities[entity] = nil
            entityCount = entityCount - 1

            for j = 1, #systems do
                system = systems[j]
                ses = system.entities
                seis = system.indices
                index = seis[entity]

                if index then
                    system.modified = true
                    tmpEntity = ses[#ses]
                    ses[index] = tmpEntity
                    seis[tmpEntity] = index
                    seis[entity] = nil
                    ses[#ses] = nil
                    onRemove = system.onRemove
                    if onRemove then
                        onRemove(system, entity)
                    end
                end

            end

        end
        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]
                ses = system.entities
                seis = system.indices
                filter = system.filter
                if filter and filter(system, entity) then
                    system.modified = true
                    index = #ses + 1
                    ses[index] = entity
                    seis[entity] = index
                    onAdd = system.onAdd
                    if onAdd then
                        onAdd(system, entity)
                    end
                end

            end

        end
        e2a[i] = nil
    end

    -- Update Entity count
    world.entityCount = entityCount
end

--- Updates the World by dt (delta time). Takes an optional parameter, `filter`,
-- which is a Filter that selects Systems from the World, and updates only those
-- Systems. If `filter` is not supplied, all Systems are updated. Put this
-- function in your main loop.
function tiny.update(world, dt, filter)

    tiny_manageSystems(world)
    tiny_manageEntities(world)

    local systems = world.systems
    local system, update, interval, onModify

    --  Iterate through Systems IN ORDER
    for i = 1, #systems do
        system = systems[i]
        if system.active and ((not filter) or filter(world, system)) then

            -- Call the modify callback on Systems that have been modified.
            onModify = system.onModify
            if onModify and system.modified then
                onModify(system, dt)
            end

            -- Update Systems that have an update method (most Systems)
            update = system.update
            if update then
                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
        end
    end
end

--- Removes all Entities from the World.
function tiny.clearEntities(world)
    for e in pairs(world.entities) do
        tiny_removeEntity(world, e)
    end
end

--- Removes all Systems from the World.
function tiny.clearSystems(world)
    local systems = world.systems
    for i = #systems, 1, -1 do
        tiny_removeSystem(world, systems[i])
    end
end

--- Gets number of Entities in the World.
function tiny.getEntityCount(world)
    return world.entityCount
end

--- Gets number of Systems in World.
function tiny.getSystemCount(world)
    return #(world.systems)
end

--- Gets the index of the System in the World.
-- A simpler alternative is `system.index`.
function tiny.getSystemIndex(world, system)
    return system.index
end

--- Sets the index of a System in the World, and returns the old index. Changes
-- the order in which they Systems processed, because lower indexed Systems are
-- processed first. Returns the old system.index.
function tiny.setSystemIndex(world, system, index)
    local oldIndex = system.index
    local systems = world.systems

    tremove(systems, oldIndex)
    tinsert(systems, index, system)

    for i = oldIndex, index, index >= oldIndex and 1 or -1 do
        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