local function WorldToGridPos(self, WorldPos)
    local CellSize = self.CellSize
    local RoundedHalfCellSize = self.RoundedHalfCellSize
    local X = WorldPos.X + RoundedHalfCellSize.X
    local Y = (WorldPos.Z or WorldPos.Y) + RoundedHalfCellSize.Y

    return Vector2.new(
        math.floor( X / CellSize.X),
        math.floor( Y / CellSize.Y)
    )
end

local function CreateCell(self, CellPos)
    local Pos_X, Pos_Y = CellPos.X, CellPos.Y
    local Cells = self.Cells
    local Cell_X = Cells[Pos_X]

    if not Cell_X then
        Cell_X = {}
        Cells[Pos_X] = Cell_X
    end

    local Cell_Y = Cell_X[Pos_Y]

    if not Cell_Y then
        Cell_Y = {}

        Cell_X[Pos_Y] = Cell_Y
    end

    return Cell_Y
end

local function GetCell(self, CellPos)
    local Cell_X = self.Cells[CellPos.X]

    return Cell_X and Cell_X[CellPos.Y]
end


local function MoveObjectToCell(self, Object, CellPos, ObjectCacheName)
    local NewCell = self:CreateCell(CellPos)
    local ObjectToCell = self.ObjectToCell
    local OldCell = ObjectToCell[Object]
    local ObjectCache = ObjectCacheName == nil and NewCell or NewCell[ObjectCacheName]

    if not ObjectCache then
        ObjectCache = {}

        NewCell[ObjectCacheName] = ObjectCacheName
    end

    if OldCell then
        if OldCell[Object] then
            --stored in dictionary
            OldCell[Object] = nil
        else
            --stored in an array
            table.remove(OldCell, table.find(OldCell, Object))
        end
    end

    table.insert(ObjectCache, Object)
    ObjectToCell[Object] = ObjectCache
end

local function RegisterObject(self, Object, WorldPos, CacheName)
    local CellPos = self:WorldToGridPos(WorldPos)

    MoveObjectToCell(self, Object, CellPos, CacheName)

    return CellPos
end

local Module = {}

function Module.new(CellSize)
    local Grid = {
        CellSize = CellSize,
        RoundedHalfCellSize = Vector2.new(math.floor(CellSize.X/2), math.floor(CellSize.Y/2)),

        Cells = {}, --[X][Z] = {}
        ObjectToCell = {}, --[Object] = Cell

        WorldToGridPos = WorldToGridPos,
        GetCell = GetCell,
        RegisterObject = RegisterObject,
        MoveObjectToCell = MoveObjectToCell,
        CreateCell = CreateCell,

    }

    return Grid
end

return Module