为什么Elixir的MapSet在32个元素之后变得无序？

Question

iex> MapSet.new(1..32) |> Enum.to_list 
[1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 
23, 24, 25, 26, 27, 28, 29, 30, 31, 32] 

iex> MapSet.new(1..33) |> Enum.to_list 
[11, 26, 15, 20, 17, 25, 13, 8, 7, 1, 32, 3, 6, 2, 33, 10, 9, 19, 14, 5, 18, 31, 
22, 29, 21, 27, 24, 30, 23, 28, 16, 4, 12] 

下面是花好月圆1.3

def new(enumerable) do 
    map = 
    enumerable 
    |> Enum.to_list 
    |> do_new([]) 

    %MapSet{map: map} 
end 

defp do_new([], acc) do 
    acc 
    |> :lists.reverse 
    |> :maps.from_list 
end 

defp do_new([item | rest], acc) do 
    do_new(rest, [{item, true} | acc]) 
end 

即使顺序不以MapSet重要的implementation，但仍然不知道为什么MapSet后32个元素变得无序？

Answer 1

这是不特定于MapSet，但同样的事情发生与正常Map（MapSet使用Map引擎盖下）：

iex(1)> for i <- Enum.shuffle(1..32), into: %{}, do: {i, i} 
%{1 => 1, 2 => 2, 3 => 3, 4 => 4, 5 => 5, 6 => 6, 7 => 7, 8 => 8, 9 => 9, 
    10 => 10, 11 => 11, 12 => 12, 13 => 13, 14 => 14, 15 => 15, 16 => 16, 
    17 => 17, 18 => 18, 19 => 19, 20 => 20, 21 => 21, 22 => 22, 23 => 23, 
    24 => 24, 25 => 25, 26 => 26, 27 => 27, 28 => 28, 29 => 29, 30 => 30, 
    31 => 31, 32 => 32} 
iex(2)> for i <- Enum.shuffle(1..33), into: %{}, do: {i, i} 
%{11 => 11, 26 => 26, 15 => 15, 20 => 20, 17 => 17, 25 => 25, 13 => 13, 8 => 8, 
    7 => 7, 1 => 1, 32 => 32, 3 => 3, 6 => 6, 2 => 2, 33 => 33, 10 => 10, 9 => 9, 
    19 => 19, 14 => 14, 5 => 5, 18 => 18, 31 => 31, 22 => 22, 29 => 29, 21 => 21, 
    27 => 27, 24 => 24, 30 => 30, 23 => 23, 28 => 28, 16 => 16, 4 => 4, 12 => 12} 

这是因为（最有可能作为优化）二郎店大小的地图至​​作为sorted by keyarray。只有在尺寸大于​​之后，Erlang才会切换到将数据存储在Hash Array Mapped Trie like data structure中。在非调试模式Erlang中，​​为defined to be 32，因此所有长度最多为32的地图都应按排序顺序进行打印。请注意，就我所知，这是一个实现细节，您不应该依赖此行为;他们可能会在将来改变常数的值，或者如果性能更好，则可以切换到完全不同的算法。