按无序集排序 [英] Sorting on unordered_sets

问题描述

我有每帧创建的项目列表，需要对其进行排序. 每个项目的第一个要排序的成员变量是unordered_set.

I have a list of items that are created each frame and need to be sorted. Each Item's first member variable to sort by is an unordered_set.

我已将其移至系统中所有位置的有序集合，因此可以在项目列表中对其进行排序.但是我遭受了另一个代码的性能打击.

I've moved this to an ordered set everywhere in the system so I can sort it in the list of items. But I'm suffering a performance hit in another are of the code foe this.

请记住，每件物品都将被销毁并在逐帧的基础上重新创建，我能做些什么来将它们保存在unordered_set中并对它们进行排序吗?

Bearing in mind that each item will be destroyed and be recreated on a per-frame basis, is there anything I can do to hold these in unordered_sets and sort them?

class item
{
    public:
    unordered_set< int > _sortUS;
    int _sortI;
    //Other members to sort
    bool operator<( const item& that ) const
    {
        if( _sortI != that._sortI )
        {
            return _sortI < that._sortI;
        }
        else if( _sortUS != that._sortUS )
        {
            return ??? // this is what I need. I don't know how to compare these without converting them to sets
        }
    }
};

推荐答案

鉴于std::unordered_set<Key, Hash>对于任意可哈希的Key，您可以定义

Given std::unordered_set<Key, Hash> for arbitrary hashable Key, you could define

template<class Key, class Hash = std::hash<Key>>
bool operator< (std::unordered_set<Key, Hash> const& L, std::unordered_set<Key, Hash> const& R)
{
    return std::lexicographical_compare(
        begin(L), end(L), begin(R), end(R), 
        [](Key const& kL, Key const& kR) {
        return Hash()(kL) < Hash()(kR);     
    });
}

，它将对Key的哈希索引使用排序.然后，您可以在item

which will use the ordering on hash indices of Key. You can then define an ordering on item

bool operator< (item const& L, item const& R)
{
     return std::tie(L.sortI, L.sortUS) < std::tie(R.sortI, R.sortUS);
}

和std::tie会根据对item成员的引用而制成std::tuple，以便您可以使用std::tuple中的operator<.

and std::tie will make a std::tuple out of references to the members of your item so that you can use the operator< from std::tuple.

注意:由于std::tuple比较和lexicographical_compare都具有此属性，因此可以轻松证明上述比较是StrictWeakOrder(对std::sort的要求).

NOTE: you can easily prove that the above comparison is a StrictWeakOrder (a requirement for std::sort) since both the std::tuple comparison and the lexicographical_compare have this property.

但是，unordered_set的顺序在其他方面非常不寻常.

However, the ordering of unordered_set is very unusual in other respects.

• 散列键索引与您对元素进行迭代的顺序不符(有一些模运算将散列键映射到容器中的索引)
• 将元素添加到unordered_set可能会导致重新哈希并导致先前的排序无效

• the hashed key index doesn't correspond to the order in which you iterate over elements (there is some modulo operation that maps hashed keys to indices in the container)
• adding elements to an unordered_set can result in rehashing and invalidation of previous orderering

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