Fastest container or algorithm for unique reusable ids in C++

Question

I have a need for unique reusable ids. The user can choose his own ids or he can ask for a free one. The API is basically

class IdManager {
public:
  int Allocat         


        

Answer 1

But I don't think you have to guarantee the id must starts from 1. You can just make sure the available id must be larger than all allocated ids.

Like if the 3 is registered first, then the next available id can just be 4. I don't think it is necessary to use 1.

Answer 2

Similar to skwllsp, I'd keep track of the ranges that have not been allocated, but my methods are slightly different. The base container would be a map, with the key being the upper bound of the range and the value being the lower bound.

IdManager::IdManager()
{
    m_map.insert(std::make_pair(std::numeric_limits<int>::max(), 1);
}

int IdManager::AllocateId()
{
    assert(!m_map.empty());
    MyMap::iterator p = m_map.begin();
    int id = p->second;
    ++p->second;
    if (p->second > p->first)
        m_map.erase(p);
    return id;
}

void IdManager::FreeId(int id)
{
    // I'll fill this in later
}

bool IdManager::MarkAsUsed(int id)
{
    MyMap::iterator p = m_map.lower_bound(id);

    // return false if the ID is already allocated
    if (p == m_map.end() || id < p->second || id > p->first)))
        return false;

    // first thunderstorm of the season, I'll leave this for now before the power glitches
}

bool IdManager::IsUsed(int id)
{
    MyMap::iterator p = m_map.lower_bound(id);
    return (p != m_map.end() && id >= p->second && id <= p->first);
}

Answer 3

Normally, i'd say stick to an simple implementation until you have an idea of which methods are used most. Premature tuning might prove wrong. Use the simple implementation, and log its use, then you can optimize from the functions that are used the most. No use in optimizing for quick removal or quick allocation if you only need a couple of hundred ids and a simple vector would be enough.

Answer 4

Compressed vector. But I don't think any container would make noticeable difference.

Answer 5

So a friend pointed out that in this case a hash might be better. Most OpenGL programs don't use more than a few thousand ids so a hash with say 4096 slots is almost guaranteed to have only 1 or 2 entries per slot. There is some degenerate case where lots of ids might go in 1 slot but that's seriously unlikely. Using a hash would make AllocateID much slower but a set could be used for that. Allocating being slower is less important than InUse being fast for my use case.

Answer 6

My idea is to use std::set and Boost.interval so IdManager will hold a set of non-overlapping intervals of free IDs. AllocateId() is very simple and very quick and just returns the left boundary of the first free interval. Other two methods are slightly more difficult because it might be necessary to split an existing interval or to merge two adjacent intervals. However they are also quite fast.

So this is an illustration of the idea of using intervals:

IdManager mgr;    // Now there is one interval of free IDs:  [1..MAX_INT]
mgr.MarkAsUsed(3);// Now there are two interval of free IDs: [1..2], [4..MAX_INT]
mgr.AllocateId(); // two intervals:                          [2..2], [4..MAX_INT]
mgr.AllocateId(); // Now there is one interval:              [4..MAX_INT]
mgr.AllocateId(); // Now there is one interval:              [5..MAX_INT]

This is code itself:

#include <boost/numeric/interval.hpp>
#include <limits>
#include <set>
#include <iostream>


class id_interval 
{
public:
    id_interval(int ll, int uu) : value_(ll,uu)  {}
    bool operator < (const id_interval& ) const;
    int left() const { return value_.lower(); }
    int right() const {  return value_.upper(); }
private:
    boost::numeric::interval<int> value_;
};

class IdManager {
public:
    IdManager();
    int AllocateId();          // Allocates an id
    void FreeId(int id);       // Frees an id so it can be used again
    bool MarkAsUsed(int id);   // Let's the user register an id. 
private: 
    typedef std::set<id_interval> id_intervals_t;
    id_intervals_t free_;
};

IdManager::IdManager()
{
    free_.insert(id_interval(1, std::numeric_limits<int>::max()));
}

int IdManager::AllocateId()
{
    id_interval first = *(free_.begin());
    int free_id = first.left();
    free_.erase(free_.begin());
    if (first.left() + 1 <= first.right()) {
        free_.insert(id_interval(first.left() + 1 , first.right()));
    }
    return free_id;
}

bool IdManager::MarkAsUsed(int id)
{
    id_intervals_t::iterator it = free_.find(id_interval(id,id));
    if (it == free_.end()) {
        return false;
    } else {
        id_interval free_interval = *(it);
        free_.erase (it);
        if (free_interval.left() < id) {
            free_.insert(id_interval(free_interval.left(), id-1));
        }
        if (id +1 <= free_interval.right() ) {
            free_.insert(id_interval(id+1, free_interval.right()));
        }
        return true;
    }
}

void IdManager::FreeId(int id)
{
    id_intervals_t::iterator it = free_.find(id_interval(id,id));
    if (it != free_.end()  && it->left() <= id && it->right() > id) {
        return ;
    }
    it = free_.upper_bound(id_interval(id,id));
    if (it == free_.end()) {
        return ;
    } else {
        id_interval free_interval = *(it);

        if (id + 1 != free_interval.left()) {
            free_.insert(id_interval(id, id));
        } else {
            if (it != free_.begin()) {
                id_intervals_t::iterator it_2 = it;
                --it_2;
                if (it_2->right() + 1 == id ) {
                    id_interval free_interval_2 = *(it_2);
                    free_.erase(it);
                    free_.erase(it_2);
                    free_.insert(
                        id_interval(free_interval_2.left(), 
                                    free_interval.right()));
                } else {
                    free_.erase(it);
                    free_.insert(id_interval(id, free_interval.right()));
                }
            } else {
                    free_.erase(it);
                    free_.insert(id_interval(id, free_interval.right()));
            }
        }
    }
}

bool id_interval::operator < (const id_interval& s) const
{
    return 
      (value_.lower() < s.value_.lower()) && 
      (value_.upper() < s.value_.lower());
}


int main()
{
    IdManager mgr;

    mgr.MarkAsUsed(3);
    printf ("%d\n", mgr.AllocateId());
    printf ("%d\n", mgr.AllocateId());
    printf ("%d\n", mgr.AllocateId());

    return 0;
}