dust3d/thirdparty/instant-meshes/instant-meshes-dust3d/ext/dset/dset.h

#if !defined(__UNIONFIND_H)
#define __UNIONFIND_H

#include <vector>
#include <atomic>
#include <iostream>

/**
 * Lock-free parallel disjoint set data structure (aka UNION-FIND)
 * with path compression and union by rank
 *
 * Supports concurrent find(), same() and unite() calls as described
 * in the paper
 *
 * "Wait-free Parallel Algorithms for the Union-Find Problem"
 * by Richard J. Anderson and Heather Woll
 *
 * In addition, this class supports optimistic locking (try_lock/unlock)
 * of disjoint sets and a combined unite+unlock operation.
 *
 * \author Wenzel Jakob
 */
class DisjointSets {
public:
    DisjointSets(uint32_t size) : mData(size) {
        for (uint32_t i=0; i<size; ++i)
            mData[i] = (uint32_t) i;
    }

    uint32_t find(uint32_t id) const {
        while (id != parent(id)) {
            uint64_t value = mData[id];
            uint32_t new_parent = parent((uint32_t) value);
            uint64_t new_value =
                (value & 0xFFFFFFFF00000000ULL) | new_parent;
            /* Try to update parent (may fail, that's ok) */
            if (value != new_value)
                mData[id].compare_exchange_weak(value, new_value);
            id = new_parent;
        }
        return id;
    }

    bool same(uint32_t id1, uint32_t id2) const {
        for (;;) {
            id1 = find(id1);
            id2 = find(id2);
            if (id1 == id2)
                return true;
            if (parent(id1) == id1)
                return false;
        }
    }

    uint32_t unite(uint32_t id1, uint32_t id2) {
        for (;;) {
            id1 = find(id1);
            id2 = find(id2);

            if (id1 == id2)
                return id1;

            uint32_t r1 = rank(id1), r2 = rank(id2);

            if (r1 > r2 || (r1 == r2 && id1 < id2)) {
                std::swap(r1, r2);
                std::swap(id1, id2);
            }

            uint64_t oldEntry = ((uint64_t) r1 << 32) | id1;
            uint64_t newEntry = ((uint64_t) r1 << 32) | id2;

            if (!mData[id1].compare_exchange_strong(oldEntry, newEntry))
                continue;

            if (r1 == r2) {
                oldEntry = ((uint64_t) r2 << 32) | id2;
                newEntry = ((uint64_t) (r2+1) << 32) | id2;
                /* Try to update the rank (may fail, that's ok) */
                mData[id2].compare_exchange_weak(oldEntry, newEntry);
            }

            break;
        }
        return id2;
    }

    /**
     * Try to lock the a disjoint union identified by one
     * of its elements (this can occasionally fail when there
     * are concurrent operations). The parameter 'id' will be
     * updated to store the current representative ID of the
     * union
     */
    bool try_lock(uint32_t &id) {
        const uint64_t lock_flag = 1ULL << 63;
        id = find(id);
        uint64_t value = mData[id];
        if ((value & lock_flag) || (uint32_t) value != id)
            return false;
        // On IA32/x64, a PAUSE instruction is recommended for CAS busy loops
        #if defined(__i386__) || defined(__amd64__)
            __asm__ __volatile__ ("pause\n");
        #endif
        return mData[id].compare_exchange_strong(value, value | lock_flag);
    }

    void unlock(uint32_t id) {
        const uint64_t lock_flag = 1ULL << 63;
        mData[id] &= ~lock_flag;
    }

    /**
     * Return the representative index of the set that results from merging
     * locked disjoint sets 'id1' and 'id2'
     */
    uint32_t unite_index_locked(uint32_t id1, uint32_t id2) const {
        uint32_t r1 = rank(id1), r2 = rank(id2);
        return (r1 > r2 || (r1 == r2 && id1 < id2)) ? id1 : id2;
    }

    /**
     * Atomically unite two locked disjoint sets and unlock them. Assumes
     * that here are no other concurrent unite() involving the same sets
     */
    uint32_t unite_unlock(uint32_t id1, uint32_t id2) {
        uint32_t r1 = rank(id1), r2 = rank(id2);

        if (r1 > r2 || (r1 == r2 && id1 < id2)) {
            std::swap(r1, r2);
            std::swap(id1, id2);
        }

        mData[id1] = ((uint64_t) r1 << 32) | id2;
        mData[id2] = ((uint64_t) (r2 + ((r1 == r2) ? 1 : 0)) << 32) | id2;

        return id2;
    }

    uint32_t size() const { return (uint32_t) mData.size(); }

    uint32_t rank(uint32_t id) const {
        return ((uint32_t) (mData[id] >> 32)) & 0x7FFFFFFFu;
    }

    uint32_t parent(uint32_t id) const {
        return (uint32_t) mData[id];
    }

    friend std::ostream &operator<<(std::ostream &os, const DisjointSets &f) {
        for (size_t i=0; i<f.mData.size(); ++i)
            os << i << ": parent=" << f.parent(i) << ", rank=" << f.rank(i) << std::endl;
        return os;
    }

    mutable std::vector<std::atomic<uint64_t>> mData;
};

#endif /* __UNIONFIND_H */