UnionFind.h

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/*
 * This file belongs to the Galois project, a C++ library for exploiting
 * parallelism. The code is being released under the terms of the 3-Clause BSD
 * License (a copy is located in LICENSE.txt at the top-level directory).
 *
 * Copyright (C) 2018, The University of Texas at Austin. All rights reserved.
 * UNIVERSITY EXPRESSLY DISCLAIMS ANY AND ALL WARRANTIES CONCERNING THIS
 * SOFTWARE AND DOCUMENTATION, INCLUDING ANY WARRANTIES OF MERCHANTABILITY,
 * FITNESS FOR ANY PARTICULAR PURPOSE, NON-INFRINGEMENT AND WARRANTIES OF
 * PERFORMANCE, AND ANY WARRANTY THAT MIGHT OTHERWISE ARISE FROM COURSE OF
 * DEALING OR USAGE OF TRADE.  NO WARRANTY IS EITHER EXPRESS OR IMPLIED WITH
 * RESPECT TO THE USE OF THE SOFTWARE OR DOCUMENTATION. Under no circumstances
 * shall University be liable for incidental, special, indirect, direct or
 * consequential damages or loss of profits, interruption of business, or
 * related expenses which may arise from use of Software or Documentation,
 * including but not limited to those resulting from defects in Software and/or
 * Documentation, or loss or inaccuracy of data of any kind.
 */

#ifndef GALOIS_UNIONFIND_H
#define GALOIS_UNIONFIND_H

#include <atomic>

#include "galois/config.h"

namespace galois {
template <typename T>
class UnionFindNode {
  T* findImpl() const {
    if (isRep())
      return m_component.load(std::memory_order_relaxed);

    T* rep = m_component;
    while (rep->m_component != rep) {
      T* next = rep->m_component.load(std::memory_order_relaxed);
      rep     = next;
    }
    return rep;
  }

protected:
  std::atomic<T*> m_component;

  UnionFindNode(T* s) : m_component(s) {}

public:
  typedef UnionFindNode<T> SuperTy;

  bool isRep() const {
    return m_component.load(std::memory_order_relaxed) == this;
  }

  T* get() const { return m_component.load(std::memory_order_relaxed); }

  const T* find() const { return findImpl(); }

  T* find() { return findImpl(); }

  void compress() {
    if (isRep())
      return;

    // my current component
    T* rep = m_component;

    // loop until rep == itself; i.e. get root
    while (rep->m_component.load(std::memory_order_relaxed) != rep) {
      // get next parent
      T* next = rep->m_component.load(std::memory_order_relaxed);
      rep     = next;
    }

    // at this point rep is the parent: save as my parent
    m_component.store(rep, std::memory_order_relaxed);
  }

  T* findAndCompress() {
    // Basic outline of race in synchronous path compression is that two path
    // compressions along two different paths to the root can create a cycle
    // in the union-find tree. Prevent that from happening by compressing
    // incrementally.
    if (isRep())
      return m_component.load(std::memory_order_relaxed);

    T* rep  = m_component;
    T* prev = 0;
    while (rep->m_component.load(std::memory_order_relaxed) != rep) {
      T* next = rep->m_component.load(std::memory_order_relaxed);

      if (prev && prev->m_component.load(std::memory_order_relaxed) == rep) {
        prev->m_component.store(next, std::memory_order_relaxed);
      }
      prev = rep;
      rep  = next;
    }

    return rep;
  }

  T* merge(T* b) {
    T* a = m_component.load(std::memory_order_relaxed);
    while (true) {
      a = a->findAndCompress();
      b = b->findAndCompress();
      if (a == b)
        return 0;
      // Avoid cycles by directing edges consistently
      if (a < b)
        std::swap(a, b);
      if (a->m_component.compare_exchange_strong(a, b)) {
        return b;
      }
    }
  }
};
} // namespace galois
#endif