UnionFind.h

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#ifndef GALOIS_UNIONFIND_H
#define GALOIS_UNIONFIND_H

namespace Galois {
template<typename T>
class UnionFindNode {
  T* findImpl() const {
    if (isRep()) return m_component;

    T* rep = m_component;
    while (rep->m_component != rep) {
      T* next = rep->m_component;
      rep = next;
    }
    return rep;
  }

protected:
  T* m_component;

  UnionFindNode(): m_component(reinterpret_cast<T*>(this)) { }

public:
  typedef UnionFindNode<T> SuperTy;

  bool isRep() const {
    return m_component == this;
  }

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

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

  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;

    T* rep = m_component;
    T* prev = 0;
    while (rep->m_component != rep) {
      T* next = rep->m_component;

      if (prev && prev->m_component == rep) {
        prev->m_component = next;
      }
      prev = rep;
      rep = next;
    }
    return rep;
  }

  T* merge(T* b) {
    T* a = m_component;
    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 (__sync_bool_compare_and_swap(&a->m_component, a, b)) {
        return b;
      }
    }
  }
};
}
#endif

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