sparsehashtable.h

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                                                ExtractKey, EqualKey, Alloc>
  destructive_iterator;


  // How full we let the table get before we resize.  Knuth says .8 is
  // good -- higher causes us to probe too much, though saves memory
  static const float HT_OCCUPANCY_FLT; // = 0.8f;

  // How empty we let the table get before we resize lower.
  // It should be less than OCCUPANCY_FLT / 2 or we thrash resizing
  static const float HT_EMPTY_FLT; // = 0.4 * HT_OCCUPANCY_FLT;

  // Minimum size we're willing to let hashtables be.
  // Must be a power of two, and at least 4.
  // Note, however, that for a given hashtable, the minimum size is
  // determined by the first constructor arg, and may be >HT_MIN_BUCKETS.
  static const size_t HT_MIN_BUCKETS = 32;

  // ITERATOR FUNCTIONS
  iterator begin()             { return iterator(this, table.nonempty_begin(),
                                                 table.nonempty_end()); }
  iterator end()               { return iterator(this, table.nonempty_end(),
                                                 table.nonempty_end()); }
  const_iterator begin() const { return const_iterator(this,
                                                       table.nonempty_begin(),
                                                       table.nonempty_end()); }
  const_iterator end() const   { return const_iterator(this,
                                                       table.nonempty_end(),
                                                       table.nonempty_end()); }

  // This is used when resizing
  destructive_iterator destructive_begin() {
    return destructive_iterator(this, table.destructive_begin(),
                                table.destructive_end());
  }
  destructive_iterator destructive_end() {
    return destructive_iterator(this, table.destructive_end(),
                                table.destructive_end());
  }


  // ACCESSOR FUNCTIONS for the things we templatize on, basically
  hasher hash_funct() const { return hash; }
  key_equal key_eq() const  { return equals; }

  // We need to copy values when we set the special marker for deleted
  // elements, but, annoyingly, we can't just use the copy assignment
  // operator because value_type might not be assignable (it's often
  // pair<const X, Y>).  We use explicit destructor invocation and
  // placement new to get around this.  Arg.
 private:
  void set_value(value_type* dst, const value_type src) {
    dst->~value_type();   // delete the old value, if any
    new(dst) value_type(src);
  }

  // This is used as a tag for the copy constructor, saying to destroy its
  // arg We have two ways of destructively copying: with potentially growing
  // the hashtable as we copy, and without.  To make sure the outside world
  // can't do a destructive copy, we make the typename private.
  enum MoveDontCopyT {MoveDontCopy, MoveDontGrow};


  // DELETE HELPER FUNCTIONS
  // This lets the user describe a key that will indicate deleted
  // table entries.  This key should be an "impossible" entry --
  // if you try to insert it for real, you won't be able to retrieve it!
  // (NB: while you pass in an entire value, only the key part is looked
  // at.  This is just because I don't know how to assign just a key.)
 private:
  void squash_deleted() {           // gets rid of any deleted entries we have
    if ( num_deleted ) {            // get rid of deleted before writing
      sparse_hashtable tmp(MoveDontGrow, *this);
      swap(tmp);                    // now we are tmp
    }
    assert(num_deleted == 0);
  }

 public:
  void set_deleted_key(const value_type &val) {
    // It's only safe to change what "deleted" means if we purge deleted guys
    squash_deleted();
    use_deleted = true;
    set_value(&delval, val);        // save the key (and rest of val too)
  }
  void clear_deleted_key() {
    squash_deleted();
    use_deleted = false;
  }

  // These are public so the iterators can use them
  // True if the item at position bucknum is "deleted" marker
  bool test_deleted(size_type bucknum) const {
    // The num_deleted test is crucial for read(): after read(), the ht values
    // are garbage, and we don't want to think some of them are deleted.
    return (use_deleted && num_deleted > 0 && table.test(bucknum) &&
            equals(get_key(delval), get_key(table.get(bucknum))));
  }
  bool test_deleted(const iterator &it) const {
    return (use_deleted && num_deleted > 0 &&
            equals(get_key(delval), get_key(*it)));
  }
  bool test_deleted(const const_iterator &it) const {
    return (use_deleted && num_deleted > 0 &&
            equals(get_key(delval), get_key(*it)));
  }
  bool test_deleted(const destructive_iterator &it) const {
    return (use_deleted && num_deleted > 0 &&
            equals(get_key(delval), get_key(*it)));
  }
  // Set it so test_deleted is true.  true if object didn't used to be deleted
  // See below (at erase()) to explain why we allow const_iterators
  bool set_deleted(const_iterator &it) {
    assert(use_deleted);             // bad if set_deleted_key() wasn't called
    bool retval = !test_deleted(it);
    // &* converts from iterator to value-type
    set_value(const_cast<value_type*>(&(*it)), delval);
    return retval;
  }
  // Set it so test_deleted is false.  true if object used to be deleted
  bool clear_deleted(const_iterator &it) {
    assert(use_deleted);             // bad if set_deleted_key() wasn't called
    // happens automatically when we assign something else in its place
    return test_deleted(it);
  }


  // FUNCTIONS CONCERNING SIZE
  size_type size() const      { return table.num_nonempty() - num_deleted; }
  // Buckets are always a power of 2
  size_type max_size() const  { return (size_type(-1) >> 1U) + 1; }
  bool empty() const          { return size() == 0; }
  size_type bucket_count() const      { return table.size(); }
  size_type max_bucket_count() const  { return max_size(); }

 private:
  // Because of the above, size_type(-1) is never legal; use it for errors
  static const size_type ILLEGAL_BUCKET = size_type(-1);

 private:
  // This is the smallest size a hashtable can be without being too crowded
  // If you like, you can give a min #buckets as well as a min #elts
  size_type min_size(size_type num_elts, size_type min_buckets_wanted) {
    size_type sz = HT_MIN_BUCKETS;
    while ( sz < min_buckets_wanted || num_elts >= sz * enlarge_resize_percent )
      sz *= 2;
    return sz;
  }

  // Used after a string of deletes
  void maybe_shrink() {
    assert(table.num_nonempty() >= num_deleted);
    assert((bucket_count() & (bucket_count()-1)) == 0); // is a power of two
    assert(bucket_count() >= HT_MIN_BUCKETS);

    if (shrink_threshold > 0
        && (table.num_nonempty()-num_deleted) < shrink_threshold &&
        bucket_count() > HT_MIN_BUCKETS ) {
      size_type sz = bucket_count() / 2;    // find how much we should shrink
      while ( sz > HT_MIN_BUCKETS &&
              (table.num_nonempty() - num_deleted) <= sz *
              shrink_resize_percent )
        sz /= 2;                            // stay a power of 2
      sparse_hashtable tmp(MoveDontCopy, *this, sz);
      swap(tmp);                            // now we are tmp
    }
    consider_shrink = false;                // because we just considered it
  }

  // We'll let you resize a hashtable -- though this makes us copy all!
  // When you resize, you say, "make it big enough for this many more elements"
  void resize_delta(size_type delta, size_type min_buckets_wanted = 0) {
    if ( consider_shrink )                   // see if lots of deletes happened
      maybe_shrink();
    if ( bucket_count() > min_buckets_wanted &&
         (table.num_nonempty() + delta) <= enlarge_threshold )
      return;                                // we're ok as we are

    // Sometimes, we need to resize just to get rid of all the
    // "deleted" buckets that are clogging up the hashtable.  So when
    // deciding whether to resize, count the deleted buckets (which
    // are currently taking up room).  But later, when we decide what
    // size to resize to, *don't* count deleted buckets, since they
    // get discarded during the resize.
    const size_type needed_size = min_size(table.num_nonempty() + delta,
                                           min_buckets_wanted);
    if ( needed_size > bucket_count() ) {      // we don't have enough buckets
      const size_type resize_to = min_size(table.num_nonempty() - num_deleted
                                           + delta, min_buckets_wanted);
      sparse_hashtable tmp(MoveDontCopy, *this, resize_to);
      swap(tmp);                             // now we are tmp
    }
  }

  // Used to actually do the rehashing when we grow/shrink a hashtable
  void copy_from(const sparse_hashtable &ht, size_type min_buckets_wanted = 0) {
    clear();            // clear table, set num_deleted to 0

    // If we need to change the size of our table, do it now
    const size_type resize_to = min_size(ht.size(), min_buckets_wanted);
    if ( resize_to > bucket_count() ) {      // we don't have enough buckets
      table.resize(resize_to);               // sets the number of buckets
      reset_thresholds();
    }

    // We use a normal iterator to get non-deleted bcks from ht
    // We could use insert() here, but since we know there are
    // no duplicates and no deleted items, we can be more efficient
    assert( (bucket_count() & (bucket_count()-1)) == 0);      // a power of two
    for ( const_iterator it = ht.begin(); it != ht.end(); ++it ) {
      size_type num_probes = 0;              // how many times we've probed
      size_type bucknum;
      const size_type bucket_count_minus_one = bucket_count() - 1;
      for (bucknum = hash(get_key(*it)) & bucket_count_minus_one;
           table.test(bucknum);                          // not empty
           bucknum = (bucknum + JUMP_(key, num_probes)) & bucket_count_minus_one) {
        ++num_probes;
        assert(num_probes < bucket_count()); // or else the hashtable is full
      }
      table.set(bucknum, *it);               // copies the value to here
    }
  }

  // Implementation is like copy_from, but it destroys the table of the
  // "from" guy by freeing sparsetable memory as we iterate.  This is
  // useful in resizing, since we're throwing away the "from" guy anyway.
  void move_from(MoveDontCopyT mover, sparse_hashtable &ht,
                 size_type min_buckets_wanted = 0) {
    clear();            // clear table, set num_deleted to 0

    // If we need to change the size of our table, do it now
    size_t resize_to;
    if ( mover == MoveDontGrow )
      resize_to = ht.bucket_count();         // keep same size as old ht
    else                                     // MoveDontCopy
      resize_to = min_size(ht.size(), min_buckets_wanted);
    if ( resize_to > bucket_count() ) {      // we don't have enough buckets
      table.resize(resize_to);               // sets the number of buckets
      reset_thresholds();
    }

    // We use a normal iterator to get non-deleted bcks from ht
    // We could use insert() here, but since we know there are
    // no duplicates and no deleted items, we can be more efficient
    assert( (bucket_count() & (bucket_count()-1)) == 0);      // a power of two
    // THIS IS THE MAJOR LINE THAT DIFFERS FROM COPY_FROM():
    for ( destructive_iterator it = ht.destructive_begin();
          it != ht.destructive_end(); ++it ) {
      size_type num_probes = 0;              // how many times we've probed
      size_type bucknum;
      for ( bucknum = hash(get_key(*it)) & (bucket_count()-1);  // h % buck_cnt
            table.test(bucknum);                          // not empty
            bucknum = (bucknum + JUMP_(key, num_probes)) & (bucket_count()-1) ) {
        ++num_probes;
        assert(num_probes < bucket_count()); // or else the hashtable is full
      }
      table.set(bucknum, *it);               // copies the value to here
    }
  }


  // Required by the spec for hashed associative container
 public:
  // Though the docs say this should be num_buckets, I think it's much
  // more useful as num_elements.  As a special feature, calling with
  // req_elements==0 will cause us to shrink if we can, saving space.
  void resize(size_type req_elements) {       // resize to this or larger
    if ( consider_shrink || req_elements == 0 )
      maybe_shrink();
    if ( req_elements > table.num_nonempty() )    // we only grow
      resize_delta(req_elements - table.num_nonempty(), 0);
  }

  // Change the value of shrink_resize_percent and
  // enlarge_resize_percent.  The description at the beginning of this
  // file explains how to choose the values.  Setting the shrink
  // parameter to 0.0 ensures that the table never shrinks.
  void set_resizing_parameters(float shrink, float grow) {
    assert(shrink >= 0.0);
    assert(grow <= 1.0);
    assert(shrink <= grow/2.0);
    shrink_resize_percent = shrink;
    enlarge_resize_percent = grow;
    reset_thresholds();
  }

  // CONSTRUCTORS -- as required by the specs, we take a size,
  // but also let you specify a hashfunction, key comparator,
  // and key extractor.  We also define a copy constructor and =.
  // DESTRUCTOR -- the default is fine, surprisingly.
  explicit sparse_hashtable(size_type n = 0,
                            const HashFcn& hf = HashFcn(),
                            const EqualKey& eql = EqualKey(),
                            const ExtractKey& ext = ExtractKey())
    : hash(hf), equals(eql), get_key(ext), num_deleted(0), use_deleted(false),
      delval(), enlarge_resize_percent(HT_OCCUPANCY_FLT),
      shrink_resize_percent(HT_EMPTY_FLT),
      table(min_size(0, n)) {  // start small
    reset_thresholds();
  }

  // As a convenience for resize(), we allow an optional second argument
  // which lets you make this new hashtable a different size than ht.
  // We also provide a mechanism of saying you want to "move" the ht argument
  // into us instead of copying.
  sparse_hashtable(const sparse_hashtable& ht, size_type min_buckets_wanted = 0)
    : hash(ht.hash), equals(ht.equals), get_key(ht.get_key),
      num_deleted(0), use_deleted(ht.use_deleted), delval(ht.delval),
      enlarge_resize_percent(ht.enlarge_resize_percent),