disjoint_set.h

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/******************************************************************************
 * Copyright 2019 The Apollo Authors. All Rights Reserved.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
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 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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/*
Copyright (C) 2006 Pedro Felzenszwalb

This program is free software; you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation; either version 2 of the License, or
(at your option) any later version.

This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
GNU General Public License for more details.

You should have received a copy of the GNU General Public License
along with this program; if not, write to the Free Software
Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307 USA
*/

#pragma once

namespace apollo {
namespace perception {
namespace lidar {

// disjoint-set forests using union-by-rank and path compression (sort of).

typedef struct {
  int rank;
  int p;
  int size;
} uni_elt;

class Universe {
 public:
  explicit Universe(int elements);
  ~Universe();
  int find(int x);
  void join(int x, int y);
  int size(int x) const { return _elts[x].size; }
  int num_sets() const { return _num; }

 private:
  uni_elt *_elts;
  int _num;
};

Universe::Universe(int elements) {
  _elts = new uni_elt[elements];
  _num = elements;
  for (int i = 0; i < elements; i++) {
    _elts[i].rank = 0;
    _elts[i].size = 1;
    _elts[i].p = i;
  }
}

Universe::~Universe() { delete[] _elts; }

int Universe::find(int x) {
  int y = x;
  while (y != _elts[y].p) {
    y = _elts[y].p;
  }
  _elts[x].p = y;
  return y;
}

void Universe::join(int x, int y) {
  if (_elts[x].rank > _elts[y].rank) {
    _elts[y].p = x;
    _elts[x].size += _elts[y].size;
  } else {
    _elts[x].p = y;
    _elts[y].size += _elts[x].size;
    if (_elts[x].rank == _elts[y].rank) {
      _elts[y].rank++;
    }
  }
  _num--;
}

}  // namespace lidar
}  // namespace perception
}  // namespace apollo