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/* SPDX-License-Identifier: GPL-2.0-or-later */
#pragma once
/** \file
* \ingroup bli
*
* This implements the disjoint set data structure with path compression and union by rank.
*/
#include "BLI_array.hh"
#include "BLI_index_range.hh"
namespace blender {
template<typename T = int64_t> class DisjointSet {
private:
Array<T> parents_;
Array<T> ranks_;
public:
/**
* Create a new disjoint set with the given size. Initially, every element is in a separate set.
*/
DisjointSet(const int64_t size) : parents_(size), ranks_(size, 0)
{
BLI_assert(size >= 0);
for (const int64_t i : IndexRange(size)) {
parents_[i] = T(i);
}
}
/**
* Join the sets containing elements x and y. Nothing happens when they have been in the same set
* before.
*/
void join(const T x, const T y)
{
T root1 = this->find_root(x);
T root2 = this->find_root(y);
/* x and y are in the same set already. */
if (root1 == root2) {
return;
}
/* Implement union by rank heuristic. */
if (ranks_[root1] < ranks_[root2]) {
std::swap(root1, root2);
}
parents_[root2] = root1;
if (ranks_[root1] == ranks_[root2]) {
ranks_[root1]++;
}
}
/**
* Return true when x and y are in the same set.
*/
bool in_same_set(const T x, const T y)
{
T root1 = this->find_root(x);
T root2 = this->find_root(y);
return root1 == root2;
}
/**
* Find the element that represents the set containing x currently.
*/
T find_root(const T x)
{
/* Find root by following parents. */
T root = x;
while (parents_[root] != root) {
root = parents_[root];
}
/* Compress path. */
T to_root = x;
while (parents_[to_root] != root) {
const T parent = parents_[to_root];
parents_[to_root] = root;
to_root = parent;
}
return root;
}
};
} // namespace blender
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