BLI: add disjoint set data structure

This can be used to find separate islands in meshes efficiently (as is
done in cycles already). Furthermore, this helps to implement some
algorithms on node trees more efficiently.

source/blender/blenlib/BLI_disjoint_set.hh

@@ -0,0 +1,105 @@
+/*
+ * 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., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
+ */
+
+#ifndef __BLI_DISJOINT_SET_HH__
+#define __BLI_DISJOINT_SET_HH__
+
+/** \file
+ * \ingroup bli
+ *
+ * This implements the disjoint set data structure with path compression and union by rank.
+ */
+
+#include "BLI_array.hh"
+
+namespace blender {
+
+class DisjointSet {
+ private:
+  Array<uint> parents_;
+  Array<uint> ranks_;
+
+ public:
+  /**
+   * Create a new disjoint set with the given size. Initially, every element is in a separate set.
+   */
+  DisjointSet(uint size) : parents_(size), ranks_(size, 0)
+  {
+    for (uint i = 0; i < size; i++) {
+      parents_[i] = i;
+    }
+  }
+
+  /**
+   * Join the sets containing elements x and y. Nothing happens when they have been in the same set
+   * before.
+   */
+  void join(uint x, uint y)
+  {
+    uint root1 = this->find_root(x);
+    uint 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(uint x, uint y)
+  {
+    uint root1 = this->find_root(x);
+    uint root2 = this->find_root(y);
+    return root1 == root2;
+  }
+
+  /**
+   * Find the element that represents the set containing x currently.
+   */
+  uint find_root(uint x)
+  {
+    /* Find root by following parents. */
+    uint root = x;
+    while (parents_[root] != root) {
+      root = parents_[root];
+    }
+
+    /* Compress path. */
+    while (parents_[x] != root) {
+      uint parent = parents_[x];
+      parents_[x] = root;
+      x = parent;
+    }
+
+    return root;
+  }
+};
+
+}  // namespace blender
+
+#endif /* __BLI_DISJOINT_SET_HH__ */

source/blender/blenlib/CMakeLists.txt

@@ -163,6 +163,7 @@ set(SRC
   BLI_convexhull_2d.h
   BLI_delaunay_2d.h
   BLI_dial_2d.h
+  BLI_disjoint_set.hh
   BLI_dlrbTree.h
   BLI_dot_export.hh
   BLI_dot_export_attribute_enums.hh

tests/gtests/blenlib/BLI_disjoint_set_test.cc

@@ -0,0 +1,34 @@
+#include "BLI_disjoint_set.hh"
+#include "BLI_strict_flags.h"
+
+#include "testing/testing.h"
+
+namespace blender {
+
+TEST(disjoint_set, Test)
+{
+  DisjointSet disjoint_set(6);
+  EXPECT_FALSE(disjoint_set.in_same_set(1, 2));
+  EXPECT_FALSE(disjoint_set.in_same_set(5, 3));
+  EXPECT_TRUE(disjoint_set.in_same_set(2, 2));
+  EXPECT_EQ(disjoint_set.find_root(3), 3u);
+
+  disjoint_set.join(1, 2);
+
+  EXPECT_TRUE(disjoint_set.in_same_set(1, 2));
+  EXPECT_FALSE(disjoint_set.in_same_set(0, 1));
+
+  disjoint_set.join(3, 4);
+
+  EXPECT_FALSE(disjoint_set.in_same_set(2, 3));
+  EXPECT_TRUE(disjoint_set.in_same_set(3, 4));
+
+  disjoint_set.join(1, 4);
+
+  EXPECT_TRUE(disjoint_set.in_same_set(1, 4));
+  EXPECT_TRUE(disjoint_set.in_same_set(1, 3));
+  EXPECT_TRUE(disjoint_set.in_same_set(2, 4));
+  EXPECT_FALSE(disjoint_set.in_same_set(0, 4));
+}
+
+}  // namespace blender

tests/gtests/blenlib/CMakeLists.txt

@@ -43,6 +43,7 @@ BLENDER_TEST(BLI_array "bf_blenlib")
 BLENDER_TEST(BLI_array_store "bf_blenlib")
 BLENDER_TEST(BLI_array_utils "bf_blenlib")
 BLENDER_TEST(BLI_delaunay_2d "bf_blenlib")
+BLENDER_TEST(BLI_disjoint_set "bf_blenlib")
 BLENDER_TEST(BLI_edgehash "bf_blenlib")
 BLENDER_TEST(BLI_expr_pylike_eval "bf_blenlib")
 BLENDER_TEST(BLI_ghash "bf_blenlib")