/*
* Disjoint-set data structure - Library (C#)
*
* Copyright (c) 2021 Project Nayuki. (MIT License)
* https://www.nayuki.io/page/disjoint-set-data-structure
*
* Permission is hereby granted, free of charge, to any person obtaining a copy of
* this software and associated documentation files (the "Software"), to deal in
* the Software without restriction, including without limitation the rights to
* use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of
* the Software, and to permit persons to whom the Software is furnished to do so,
* subject to the following conditions:
* - The above copyright notice and this permission notice shall be included in
* all copies or substantial portions of the Software.
* - The Software is provided "as is", without warranty of any kind, express or
* implied, including but not limited to the warranties of merchantability,
* fitness for a particular purpose and noninfringement. In no event shall the
* authors or copyright holders be liable for any claim, damages or other
* liability, whether in an action of contract, tort or otherwise, arising from,
* out of or in connection with the Software or the use or other dealings in the
* Software.
*/
using System;
/*
* Represents a set of disjoint sets. Also known as the union-find data structure.
* Main operations are querying if two elements are in the same set, and merging two sets together.
* Useful for testing graph connectivity, and is used in Kruskal's algorithm.
*/
public sealed class DisjointSet {
/*---- Fields ----*/
// The number of disjoint sets overall. This number decreases monotonically as time progresses;
// each call to MergeSets() either decrements the number by one or leaves it unchanged. 0 <= NumSets <= NumElements.
public int NumSets {
get;
private set;
}
private Node[] nodes;
/*---- Constructors ----*/
// Constructs a new set containing the given number of singleton sets.
// For example, new DisjointSet(3) --> {{0}, {1}, {2}}.
public DisjointSet(int numElems) {
if (numElems < 0)
throw new ArgumentOutOfRangeException("Number of elements must be non-negative");
nodes = new Node[numElems];
for (int i = 0; i < numElems; i++) {
nodes[i].Parent = i;
nodes[i].Size = 1;
}
NumSets = numElems;
}
/*---- Methods ----*/
// Returns the number of elements among the set of disjoint sets; this was the number passed
// into the constructor and is constant for the lifetime of the object. All the other methods
// require the argument elemIndex to satisfy 0 <= elemIndex < NumElements.
public int NumElements {
get {
return nodes.Length;
}
}
// (Private) Returns the representative element for the set containing the given element. This method is also
// known as "find" in the literature. Also performs path compression, which alters the internal state to
// improve the speed of future queries, but has no externally visible effect on the values returned.
private int getRepr(int elemIndex) {
if (elemIndex < 0 || elemIndex >= nodes.Length)
throw new IndexOutOfRangeException();
// Follow parent pointers until we reach a representative
int parent = nodes[elemIndex].Parent;
while (true) {
int grandparent = nodes[parent].Parent;
if (grandparent == parent)
return parent;
nodes[elemIndex].Parent = grandparent; // Partial path compression
elemIndex = parent;
parent = grandparent;
}
}
// Returns the size of the set that the given element is a member of. 1 <= result <= NumElements.
public int GetSizeOfSet(int elemIndex) {
return nodes[getRepr(elemIndex)].Size;
}
// Tests whether the given two elements are members of the same set. Note that the arguments are orderless.
public bool AreInSameSet(int elemIndex0, int elemIndex1) {
return getRepr(elemIndex0) == getRepr(elemIndex1);
}
// Merges together the sets that the given two elements belong to. This method is also known as "union" in the literature.
// If the two elements belong to different sets, then the two sets are merged and the method returns true.
// Otherwise they belong in the same set, nothing is changed and the method returns false. Note that the arguments are orderless.
public bool MergeSets(int elemIndex0, int elemIndex1) {
// Get representatives
int repr0 = getRepr(elemIndex0);
int repr1 = getRepr(elemIndex1);
if (repr0 == repr1)
return false;
// Compare sizes to choose parent node
if (nodes[repr0].Size < nodes[repr1].Size) {
int temp = repr0;
repr0 = repr1;
repr1 = temp;
}
// Now repr0's size >= repr1's size
// Graft repr1's subtree onto node repr0
nodes[repr1].Parent = repr0;
nodes[repr0].Size += nodes[repr1].Size;
nodes[repr1].Size = 0;
NumSets--;
return true;
}
// For unit tests. This detects many but not all invalid data structures, throwing a SystemException
// if a structural invariant is known to be violated. This always returns silently on a valid object.
public void CheckStructure() {
int numRepr = 0;
int sizeSum = 0;
for (int i = 0; i < nodes.Length; i++) {
int parent = nodes[i].Parent;
int size = nodes[i].Size;
bool isRepr = parent == i;
if (isRepr)
numRepr++;
bool ok = true;
ok &= 0 <= parent && parent < nodes.Length;
ok &= !isRepr && size == 0 || isRepr && 1 <= size && size <= nodes.Length && size <= int.MaxValue - sizeSum;
if (!ok)
throw new SystemException();
sizeSum += size;
}
if (!(0 <= NumSets && NumSets == numRepr && NumSets <= nodes.Length && nodes.Length == sizeSum))
throw new SystemException();
}
private struct Node {
// The index of the parent element. An element is a representative iff its parent is itself.
public int Parent;
// Positive number if the element is a representative, otherwise zero.
public int Size;
}
}