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Datastructures and utility classes for non-negative integers
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/*
 * Copyright (C) 2017 Tobias Meggendorfer
 *
 * 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 3 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, see .
 */

package de.tum.in.naturals.unionfind;

import java.util.Arrays;

public class IntArrayUnionFind implements IntUnionFind {
    private int componentCount;
    private int[] parent;
    private int[] size;

    public IntArrayUnionFind(int size) {
        parent = new int[size];
        this.size = new int[size];
        componentCount = size;
        for (int i = 0; i < size; i++) {
            parent[i] = i;
        }
        Arrays.fill(this.size, 1);
    }

    @Override
    public void add(int num) {
        componentCount += num;

        int currentSize = parent.length;
        int newSize = currentSize + num;
        parent = Arrays.copyOf(parent, newSize);
        for (int i = currentSize; i < newSize; i++) {
            parent[i] = i;
        }
        size = Arrays.copyOf(size, newSize);
        Arrays.fill(size, currentSize, newSize, 1);
    }

    @Override
    public int componentCount() {
        return componentCount;
    }

    @Override
    public int find(int p) {
        assert isValid(p);

        // Search parent
        int root = p;
        while (root != parent[root]) {
            root = parent[root];
        }

        // Compress path
        int current = p;
        while (current != root) {
            int newParent = parent[current];
            parent[current] = root;
            current = newParent;
        }
        return root;
    }

    private boolean isValid(int p) {
        return 0 <= p && p < size();
    }

    @Override
    public int size() {
        return parent.length;
    }

    @Override
    public void union(int p, int q) {
        int rootP = find(p);
        int rootQ = find(q);
        if (rootP == rootQ) {
            return;
        }

        // make smaller root point to larger one
        int sizeP = size[rootP];
        int sizeQ = size[rootQ];
        if (sizeP < sizeQ) {
            parent[rootP] = rootQ;
            size[rootQ] = sizeP + sizeQ;
        } else {
            parent[rootQ] = rootP;
            size[rootP] = sizeP + sizeQ;
        }
        componentCount--;
    }
}