IndexMinPQ.java

package AlgorithmsProject;
 /*https://algs4.cs.princeton.edu/code/edu/princeton/cs/algs4/IndexMinPQ.java */

import java.util.Iterator;
import java.util.NoSuchElementException;

/**
 *  The {@code IndexMinPQ} class represents an indexed priority queue of generic keys.
 *  It supports the usual <em>insert</em> and <em>delete-the-minimum</em>
 *  operations, along with <em>delete</em> and <em>change-the-key</em> 
 *  methods. In order to let the client refer to keys on the priority queue,
 *  an integer between {@code 0} and {@code maxN - 1}
 *  is associated with each key—the client uses this integer to specify
 *  which key to delete or change.
 *  It also supports methods for peeking at the minimum key,
 *  testing if the priority queue is empty, and iterating through
 *  the keys.
 *  <p>
 *  This implementation uses a binary heap along with an array to associate
 *  keys with integers in the given range.
 *  The <em>insert</em>, <em>delete-the-minimum</em>, <em>delete</em>,
 *  <em>change-key</em>, <em>decrease-key</em>, and <em>increase-key</em>
 *  operations take &Theta;(log <em>n</em>) time in the worst case,
 *  where <em>n</em> is the number of elements in the priority queue.
 *  Construction takes time proportional to the specified capacity.
 *  <p>
 *  For additional documentation, see
 *  <a href="https://algs4.cs.princeton.edu/24pq">Section 2.4</a> of
 *  <i>Algorithms, 4th Edition</i> by Robert Sedgewick and Kevin Wayne.
 *
 *  @author Robert Sedgewick
 *  @author Kevin Wayne
 *
 *  @param <Key> the generic type of key on this priority queue
 */
public class IndexMinPQ<Key extends Comparable<Key>>  {
    private int maxN;        // maximum number of elements on PQ
    private int n;           // number of elements on PQ
    private int[] pq;        // binary heap using 1-based indexing
    private int[] qp;        // inverse of pq - qp[pq[i]] = pq[qp[i]] = i
    private Key[] keys;      // keys[i] = priority of i

    /**
     * Initializes an empty indexed priority queue with indices between {@code 0}
     * and {@code maxN - 1}.
     * @param  maxN the keys on this priority queue are index from {@code 0}
     *         {@code maxN - 1}
     * @throws IllegalArgumentException if {@code maxN < 0}
     */
    public IndexMinPQ(int maxN) {
        if (maxN < 0) throw new IllegalArgumentException();
        this.maxN = maxN;
        n = 0;
        keys = (Key[]) new Comparable[maxN + 1];    // make this of length maxN??
        pq   = new int[maxN + 1];
        qp   = new int[maxN + 1];                   // make this of length maxN??
        for (int i = 0; i <= maxN; i++)
            qp[i] = -1;
    }

    /**
     * Returns true if this priority queue is empty.
     *
     * @return {@code true} if this priority queue is empty;
     *         {@code false} otherwise
     */
    public boolean isEmpty() {
        return n == 0;
    }

    /**
     * Is {@code i} an index on this priority queue?
     *
     * @param  i an index
     * @return {@code true} if {@code i} is an index on this priority queue;
     *         {@code false} otherwise
     * @throws IllegalArgumentException unless {@code 0 <= i < maxN}
     */
    public boolean contains(int i) {
        validateIndex(i);
        return qp[i] != -1;
    }

    /**
     * Returns the number of keys on this priority queue.
     *
     * @return the number of keys on this priority queue
     */
    public int size() {
        return n;
    }

    /**
     * Associates key with index {@code i}.
     *
     * @param  i an index
     * @param  key the key to associate with index {@code i}
     * @throws IllegalArgumentException unless {@code 0 <= i < maxN}
     * @throws IllegalArgumentException if there already is an item associated
     *         with index {@code i}
     */
    public void insert(int i, Key key) {
        validateIndex(i);
        if (contains(i)) throw new IllegalArgumentException("index is already in the priority queue");
        n++;
        qp[i] = n;
        pq[n] = i;
        keys[i] = key;
        swim(n);
    }

    /**
     * Returns an index associated with a minimum key.
     *
     * @return an index associated with a minimum key
     * @throws NoSuchElementException if this priority queue is empty
     */
    public int minIndex() {
        if (n == 0) throw new NoSuchElementException("Priority queue underflow");
        return pq[1];
    }

    /**
     * Returns a minimum key.
     *
     * @return a minimum key
     * @throws NoSuchElementException if this priority queue is empty
     */
    public Key minKey() {
        if (n == 0) throw new NoSuchElementException("Priority queue underflow");
        return keys[pq[1]];
    }

    /**
     * Removes a minimum key and returns its associated index.
     * @return an index associated with a minimum key
     * @throws NoSuchElementException if this priority queue is empty
     */
    public int delMin() {
        if (n == 0) throw new NoSuchElementException("Priority queue underflow");
        int min = pq[1];
        exch(1, n--);
        sink(1);
        assert min == pq[n+1];
        qp[min] = -1;        // delete
        keys[min] = null;    // to help with garbage collection
        pq[n+1] = -1;        // not needed
        return min;
    }



    /**
     * Decrease the key associated with index {@code i} to the specified value.
     *
     * @param  i the index of the key to decrease
     * @param  key decrease the key associated with index {@code i} to this key
     * @throws IllegalArgumentException unless {@code 0 <= i < maxN}
     * @throws IllegalArgumentException if {@code key >= keyOf(i)}
     * @throws NoSuchElementException no key is associated with index {@code i}
     */
    public void decreaseKey(int i, Key key) {
        validateIndex(i);
        if (!contains(i)) throw new NoSuchElementException("index is not in the priority queue");
        if (keys[i].compareTo(key) == 0)
            throw new IllegalArgumentException("Calling decreaseKey() with a key equal to the key in the priority queue");
        if (keys[i].compareTo(key) < 0)
            throw new IllegalArgumentException("Calling decreaseKey() with a key strictly greater than the key in the priority queue");
        keys[i] = key;
        swim(qp[i]);
    }

    /**
     * Increase the key associated with index {@code i} to the specified value.
     *
     * @param  i the index of the key to increase
     * @param  key increase the key associated with index {@code i} to this key
     * @throws IllegalArgumentException unless {@code 0 <= i < maxN}
     * @throws IllegalArgumentException if {@code key <= keyOf(i)}
     * @throws NoSuchElementException no key is associated with index {@code i}
     */
    public void increaseKey(int i, Key key) {
        validateIndex(i);
        if (!contains(i)) throw new NoSuchElementException("index is not in the priority queue");
        if (keys[i].compareTo(key) == 0)
            throw new IllegalArgumentException("Calling increaseKey() with a key equal to the key in the priority queue");
        if (keys[i].compareTo(key) > 0)
            throw new IllegalArgumentException("Calling increaseKey() with a key strictly less than the key in the priority queue");
        keys[i] = key;
        sink(qp[i]);
    }

    /**
     * Remove the key associated with index {@code i}.
     *
     * @param  i the index of the key to remove
     * @throws IllegalArgumentException unless {@code 0 <= i < maxN}
     * @throws NoSuchElementException no key is associated with index {@code i}
     */
    public void delete(int i) {
        validateIndex(i);
        if (!contains(i)) throw new NoSuchElementException("index is not in the priority queue");
        int index = qp[i];
        exch(index, n--);
        swim(index);
        sink(index);
        keys[i] = null;
        qp[i] = -1;
    }

    // throw an IllegalArgumentException if i is an invalid index
    private void validateIndex(int i) {
        if (i < 0) throw new IllegalArgumentException("index is negative: " + i);
        if (i >= maxN) throw new IllegalArgumentException("index >= capacity: " + i);
    }

   /***************************************************************************
    * General helper functions.
    ***************************************************************************/
    private boolean greater(int i, int j) {
        return keys[pq[i]].compareTo(keys[pq[j]]) > 0;
    }

    private void exch(int i, int j) {
        int swap = pq[i];
        pq[i] = pq[j];
        pq[j] = swap;
        qp[pq[i]] = i;
        qp[pq[j]] = j;
    }


   /***************************************************************************
    * Heap helper functions.
    ***************************************************************************/
    private void swim(int k) {
        while (k > 1 && greater(k/2, k)) {
            exch(k, k/2);
            k = k/2;
        }
    }

    private void sink(int k) {
        while (2*k <= n) {
            int j = 2*k;
            if (j < n && greater(j, j+1)) j++;
            if (!greater(k, j)) break;
            exch(k, j);
            k = j;
        }
    }


}