Data Structure - Quadratic Probing Hash Table (Java)

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package chimomo.learning.java.datastructure;/*** Probing table implementation of hash tables.* Note that all "matching" is based on the equals method.** @author Created by Chimomo*/
public class QuadraticProbingHashTable<T> {private static final int DEFAULT_TABLE_SIZE = 101;// The array of elements.private HashEntry<T>[] array;// The number of occupied cells.private int occupied;// Current size.private int size;/*** Construct the hash table.*/public QuadraticProbingHashTable() {this(DEFAULT_TABLE_SIZE);}/*** Construct the hash table.** @param size The approximate initial size.*/public QuadraticProbingHashTable(int size) {allocateArray(size);doClear();}/*** Internal method to find a prime number at least as large as n.** @param n The starting number (must be positive).* @return A prime number larger than or equal to n.*/private static int nextPrime(int n) {if (n % 2 == 0) {n++;}for (; !isPrime(n); n += 2) {}return n;}/*** Internal method to test if a number is prime.* Not an efficient algorithm.** @param n The number to test.* @return The result of the test.*/private static boolean isPrime(int n) {if (n == 2 || n == 3) {return true;}if (n == 1 || n % 2 == 0) {return false;}for (int i = 3; i * i <= n; i += 2) {if (n % i == 0) {return false;}}return true;}// Test program.public static void main(String[] args) {// Create quadratic probing hash table.QuadraticProbingHashTable<String> H = new QuadraticProbingHashTable<>();long startTime = System.currentTimeMillis();final int NUMS = 2000000;final int GAP = 37;System.out.println("Checking... (no more output means success)");// Insert.for (int i = GAP; i != 0; i = (i + GAP) % NUMS) {H.insert("" + i);}for (int i = GAP; i != 0; i = (i + GAP) % NUMS) {if (H.insert("" + i)) {System.out.println("OOPS!!! " + i);}}// Remove.for (int i = 1; i < NUMS; i += 2) {H.remove("" + i);}// Contains.for (int i = 2; i < NUMS; i += 2) {if (!H.contains("" + i)) {System.out.println("Find fails " + i);}}for (int i = 1; i < NUMS; i += 2) {if (H.contains("" + i)) {System.out.println("OOPS!!! " + i);}}long endTime = System.currentTimeMillis();System.out.println("Elapsed time: " + (endTime - startTime));}/*** Insert into the hash table.* If the item is already present, do nothing.** @param x The item to insert.*/public boolean insert(T x) {// Insert x as active.int currentPos = findPos(x);if (isActive(currentPos)) {return false;}if (array[currentPos] == null) {++occupied;}array[currentPos] = new HashEntry<>(x, true);size++;// Rehash.if (occupied > array.length / 2) {rehash();}return true;}/*** Expand the hash table.*/private void rehash() {HashEntry<T>[] oldArray = array;// Create a new double-sized, empty table.allocateArray(2 * oldArray.length);occupied = 0;size = 0;// Copy table over.for (HashEntry<T> entry : oldArray) {if (entry != null && entry.isActive) {insert(entry.element);}}}/*** Method that performs quadratic probing resolution.** @param x The item to search for.* @return The position where the search terminates.*/private int findPos(T x) {int offset = 1;int currentPos = myHash(x);while (array[currentPos] != null && !array[currentPos].element.equals(x)) {// Compute ith probe.currentPos += offset;offset += 2;if (currentPos >= array.length) {currentPos -= array.length;}}return currentPos;}/*** Remove from the hash table.** @param x The item to remove.* @return True if item removed.*/public boolean remove(T x) {int currentPos = findPos(x);if (isActive(currentPos)) {array[currentPos].isActive = false;size--;return true;} else {return false;}}/*** Get current size.** @return The current size.*/public int size() {return size;}/*** Get length of internal table.** @return The array length.*/public int capacity() {return array.length;}/*** Find an item in the hash table.** @param x The item to search for.* @return The matching item.*/public boolean contains(T x) {int currentPos = findPos(x);return isActive(currentPos);}/*** Return true if current position exists and is active.** @param currentPosition The result of a call to find position.* @return True if current position is active.*/private boolean isActive(int currentPosition) {return array[currentPosition] != null && array[currentPosition].isActive;}/*** Make the hash table logically empty.*/public void makeEmpty() {doClear();}/*** Do clear.*/private void doClear() {occupied = 0;for (int i = 0; i < array.length; i++) {array[i] = null;}}private int myHash(T x) {int hashVal = x.hashCode();hashVal %= array.length;if (hashVal < 0) {hashVal += array.length;}return hashVal;}/*** Internal method to allocate array.** @param arraySize The size of the array.*/private void allocateArray(int arraySize) {array = new HashEntry[nextPrime(arraySize)];}/*** Hash entry class for QuadraticProbingHashTable.** @param <AnyType> Any type.*/private static class HashEntry<AnyType> {public AnyType element; // The element.public boolean isActive; // False if marked deleted.public HashEntry(AnyType e) {this(e, true);}public HashEntry(AnyType e, boolean i) {element = e;isActive = i;}}
}/*
Output:
Checking... (no more output means success)
Elapsed time: 3968
*/

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