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DoubleHashing.java
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DoubleHashing.java
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package aab180004;
/**
@author Achyut Arun Bhandiwad - aab180004
Implementation of double hasing.
*/
import java.util.NoSuchElementException;
public class DoubleHashing< K extends Comparable< ? super K> & DoubleHashing.HashCodeInterface,V extends Comparable< ? super V>> {
int capacity;
Entry<K,V>[] hashTable;
int size;
public interface HashCodeInterface {
public int hashCode2();
}
static class Entry< K extends Comparable< ? super K> & DoubleHashing.HashCodeInterface,V extends Comparable< ? super V>>{
K key;
V value;
boolean isDeleted;
public Entry(K key, V value){
this.key = key;
this.value = value;
this.isDeleted = false;
}
@Override
public boolean equals(Object obj) {
if(obj instanceof Entry){
if(((Entry) obj).key.compareTo(this.key) == 0){
return true;
}
}
else{
if(key.compareTo((K)obj) ==0){
return true;
}
}
return false;
}
}
/**
* check if full
* @return isFull
*/
public boolean isFull(){
return size == capacity;
}
/**
* Default constructor
*/
public DoubleHashing(){
this.capacity = 16;
this.hashTable = new Entry[capacity];
this.size = 0;
}
// Code extracted from Java’s HashMap:
static int hash(int h) {
// This function ensures that hashCodes that differ only by
// constant multiples at each bit position have a bounded
// number of collisions (approximately 8 at default load factor).
h ^= (h >>> 20) ^ (h >>> 12);
return h ^ (h >>> 7) ^ (h >>> 4);
}
static int indexFor(int h, int length) { // length = table.length is a power of 2
return h & (length-1);
}
// Key x is stored at table[ hash( x.hashCode( ) ) & ( table.length − 1 ) ]
/**
* hash1 function for double hashing
* @param key
* @return hash1
*/
private int hash1(K key){
return indexFor(hash(key.hashCode()), capacity);
}
/**
* hash2 function for double hashing
* @param key
* @return hash2
*/
private int hash2(K key){
return indexFor(hash(key.hashCode2()), capacity);
}
private int getIndex(int k,K key){
return (hash1(key)+ k*hash2(key))% capacity;
}
/**
* find key in the hash table
* @param key
* @return location
*/
public int find(K key){
int k =0;
int ik;
while(true){
ik = getIndex(k,key);
if(hashTable[ik] == null || hashTable[ik].equals(key))
return ik;
else if(hashTable[ik].isDeleted){ // deleted
break;
}else{
k++;
}
}
int xspot = ik;
while(true){
k++;
if(hashTable[ik]!=null && hashTable[ik].equals(key)){
return ik;
}
if(hashTable[ik] == null){
return xspot;
}
}
}
/**
* check if hashtable contains key
* @param key
* @return
*/
public boolean contains(K key){
int loc = find(key);
if((hashTable[loc]!= null) && hashTable[loc].equals(key))
return true;
else
return false;
}
/**
* add key,value to the hash table
* @param key
* @param value
* @return true if success, false if failed
*/
public boolean add(K key, V value){
int loc = find(key);
if(needToRehash()){
rehash();
}
Entry<K,V> newEntry = new Entry<>(key,value);
if(hashTable[loc]!= null && hashTable[loc].equals(key))
return false;
else{
hashTable[loc] = newEntry;
size++;
return true;
}
}
/**
* find key and remove if exists
* @param key
* @return
*/
public Entry remove(K key){
int loc = find(key);
if(hashTable[loc]!=null && hashTable[loc].equals(key)){
Entry<K,V> result = hashTable[loc];
hashTable[loc].isDeleted = true;
size--;
return result;
}
else
return null;
}
/**
* rehash the hashtable if load factor is greater than threshold
*/
private void rehash() {
Entry[] oldHashTable = this.hashTable;
this.capacity = this.capacity * 2;
this.hashTable = new Entry[this.capacity];
this.size = 0;
for (Entry eachEntry : oldHashTable) {
if (eachEntry != null) {
if (!eachEntry.isDeleted) {
this.add((K)eachEntry.key,(V)eachEntry.value);
}
}
}
return;
}
/**
* calculate load factor
* @return
*/
private float loadFactor() {
return (float) this.size / this.capacity;
}
/**
* check loadfactor
* @return
*/
private boolean needToRehash() {
if (this.loadFactor() >= 0.75) {
return true;
}
return false;
}
public int numberOfKeys() {
int numberOfKey = 0;
for (Entry eachEntry : this.hashTable) {
if (eachEntry != null) {
if (eachEntry.isDeleted == false) {
numberOfKey++;
}
}
}
return numberOfKey;
}
/**
* @param arr : Return the number of unique elements
* @return
*/
public int distinctElements(Integer[] arr) {
DoubleHashing<K,V> hashMap = new DoubleHashing<>();
for (int i = 0; i < arr.length; i++) {
hashMap.add((K)new MyInteger(i),(V)new MyInteger(i));
}
return hashMap.numberOfKeys();
}
}