Computer Science/61b/Homework/hw6/dict/HashTableChained.java
< Computer Science | 61b | Homework | hw6- This page contains computer code. Unlike all articles on the lensowiki, which are released under the GFDL, this code is released under the GPL.
Copyright 2006, 2007 Paul Borokhov. All rights reserved.
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/* HashTableChained.java */
package dict;
/**
* HashTableChained implements a Dictionary as a hash table with chaining.
* All objects used as keys must have a valid hashCode() method, which is
* used to determine which bucket of the hash table an entry is stored in.
* Each object's hashCode() is presumed to return an int between
* Integer.MIN_VALUE and Integer.MAX_VALUE. The HashTableChained class
* implements only the compression function, which maps the hash code to
* a bucket in the table's range.
*
* DO NOT CHANGE ANY PROTOTYPES IN THIS FILE.
**/
public class HashTableChained implements Dictionary {
/**
* Place any data fields here.
**/
list.DList[] table;
int tablesize;
/**
* Construct a new empty hash table intended to hold roughly sizeEstimate
* entries. (The precise number of buckets is up to you, but we recommend
* you use a prime number, and shoot for a load factor between 0.5 and 1.)
**/
public HashTableChained(int sizeEstimate) {
int targetsize = (int) Math.round(sizeEstimate); // originally this actually gave a load factor of ~1.25
table = new list.DList[targetsize];
}
/**
* Construct a new empty hash table with a default size. Say, a prime in
* the neighborhood of 100.
**/
public HashTableChained() {
table = new list.DList[101];
}
/**
* Converts a hash code in the range Integer.MIN_VALUE...Integer.MAX_VALUE
* to a value in the range 0...(size of hash table) - 1.
*
* This function should have package protection (so we can test it), and
* should be used by insert, find, and remove.
**/
int compFunction(int code) {
/* int returnval = (((6*code + 11) % 536870911) % table.length);
if (returnval < 0) {
returnval = returnval + table.length;
} */
int returnval = Math.abs((((6*code + 11) % 67108865) % table.length));
return returnval;
}
/**
* Returns the number of entries stored in the dictionary. Entries with
* the same key (or even the same key and value) each still count as
* a separate entry.
* @return number of entries in the dictionary.
**/
public int size() {
return tablesize;
}
/**
* Tests if the dictionary is empty.
*
* @return true if the dictionary has no entries; false otherwise.
**/
public boolean isEmpty() {
if (tablesize == 0) {
return true;
} else {
return false;
}
}
/**
* Create a new Entry object referencing the input key and associated value,
* and insert the entry into the dictionary. Return a reference to the new
* entry. Multiple entries with the same key (or even the same key and
* value) can coexist in the dictionary.
*
* This method should run in O(1) time if the number of collisions is small.
*
* @param key the key by which the entry can be retrieved.
* @param value an arbitrary object.
* @return an entry containing the key and value.
**/
public Entry insert(Object key, Object value) {
Entry insertion = new Entry();
insertion.key = key;
insertion.value = value;
int targetindex = compFunction(key.hashCode());
if (table[targetindex] == null) {
list.DList newlist = new list.DList();
newlist.insertFront(insertion);
table[targetindex] = newlist;
} else {
table[targetindex].insertFront(insertion);
}
tablesize++;
return insertion;
}
/**
* Search for an entry with the specified key. If such an entry is found,
* return it; otherwise return null. If several entries have the specified
* key, choose one arbitrarily and return it.
*
* This method should run in O(1) time if the number of collisions is small.
*
* @param key the search key.
* @return an entry containing the key and an associated value, or null if
* no entry contains the specified key.
**/
public Entry find(Object key) {
int targetindex = compFunction(key.hashCode());
list.DList targetlist = table[targetindex];
if (targetlist != null) {
list.ListNode curnode = null;
try { // since we're using a DList implementation that throws exceptions, this is required
curnode = targetlist.front();
while (!((Entry) curnode.item()).key().equals(key)) { // if the current key is not equals() to the one we're looking for, look in the next node
curnode = curnode.next();
}
return (Entry) curnode.item(); // found a match w/out throwing exceptions
} catch (list.InvalidNodeException e) { // no match for key in the list
return null;
}
} else {
return null;
}
}
/**
* Remove an entry with the specified key. If such an entry is found,
* remove it from the table and return it; otherwise return null.
* If several entries have the specified key, choose one arbitrarily, then
* remove and return it.
*
* This method should run in O(1) time if the number of collisions is small.
*
* @param key the search key.
* @return an entry containing the key and an associated value, or null if
* no entry contains the specified key.
*/
public Entry remove(Object key) {
int targetindex = compFunction(key.hashCode());
list.DList targetlist = table[targetindex];
if (targetlist != null) {
list.ListNode curnode = null;
try { // since we're using a DList implementation that throws exceptions, this is required
curnode = targetlist.front();
while (!((Entry) curnode.item()).key().equals(key)) { // if the current key is not equals() to the one we're looking for, look in the next node
curnode = curnode.next();
}
Entry returned = (Entry) curnode.item(); // found a match w/out throwing exceptions
curnode.remove();
tablesize--;
return returned;
} catch (list.InvalidNodeException e) { // no match for key in the list
return null;
}
} else {
return null;
}
}
/**
* Remove all entries from the dictionary.
*/
public void makeEmpty() {
int targetsize = table.length - 1;
table = new list.DList[targetsize];
tablesize = 0;
}
/**
* Print statistical info about the hashtable: the number of keys stored in each bucket and the total number of collisions.
*/
public void printHistogram() {
String histog = "";
int cols = 0;
for (int i=0; i<table.length; i++) {
if (table[i] == null) {
histog = histog + " 0";
} else {
histog = histog + " " + table[i].length();
cols = cols + table[i].length() - 1;
}
}
System.out.println(histog);
System.out.println("Number of collisions is " + cols + " and table size is " + table.length);
}
}