Pair.java

package ca.pfv.spmf.algorithms.sequentialpatterns.BIDE_and_prefixspan_with_strings;
/* This file is copyright (c) 2008-2013 Philippe Fournier-Viger
* 
* This file is part of the SPMF DATA MINING SOFTWARE
* (http://www.philippe-fournier-viger.com/spmf).
* 
* SPMF 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.
* 
* SPMF 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
* SPMF. If not, see <http://www.gnu.org/licenses/>.
*/

import java.util.HashSet;
import java.util.Set;

/**
 * This class, used by the PrefixSpan algorithm, represents a pair of an (1) item  
 * (2)  a boolean indicating if the item is contained in an itemset that was cut or not (a postfix). 
 * and (3) the sequence IDs containing this item.
 * <br/><br/>
 * 
 * This class is used by PrefixSpan.
 * <br/><br/>
 * 
 * It is used for calculating the support of an item in a database.
 * 
 @see AlgoPrefixSpan_with_Strings
* @author Philippe Fournier-Viger
 */
class Pair{
	/** the item */
	protected final String item;
	/** indicate if this represents the item appearing 
	 in an itemset that is cut at the left or not */
	protected final boolean postfix; 
	
	/** List of the sequence IDs that contains this item. */
	private Set<Integer> sequencesID = new HashSet<Integer>();

	/**
	 * Constructor
	 * @param postfix indicate if this is the case of an item appearing
	 *  in an itemset that is cut at the left because of a projection
	 * @param item the item
	 */
	Pair(boolean postfix, String item){
		this.postfix = postfix;
		this.item = item;
	}
	
	/**
	 * Check if two pairs are equal (same item and both appears in a postfix or not).
	 * @return true if equals.
	 */
	public boolean equals(Object object){
		Pair paire = (Pair) object;
		if((paire.postfix == this.postfix) 
				&& (paire.item.equals(this.item))){
			return true;
		}
		return false;
	}
	
	/**
	 * Method to calculate an hashcode (because pairs are stored in a map).
	 */
	public int hashCode()
	{// Ex: 127333,P,X,1  127333,N,Z,2
		// transform it into a string
		StringBuilder r = new StringBuilder();
		r.append((postfix ? 'P' : 'N')); // the letters here have no meanings. they are just used for the hashcode
		r.append(item);
		// then use the hashcode method from the string class
		return r.toString().hashCode();
	}
	
	/**
	 * Check if this is the case of the item appearing in a postfix
	 * @return true if this is the case.
	 */
	public boolean isPostfix() {
		return postfix;
	}

	/**
	 * Get the item represented by this pair
	 * @return the item.
	 */
	public String getItem() {
		return item;
	}

	/**
	 * Get the support of this item (the number of sequences 
	 * containing it).
	 * @return the support (an integer)
	 */
	public int getCount() {
		return sequencesID.size();
	}		

	/**
	 * Get the list of sequence IDs associated with this item.
	 * @return  the list of sequence IDs.
	 */
	public Set<Integer> getSequencesID() {
		return sequencesID;
	}
}