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///////////////////////////////////////////////////////////////////////////////
// For information as to what this class does, see the Javadoc, below.       //
// Copyright (C) 1998, 1999, 2000, 2001, 2002, 2003, 2004, 2005, 2006,       //
// 2007, 2008, 2009, 2010, 2014, 2015, 2022 by Peter Spirtes, Richard        //
// Scheines, Joseph Ramsey, and Clark Glymour.                               //
//                                                                           //
// This program 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 2 of the License, or         //
// (at your option) any later version.                                       //
//                                                                           //
// This program 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 this program; if not, write to the Free Software               //
// Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA //
///////////////////////////////////////////////////////////////////////////////

package edu.cmu.tetrad.data;

import edu.cmu.tetrad.util.TetradSerializable;

import java.io.IOException;
import java.io.ObjectInputStream;

/**
 * Implements a knowledge edge X-->Y as a simple ordered pair of strings.
 *
 * @author josephramsey
 */
public final class KnowledgeEdge implements TetradSerializable {
    private static final long serialVersionUID = 23L;

    /**
     * @serial
     */
    private final String from;

    /**
     * @serial
     */
    private final String to;

    /**
     * Constructs a knowledge edge for from-->to.
     */
    public KnowledgeEdge(String from, String to) {
        if (from == null || to == null) {
            throw new NullPointerException();
        }

        this.from = from;
        this.to = to;
    }

    /**
     * Generates a simple exemplar of this class to test serialization.
     */
    public static KnowledgeEdge serializableInstance() {
        return new KnowledgeEdge("X", "Y");
    }

    /**
     * @return the tail node of the edge.
     */
    public String getFrom() {
        return this.from;
    }

    /**
     * @return the head node of the edge.
     */
    public String getTo() {
        return this.to;
    }

    /**
     * Reteurns true if (from1, to1) == (from2, to2).
     */
    public boolean equals(Object object) {
        if (object == null) {
            return false;
        }

        if (!(object instanceof KnowledgeEdge)) {
            return false;
        }

        KnowledgeEdge pair = (KnowledgeEdge) object;
        return this.from.equals(pair.from) && this.to.equals(pair.to);
    }

    /**
     * @return a good hashcode.
     */
    public int hashCode() {
        int hashCode = 31 + this.from.hashCode();
        return 37 * hashCode + this.to.hashCode();
    }

    public String toString() {
        return this.from + "-->" + this.to;
    }

    /**
     * Adds semantic checks to the default deserialization method. This method must have the standard signature for a
     * readObject method, and the body of the method must begin with "s.defaultReadObject();". Other than that, any
     * semantic checks can be specified and do not need to stay the same from version to version. A readObject method of
     * this form may be added to any class, even if Tetrad sessions were previously saved out using a version of the
     * class that didn't include it. (That's what the "s.defaultReadObject();" is for. See J. Bloch, Effective Java, for
     * help.
     */
    private void readObject(ObjectInputStream s)
            throws IOException, ClassNotFoundException {
        s.defaultReadObject();
    }
}