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/*
 * Bytecode Analysis Framework
 * Copyright (C) 2003,2004 University of Maryland
 *
 * This library is free software; you can redistribute it and/or
 * modify it under the terms of the GNU Lesser General Public
 * License as published by the Free Software Foundation; either
 * version 2.1 of the License, or (at your option) any later version.
 *
 * This library 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
 * Lesser General Public License for more details.
 *
 * You should have received a copy of the GNU Lesser General Public
 * License along with this library; if not, write to the Free Software
 * Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
 */

package edu.umd.cs.findbugs.ba.vna;

import edu.umd.cs.findbugs.ba.XField;

/**
 * An AvailableLoad indicates a field and (optionally) object reference for
 * which a value is available. It is used to implement redundant load
 * elimination and forward substitution in ValueNumberAnalysis. The idea is that
 * programmers often reload fields unnecessarily when they "know" that the value
 * will not change. In order to deduce the intended meaning of such code, our
 * analyses need to figure out that such loads return the same value.
 * 
 * 
 * AvailableLoad objects may be used as keys in both hash and tree sets and
 * maps.
 * 
 * @author David Hovemeyer
 * @see ValueNumberAnalysis
 */
public class AvailableLoad implements Comparable {
    private final ValueNumber reference;

    private final XField field;

    /**
     * Constructor from static field.
     *
     * @param staticField
     *            the StaticField
     */
    public AvailableLoad(XField staticField) {
        this.reference = null;
        this.field = staticField;
    }

    /**
     * Constructor from object reference and instance field.
     *
     * @param reference
     *            the ValueNumber of the object reference
     * @param field
     *            the InstanceField
     */
    public AvailableLoad(ValueNumber reference, XField field) {
        if (reference == null) {
            throw new IllegalArgumentException();
        }
        this.reference = reference;
        this.field = field;
    }

    /**
     * Get the ValueNumber of the object reference.
     *
     * @return the ValueNumber, or null if this is a an available static field
     *         load
     */
    public ValueNumber getReference() {
        return reference;
    }

    public boolean matchesReference(ValueNumber v) {
        if (v == reference) {
            return true;
        }
        if (reference == null) {
            return false;
        }
        return reference.equals(v);
    }

    /**
     * Get the field for which a load is available.
     *
     * @return the XField
     */
    public XField getField() {
        return field;
    }

    @Override
    public int compareTo(AvailableLoad other) {
        int cmp = field.compareTo(other.field);
        if (cmp != 0) {
            return cmp;
        } else if (reference == other.reference) {
            return 0;
        } else if (reference == null) {
            return -1;
        } else if (other.reference == null) {
            return 1;
        } else {
            return reference.compareTo(other.reference);
        }
    }

    @Override
    public int hashCode() {
        return (reference == null ? 0 : reference.hashCode()) ^ field.hashCode();
    }

    @Override
    public boolean equals(Object o) {
        if (o == null || this.getClass() != o.getClass()) {
            return false;
        }
        AvailableLoad other = (AvailableLoad) o;
        return (reference == other.reference || (reference != null && other.reference != null && reference
                .equals(other.reference))) && field.equals(other.field);
    }

    @Override
    public String toString() {
        return (reference == null ? "" : reference.getNumber() + ".") + field;
    }
}