evaluation.java

国外的一套开源CRM

/*
 * $Id: Evaluation.java,v 1.1 2003/08/19 01:12:57 jonesde Exp $
 *
 * Copyright (c) 1999 Steven J. Metsker.
 * Copyright (c) 2001 The Open For Business Project - www.ofbiz.org
 *
 *  Permission is hereby granted, free of charge, to any person obtaining a
 *  copy of this software and associated documentation files (the "Software"),
 *  to deal in the Software without restriction, including without limitation
 *  the rights to use, copy, modify, merge, publish, distribute, sublicense,
 *  and/or sell copies of the Software, and to permit persons to whom the
 *  Software is furnished to do so, subject to the following conditions:
 *
 *  The above copyright notice and this permission notice shall be included
 *  in all copies or substantial portions of the Software.
 *
 *  THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS
 *  OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
 *  MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.
 *  IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY
 *  CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT
 *  OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR
 *  THE USE OR OTHER DEALINGS IN THE SOFTWARE.
 */

package org.ofbiz.rules.engine;


/**
 * An Evaluation unifies a term with the value of another term.
 * <p>
 * For example,
 *
 * <blockquote><pre>
 *     NumberFact two = new NumberFact(2);
 *     ArithmeticOperator x, y;
 *     x = new ArithmeticOperator('*', two, two);
 *     y = new ArithmeticOperator('+', x, two);
 *
 *     Variable result = new Variable("Result");
 *     Evaluation e = new Evaluation(result, y);
 *     System.out.println(e);
 *     System.out.println(e.canFindNextProof());
 *     System.out.println(result);
 *
 * </pre></blockquote>
 *
 * prints out:
 *
 * <blockquote><pre>
 *     #(Result, +(*(2.0, 2.0), 2.0))
 *     true
 *     6.0
 * </pre></blockquote>
 *
 * <p>
 * Since an Evaluation <i>unifies</i> the first term with the
 * arithmetic value, the second term may have a value before
 * the Evaluation proves itself. In this case, the Evaluation
 * checks that the second term's ground value equals the
 * arithmetic value of the first term.
 * <p>
 * The Evaluation fails if the arithmetic value is invalid for
 * any reason.
 *
 * @author Steven J. Metsker
 * @version 1.0
 */

public class Evaluation extends Gateway {
    Term term0;
    Term term1;
    protected Unification currentUnification;

    /**
     * Constructs an Evaluation that will unify the first term
     * with the second term during proofs.
     *
     * @param Term the first term to unify
     *
     * @param Term the term whose value should unify
     *             with the first term
     */
    public Evaluation(Term term0, Term term1) {
        super("#", new Term[] {term0, term1}
        );
        this.term0 = term0;
        this.term1 = term1;
    }

    /**
     * Returns true if this Evaluation can unify its first term
     * with the value of its second term.
     * <p>
     * If the attempt to evaluate the second term causes an
     * exception, this method swallows it and simply fails.
     *
     * @return <code>true<</code>, if this Evaluation can unify
     *         its first term with the arithmetic value of its
     *         second term
     */
    public boolean canProveOnce() {
        Object o;

        try {
            o = term1.eval();
        } catch (EvaluationException e) {
            return false;
        }
        currentUnification = term0.unify(new Atom(o));
        return currentUnification != null;
    }

    /**
     * The superclass calls this after the evaluation has
     * succeeded once, and rule is now failing backwards. The
     * assigment needs to undo any binding it did on the way
     * forward.
     */
    protected void cleanup() {
        unbind();
    }

    /**
     * Create a copy that uses the provided scope.
     *
     * @param AxiomSource ignored
     *
     * @param Scope the scope to use for variables in the
     *              copy
     *
     * @return a copy that uses the provided scope
     */
    public Term copyForProof(AxiomSource ignored, Scope scope) {
        return new Evaluation(
                term0.copyForProof(null, scope),
                term1.copyForProof(null, scope));
    }

    /**
     * Releases the variable bindings that the last unification produced.
     *
     */
    public void unbind() {
        if (currentUnification != null) {
            currentUnification.unbind();
        }
        currentUnification = null;
    }
}