////////////////////////////////////////////////////////////////////////////////////////////////////////////////////
// Copyright (c) 2013 Saxonica Limited.
// This Source Code Form is subject to the terms of the Mozilla Public License, v. 2.0.
// If a copy of the MPL was not distributed with this file, You can obtain one at http://mozilla.org/MPL/2.0/.
// This Source Code Form is "Incompatible With Secondary Licenses", as defined by the Mozilla Public License, v. 2.0.
////////////////////////////////////////////////////////////////////////////////////////////////////////////////////

package net.sf.saxon.expr.flwor;

import net.sf.saxon.expr.Expression;
import net.sf.saxon.expr.XPathContext;
import net.sf.saxon.trans.XPathException;

/**
 * This class represents the tuple stream delivered as the output of a where clause in a
 * FLWOR expression: that is, it returns all the tuples in its input stream that satisfy
 * a specified predicate. It does not change the values of any variables in the tuple stream.
 */
public class WhereClausePull extends TuplePull {

    TuplePull base;
    Expression predicate;

    public WhereClausePull(TuplePull base, Expression predicate) {
        this.base = base;
        this.predicate = predicate;
    }

    /**
     * Move on to the next tuple. Before returning, this method must set all the variables corresponding
     * to the "returned" tuple in the local stack frame associated with the context object
     *
     * @param context the dynamic evaluation context
     * @return true if another tuple has been generated; false if the tuple stream is exhausted. If the
     *         method returns false, the values of the local variables corresponding to this tuple stream
     *         are undefined.
     */
    @Override
    public boolean nextTuple(XPathContext context) throws XPathException {
        while (base.nextTuple(context)) {
            if (predicate.effectiveBooleanValue(context)) {
                return true;
            }
        }
        return false;
    }

    /**
     * Close the tuple stream, indicating that although not all tuples have been read,
     * no further tuples are required and resources can be released
     */
    @Override
    public void close() {
        base.close();
    }
}

