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Iterable of the cartesian product of Iterables
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/*
 * Copyright 2019 Information Retrieval Lab - University of A Coruña
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package org.irlab.cartesianproduct;

import java.util.Iterator;
import java.util.NoSuchElementException;

/**
 * Cartesian product of two iterables.
 *
 * Makes the cartesian product of two iterables of type E1 and E2, defining an
 * operation to join elements of both types together, that subclasses have to
 * define.
 *
 * @param  the type of the elements of the first {@link Iterable}
 * @param  the type of the elements of the second {@link Iterable}
 * @param   the type for the result of joining both elements
 * 
 * @author [email protected]
 */
abstract class ProductIterator implements Iterator {

    /** Iterator of the first iterable. */
    private final Iterator it1;

    /** The second iterable. */
    private final Iterable col2;

    /** Iterator of the second iterable. */
    private Iterator it2;

    /** The current element in the iteration of the first iterable */
    private E1 curr1;

    /**
     * Instantiates a new product iterator.
     *
     * @param col1 the first iterable
     * @param col2 the second iterable
     */
    ProductIterator(final Iterable col1, final Iterable col2) {
        this.it1 = col1.iterator();
        this.col2 = col2;
        this.it2 = col2.iterator();
        if (it1.hasNext() && it2.hasNext()) {
            curr1 = it1.next();
        }
    }

    /**
     * {@inheritDoc}
     */
    @Override
    public boolean hasNext() {
        return (curr1 != null) && (it1.hasNext() || it2.hasNext());
    }

    /**
     * Joins two elements from each iterable into an object of the final type..
     *
     * @param e1 the element of the first iterable
     * @param e2 the element of the second iterable
     * @return the resulting object
     */
    protected abstract T join(E1 e1, E2 e2);

    /**
     * {@inheritDoc}
     */
    @Override
    public T next() {
        if (curr1 == null) {
            throw new NoSuchElementException();
        }

        if (it2.hasNext()) {
            return join(curr1, it2.next());
        } else if (it1.hasNext()) {
            curr1 = it1.next();
            it2 = col2.iterator();
            return join(curr1, it2.next());
        } else {
            curr1 = null;
            throw new NoSuchElementException();
        }
    }
}