/*

 * Licensed to the Apache Software Foundation (ASF) under one or more

 * contributor license agreements. See the NOTICE file distributed with this

 * work for additional information regarding copyright ownership. The ASF

 * licenses this file to You 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.apache.commons.collections4.iterators;

import java.util.Iterator;

/**

 * Decorates another iterator to skip the first N elements.

 * <p>

 * In case an offset parameter other than 0 is provided, the decorated

 * iterator is immediately advanced to this position, skipping all elements

 * before that position.

 * </p>

 *

 * @since 4.1

 */

public class SkippingIterator<E> extends AbstractIteratorDecorator<E> {

    /** The offset to bound the first element return */

    private final long offset;

    /** The position of the current element */

    private long pos;

    //-----------------------------------------------------------------------

    /**

     * Decorates the specified iterator to skip all elements until the iterator

     * reaches the position at {@code offset}.

     * <p>

     * The iterator is immediately advanced until it reaches the position at {@code offset},

     * incurring O(n) time.

     *

     * @param iterator  the iterator to be decorated

     * @param offset  the index of the first element of the decorated iterator to return

     * @throws NullPointerException if iterator is null

     * @throws IllegalArgumentException if offset is negative

     */

    public SkippingIterator(final Iterator<E> iterator, final long offset) {

        super(iterator);

        if (offset < 0) {

            throw new IllegalArgumentException("Offset parameter must not be negative.");

        }

        this.offset = offset;

        this.pos = 0;

        init();

    }

    /**

     * Skips the given number of elements.

     */

    private void init() {

        while (pos < offset && hasNext()) {

            next();

        }

    }

    //-----------------------------------------------------------------------

    @Override

    public E next() {

        final E next = super.next();

        pos++;

        return next;

    }

    /**

     * {@inheritDoc}

     * <p>

     * In case an offset other than 0 was specified, the underlying iterator will be advanced

     * to this position upon creation. A call to {@link #remove()} will still result in an

     * {@link IllegalStateException} if no explicit call to {@link #next()} has been made prior

     * to calling {@link #remove()}.

     */

    @Override

    public void remove() {

        if (pos <= offset) {

            throw new IllegalStateException("remove() can not be called before calling next()");

        }

        super.remove();

    }

}