ResettableRandomizedIterator.java

/*
 * 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 com.facebook.presto.execution.scheduler;

import java.util.ArrayList;
import java.util.Collection;
import java.util.Iterator;
import java.util.List;
import java.util.NoSuchElementException;
import java.util.concurrent.ThreadLocalRandom;

public class ResettableRandomizedIterator<T>
        implements Iterator<T>
{
    private final List<T> list;
    private int position;

    public ResettableRandomizedIterator(Collection<T> elements)
    {
        this.list = new ArrayList<>(elements);
    }

    public void reset()
    {
        position = 0;
    }

    @Override
    public boolean hasNext()
    {
        return position < list.size();
    }

    @Override
    public T next()
    {
        if (!hasNext()) {
            throw new NoSuchElementException();
        }

        int position = ThreadLocalRandom.current().nextInt(this.position, list.size());

        T result = list.set(position, list.get(this.position));
        list.set(this.position, result);
        this.position++;

        return result;
    }
}