org.apache.beam.fn.harness.state.LazyCachingIteratorToIterable.java Source code

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Here is the source code for org.apache.beam.fn.harness.state.LazyCachingIteratorToIterable.java

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/*
 * 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.beam.fn.harness.state;

import com.google.common.collect.Iterables;
import java.util.ArrayList;
import java.util.Iterator;
import java.util.List;
import java.util.NoSuchElementException;

/**
 * Converts an iterator to an iterable lazily loading values from the underlying iterator and
 * caching them to support reiteration.
 */
class LazyCachingIteratorToIterable<T> implements Iterable<T> {
    private final List<T> cachedElements;
    private final Iterator<T> iterator;

    public LazyCachingIteratorToIterable(Iterator<T> iterator) {
        this.cachedElements = new ArrayList<>();
        this.iterator = iterator;
    }

    @Override
    public Iterator<T> iterator() {
        return new CachingIterator();
    }

    /** An {@link Iterator} which adds and fetched values into the cached elements list. */
    private class CachingIterator implements Iterator<T> {
        private int position = 0;

        private CachingIterator() {
        }

        @Override
        public boolean hasNext() {
            // The order of the short circuit is important below.
            return position < cachedElements.size() || iterator.hasNext();
        }

        @Override
        public T next() {
            if (position < cachedElements.size()) {
                return cachedElements.get(position++);
            }

            if (!iterator.hasNext()) {
                throw new NoSuchElementException();
            }

            T rval = iterator.next();
            cachedElements.add(rval);
            position += 1;
            return rval;
        }
    }

    @Override
    public int hashCode() {
        return iterator.hasNext() ? iterator.next().hashCode() : -1789023489;
    }

    @Override
    public boolean equals(Object obj) {
        return obj instanceof Iterable && Iterables.elementsEqual(this, (Iterable) obj);
    }

    @Override
    public String toString() {
        return Iterables.toString(this);
    }
}