001package ca.uhn.fhir.model.api;
002
003/*-
004 * #%L
005 * HAPI FHIR - Core Library
006 * %%
007 * Copyright (C) 2014 - 2022 Smile CDR, Inc.
008 * %%
009 * Licensed under the Apache License, Version 2.0 (the "License");
010 * you may not use this file except in compliance with the License.
011 * You may obtain a copy of the License at
012 *
013 *      http://www.apache.org/licenses/LICENSE-2.0
014 *
015 * Unless required by applicable law or agreed to in writing, software
016 * distributed under the License is distributed on an "AS IS" BASIS,
017 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
018 * See the License for the specific language governing permissions and
019 * limitations under the License.
020 * #L%
021 */
022
023import ca.uhn.fhir.i18n.Msg;
024
025import java.util.Iterator;
026import java.util.LinkedList;
027import java.util.NoSuchElementException;
028import java.util.function.Consumer;
029
030public class PagingIterator<T> implements Iterator<T> {
031
032        public interface PageFetcher<T> {
033                void fetchNextPage(int thePageIndex, int theBatchSize, Consumer<T> theConsumer);
034        }
035
036        static final int PAGE_SIZE = 100;
037
038        private int myPage;
039
040        private boolean myIsFinished;
041
042        private final LinkedList<T> myCurrentBatch = new LinkedList<>();
043
044        private final PageFetcher<T> myFetcher;
045
046        public PagingIterator(PageFetcher<T> theFetcher) {
047                myFetcher = theFetcher;
048        }
049
050        @Override
051        public boolean hasNext() {
052                fetchNextBatch();
053
054                return !myCurrentBatch.isEmpty();
055        }
056
057        @Override
058        public T next() {
059                fetchNextBatch();
060
061                if (myCurrentBatch.isEmpty()) {
062                        throw new NoSuchElementException(Msg.code(2098) + " Nothing to fetch");
063                }
064
065                return myCurrentBatch.remove(0);
066        }
067
068        private void fetchNextBatch() {
069                if (!myIsFinished && myCurrentBatch.isEmpty()) {
070                        myFetcher.fetchNextPage(myPage, PAGE_SIZE, myCurrentBatch::add);
071                        myPage++;
072                        myIsFinished = myCurrentBatch.size() < PAGE_SIZE;
073                }
074        }
075}