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/*
 * Copyright (c) 2017-2021 Nitrite author or authors.
 *
 * 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.dizitart.no2.common.streams;

import org.dizitart.no2.collection.Document;
import org.dizitart.no2.collection.NitriteId;
import org.dizitart.no2.common.RecordStream;
import org.dizitart.no2.common.tuples.Pair;

import java.util.*;

/**
 * @author Anindya Chatterjee
 * @since 4.0
 */
public class DistinctStream implements RecordStream> {
    private final RecordStream> rawStream;

    public DistinctStream(RecordStream> rawStream) {
        this.rawStream = rawStream;
    }

    @Override
    public Iterator> iterator() {
        Iterator> iterator = rawStream == null ? Collections.emptyIterator()
            : rawStream.iterator();
        return new DistinctStreamIterator(iterator);
    }

    private static class DistinctStreamIterator implements Iterator> {
        private final Iterator> iterator;
        private final Set scannedIds;
        private Pair nextPair;
        private boolean nextPairSet = false;

        public DistinctStreamIterator(Iterator> iterator) {
            this.iterator = iterator;
            this.scannedIds = new HashSet<>(); // fastest lookup for ids - O(1)
        }

        @Override
        public boolean hasNext() {
            return nextPairSet || setNextId();
        }

        @Override
        public Pair next() {
            if (!nextPairSet && !setNextId()) {
                throw new NoSuchElementException();
            }
            nextPairSet = false;
            return nextPair;
        }

        private boolean setNextId() {
            while (iterator.hasNext()) {
                final Pair pair = iterator.next();
                if (!scannedIds.contains(pair.getFirst())) {
                    scannedIds.add(pair.getFirst());
                    nextPair = pair;
                    nextPairSet = true;
                    return true;
                }
            }
            return false;
        }
    }
}