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QuestDB is High Performance Time Series Database
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 * Copyright (C) 2014-2016 Appsicle
 *
 * This program is free software: you can redistribute it and/or  modify
 * it under the terms of the GNU Affero General Public License, version 3,
 * as published by the Free Software Foundation.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU Affero General Public License for more details.
 *
 * You should have received a copy of the GNU Affero General Public License
 * along with this program.  If not, see .
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package com.questdb.iter;

import com.questdb.std.AbstractImmutableIterator;
import com.questdb.std.ImmutableIterator;

import java.util.Comparator;
import java.util.Iterator;
import java.util.List;

public class MergingIterator extends AbstractImmutableIterator {

    Iterator a;
    Iterator b;
    Comparator comparator;
    T nextA;
    T nextB;

    public static > ImmutableIterator merge(List iterators, Comparator comparator) {
        return merge(iterators, comparator, 0);
    }

    public MergingIterator $new(Iterator a, Iterator b, Comparator comparator) {
        this.a = a;
        this.b = b;
        this.comparator = comparator;
        this.nextA = null;
        this.nextB = null;
        return this;
    }

    @Override
    public boolean hasNext() {
        return nextA != null || a.hasNext() || nextB != null || b.hasNext();
    }

    @Override
    public T next() {
        T result;

        if (nextA == null && a.hasNext()) {
            nextA = a.next();
        }

        if (nextB == null && b.hasNext()) {
            nextB = b.next();
        }

        if (nextB == null || (nextA != null && comparator.compare(nextA, nextB) < 0)) {
            result = nextA;
            nextA = null;
        } else {
            result = nextB;
            nextB = null;
        }

        return result;
    }

    private static > ImmutableIterator merge(List iterators, Comparator comparator, final int index) {
        if (iterators == null || iterators.isEmpty()) {
            throw new IllegalArgumentException();
        }

        if (iterators.size() - index == 1) {
            return iterators.get(index);
        }

        return new MergingIterator().$new(iterators.get(index), merge(iterators, comparator, index + 1), comparator);
    }
}