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Neo4j kernel is a lightweight, embedded Java database designed to store data structured as graphs rather than tables. For more information, see http://neo4j.org.
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/*
 * Copyright (c) "Neo4j"
 * Neo4j Sweden AB [https://neo4j.com]
 *
 * This file is part of Neo4j.
 *
 * Neo4j is free software: you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation, either version 3 of the License, or
 * (at your option) any later version.
 *
 * 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 General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program.  If not, see .
 */
package org.neo4j.kernel.impl.index.schema;

import static org.neo4j.io.IOUtils.closeAll;

import java.io.Closeable;
import java.io.IOException;
import java.util.ArrayList;
import java.util.List;
import org.neo4j.index.internal.gbptree.Layout;

/**
 * Take multiple {@link BlockEntryCursor} that each by themselves provide block entries in sorted order and lazily merge join, providing a view over all
 * entries from given cursors in sorted order.
 * Merging is done by keeping the cursors in an array amd pick the next lowest among them until all are exhausted, comparing
 * {@link BlockEntryCursor#key()} (current key on each cursor).
 * Instances handed out from {@link #key()} and {@link #value()} are reused, consumer is responsible for creating copy if there is a need to cache results.
 */
public class MergingBlockEntryReader implements BlockEntryCursor {
    // Means that a cursor needs to be advanced, i.e. its current head has already been used, or that it has no head yet
    private static final byte STATE_NEED_ADVANCE = 0;
    // Means that a cursor has been advanced and its current key() contains its current head
    private static final byte STATE_HAS = 1;
    // Means that a cursor has been exhausted and has no more entries in it
    private static final byte STATE_EXHAUSTED = 2;

    private final Layout layout;
    private List sources = new ArrayList<>();
    private Source lastReturned;

    MergingBlockEntryReader(Layout layout) {
        this.layout = layout;
    }

    void addSource(BlockEntryCursor source) {
        sources.add(new Source(source));
    }

    @Override
    public boolean next() throws IOException {
        // Figure out lowest among cursor heads
        KEY lowest = null;
        Source lowestSource = null;
        for (Source source : sources) {
            KEY candidate = source.tryNext();
            if (candidate != null && (lowest == null || layout.compare(candidate, lowest) < 0)) {
                lowest = candidate;
                lowestSource = source;
            }
        }

        // Make state transitions so that this entry is now considered used
        if (lowest != null) {
            lastReturned = lowestSource.takeHead();
            return true;
        }
        return false;
    }

    @Override
    public KEY key() {
        return lastReturned.cursor.key();
    }

    @Override
    public VALUE value() {
        return lastReturned.cursor.value();
    }

    @Override
    public void close() throws IOException {
        closeAll(sources);
    }

    private class Source implements Closeable {
        private final BlockEntryCursor cursor;
        private byte state;

        Source(BlockEntryCursor cursor) {
            this.cursor = cursor;
        }

        KEY tryNext() throws IOException {
            if (state == STATE_NEED_ADVANCE) {
                if (cursor.next()) {
                    state = STATE_HAS;
                    return cursor.key();
                } else {
                    state = STATE_EXHAUSTED;
                }
            } else if (state == STATE_HAS) {
                return cursor.key();
            }
            return null;
        }

        Source takeHead() {
            state = STATE_NEED_ADVANCE;
            return this;
        }

        @Override
        public void close() throws IOException {
            cursor.close();
        }
    }
}