/*
 * 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.cassandra.index.sasi.utils;

import java.io.IOException;
import java.util.*;
import java.util.stream.Collectors;

import org.apache.cassandra.io.util.FileUtils;

Range Union Iterator is used to return sorted stream of elements from multiple RangeIterator instances. PriorityQueue is used as a sorting mechanism for the ranges, where each computeNext() operation would poll from the queue (and push when done), which returns range that contains the smallest element, because sorting is done on the moving window of range iteration RangeIterator.getCurrent(). Once retrieved the smallest element (return candidate) is attempted to be merged with other ranges, because there could be equal elements in adjacent ranges, such ranges are poll'ed only if their RangeIterator.getCurrent() equals to the return candidate. 
Type parameters:
<K> – The type used to sort ranges.
<D> – The container type which is going to be returned by Iterator.next()./**
 * Range Union Iterator is used to return sorted stream of elements from multiple RangeIterator instances.
 *
 * PriorityQueue is used as a sorting mechanism for the ranges, where each computeNext() operation would poll
 * from the queue (and push when done), which returns range that contains the smallest element, because
 * sorting is done on the moving window of range iteration {@link RangeIterator#getCurrent()}. Once retrieved
 * the smallest element (return candidate) is attempted to be merged with other ranges, because there could
 * be equal elements in adjacent ranges, such ranges are poll'ed only if their {@link RangeIterator#getCurrent()}
 * equals to the return candidate.
 *
 * @param <K> The type used to sort ranges.
 * @param <D> The container type which is going to be returned by {@link Iterator#next()}.
 */
@SuppressWarnings("resource")
public class RangeUnionIterator<K extends Comparable<K>, D extends CombinedValue<K>> extends RangeIterator<K, D>
{
    private final PriorityQueue<RangeIterator<K, D>> ranges;

    private RangeUnionIterator(Builder.Statistics<K, D> statistics, PriorityQueue<RangeIterator<K, D>> ranges)
    {
        super(statistics);
        this.ranges = ranges;
    }

    public D computeNext()
    {
        RangeIterator<K, D> head = null;

        while (!ranges.isEmpty())
        {
            head = ranges.poll();
            if (head.hasNext())
                break;

            FileUtils.closeQuietly(head);
        }

        if (head == null || !head.hasNext())
            return endOfData();

        D candidate = head.next();

        List<RangeIterator<K, D>> processedRanges = new ArrayList<>();

        if (head.hasNext())
            processedRanges.add(head);
        else
            FileUtils.closeQuietly(head);

        while (!ranges.isEmpty())
        {
            // peek here instead of poll is an optimization
            // so we can re-insert less ranges back if candidate
            // is less than head of the current range.
            RangeIterator<K, D> range = ranges.peek();

            int cmp = candidate.get().compareTo(range.getCurrent());

            assert cmp <= 0;

            if (cmp < 0)
            {
                break; // candidate is smaller than next token, return immediately
            }
            else if (cmp == 0)
            {
                candidate.merge(range.next()); // consume and merge

                range = ranges.poll();
                // re-prioritize changed range

                if (range.hasNext())
                    processedRanges.add(range);
                else
                    FileUtils.closeQuietly(range);
            }
        }

        ranges.addAll(processedRanges);
        return candidate;
    }

    protected void performSkipTo(K nextToken)
    {
        List<RangeIterator<K, D>> changedRanges = new ArrayList<>();

        while (!ranges.isEmpty())
        {
            if (ranges.peek().getCurrent().compareTo(nextToken) >= 0)
                break;

            RangeIterator<K, D> head = ranges.poll();

            if (head.getMaximum().compareTo(nextToken) >= 0)
            {
                head.skipTo(nextToken);
                changedRanges.add(head);
                continue;
            }

            FileUtils.closeQuietly(head);
        }

        ranges.addAll(changedRanges.stream().collect(Collectors.toList()));
    }

    public void close() throws IOException
    {
        ranges.forEach(FileUtils::closeQuietly);
    }

    public static <K extends Comparable<K>, D extends CombinedValue<K>> Builder<K, D> builder()
    {
        return new Builder<>();
    }

    public static <K extends Comparable<K>, D extends CombinedValue<K>> RangeIterator<K, D> build(List<RangeIterator<K, D>> tokens)
    {
        return new Builder<K, D>().add(tokens).build();
    }

    public static class Builder<K extends Comparable<K>, D extends CombinedValue<K>> extends RangeIterator.Builder<K, D>
    {
        public Builder()
        {
            super(IteratorType.UNION);
        }

        protected RangeIterator<K, D> buildIterator()
        {
            switch (rangeCount())
            {
                case 1:
                    return ranges.poll();

                default:
                    return new RangeUnionIterator<>(statistics, ranges);
            }
        }
    }
}