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Basic configurable disk-backed N-way merge sort
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package com.fasterxml.sort;

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

/**
 * Object used to merge items from multiple input sources into one.
 */
public abstract class Merger
    extends DataReader
{
    protected final Comparator _comparator;

    /*
    /********************************************************************** 
    /* Construction
    /********************************************************************** 
     */
    
    public Merger(Comparator cmp) {
        _comparator = cmp;
    }
    
    public static  DataReader mergedReader(Comparator cmp, List> inputs)
        throws IOException
    {
        switch (inputs.size()) {
        case 0:
            throw new IllegalArgumentException("Can not pass empty DataReader array");
        case 1:
            return inputs.get(0);
        case 2:
            return new PairwiseMerger(cmp, inputs.get(0), inputs.get(1));
        }
        
        // otherwise, divide and conquer
        ArrayList> readers = new ArrayList>(1 + (inputs.size() >> 1));
        int i = 0;
        final int end = inputs.size()-1;
        for (; i < end; i += 2) {
            readers.add(new PairwiseMerger(cmp, inputs.get(i), inputs.get(i+1)));
        }
        // and for odd number of readers, add last one as is without merging
        if (i < inputs.size()) {
            readers.add(inputs.get(i));
        }
        return mergedReader(cmp, readers);
    }

    /*
    /********************************************************************** 
    /* Concrete implementations
    /********************************************************************** 
     */

    protected static class PairwiseMerger
        extends Merger
    {
        protected final DataReader _reader1;
        protected final DataReader _reader2;

        protected T _data1;
        protected T _data2;

        protected boolean _closed;
        
        public PairwiseMerger(Comparator comparator,
                DataReader reader1, DataReader reader2)
            throws IOException
        {
            super(comparator);
            _reader1 = reader1;
            _data1 = reader1.readNext();
            _reader2 = reader2;
            _data2 = reader2.readNext();
        }

        @Override
        public T readNext() throws IOException
        {
            if (_data1 == null) {
                if (_data2 == null) {
                    // [Issue#8]: Should auto-close merged input when there is no more data
                    close();
                    return null;
                }
                T result = _data2;
                _data2 = _reader2.readNext();
                return result;
            }
            if (_data2 == null) {
                T result = _data1;
                _data1 = _reader1.readNext();
                return result;
            }
            // neither is null, compare
            T result;
            if (_comparator.compare(_data1, _data2) <= 0) {
                result = _data1;
                _data1 = _reader1.readNext();
            } else {
                result = _data2;
                _data2 = _reader2.readNext();
            }
            return result;
        }

        @Override
        public int estimateSizeInBytes(T item) {
            // should not matter so
            return _reader1.estimateSizeInBytes(item);
        }

        @Override
        public void close() throws IOException
        {
            if (!_closed) {
                _reader1.close();
                _reader2.close();
                _closed = true;
            }
        }
    }
}