com.github.ygf.pagerank.PageRankMatrixReducer.java Source code

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Here is the source code for com.github.ygf.pagerank.PageRankMatrixReducer.java

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/*
 * Copyright 2014 Yasser Gonzalez Fernandez <ygonzalezfernandez@gmail.com>.
 * 
 * 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 com.github.ygf.pagerank;

import java.io.IOException;
import java.util.ArrayList;
import java.util.List;

import org.apache.hadoop.conf.Configuration;
import org.apache.hadoop.io.ShortWritable;
import org.apache.hadoop.io.Writable;
import org.apache.hadoop.mapreduce.Reducer;

public class PageRankMatrixReducer
        extends Reducer<ShortArrayWritable, ShortArrayWritable, ShortArrayWritable, MatrixBlockWritable> {

    @Override
    public void reduce(ShortArrayWritable inKey, Iterable<ShortArrayWritable> inValues, Context context)
            throws IOException, InterruptedException {

        // This task receives all the entries in M_{i,j} and builds the compact
        // representation of the block. See Section 5.2.4 of Mining of Massive
        // Datasets (http://infolab.stanford.edu/~ullman/mmds.html) for details.
        // Only blocks with at least one nonzero entry are generated. 

        Configuration conf = context.getConfiguration();
        short blockSize = Short.parseShort(conf.get("pagerank.block_size"));

        short vIndexInBlock, wIndexInBlock, vDegree;
        List<List<Short>> blockColumns = new ArrayList<List<Short>>(blockSize);
        for (int k = 0; k < blockSize; k++) {
            blockColumns.add(new ArrayList<Short>());
        }

        for (ShortArrayWritable inValue : inValues) {
            Writable[] blockEntry = inValue.get();
            vIndexInBlock = ((ShortWritable) blockEntry[0]).get();
            wIndexInBlock = ((ShortWritable) blockEntry[1]).get();
            vDegree = ((ShortWritable) blockEntry[2]).get();

            if (blockColumns.get(vIndexInBlock).isEmpty()) {
                blockColumns.get(vIndexInBlock).add(vDegree);
            }
            blockColumns.get(vIndexInBlock).add(wIndexInBlock);
        }

        ShortWritable[][] blockColumnWritables = new ShortWritable[blockColumns.size()][];
        for (int k = 0; k < blockColumns.size(); k++) {
            List<Short> column = blockColumns.get(k);
            blockColumnWritables[k] = new ShortWritable[column.size()];
            for (int l = 0; l < column.size(); l++) {
                blockColumnWritables[k][l] = new ShortWritable();
                blockColumnWritables[k][l].set(column.get(l).shortValue());
            }
        }

        context.write(inKey, new MatrixBlockWritable(blockColumnWritables));
    }
}