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annotate src/goodjava/util/MapReduce.java @ 2171:8b77bd42864d

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add add Thread.map_reduce
author Franklin Schmidt <fschmidt@gmail.com>
date Sun, 22 Mar 2026 19:24:52 -0600
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children 75c45f1a743e
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1 package goodjava.util;
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2
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3 import java.util.Collections;
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4 import java.util.List;
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5 import java.util.ArrayList;
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6 import java.util.Map;
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7 import java.util.HashMap;
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8 import java.util.WeakHashMap;
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9 import java.util.Set;
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10
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11
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12 public class MapReduce {
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13
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14 public interface Mapper {
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15 public List map(Object arg);
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16 }
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17
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18 public interface Handler extends Mapper {
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19 public List reduce(List<List> lists);
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20 }
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21
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22 private static Map<String,MapReduce> instances = new HashMap<String,MapReduce>();
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23
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24 // an active handler will be arbitrarily chosen for reduce
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25 public static synchronized MapReduce register(String key,Mapper mapper) {
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26 MapReduce mr = instances.get(key);
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27 if( mr == null ) {
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28 mr = new MapReduce();
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29 instances.put(key,mr);
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30 }
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31 mr.mappers.add(mapper);
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32 return mr;
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33 }
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34
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35 private Set<Mapper> mappers = Collections.newSetFromMap(new WeakHashMap<Mapper, Boolean>());
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36
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37 public List run(Object arg) {
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38 List<List> lists = new ArrayList<List>();
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39 Handler lastHandler = null;
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40 Mapper[] snapshot = mappers.toArray(new Mapper[0]);
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41 for( Mapper m : snapshot ) {
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42 lists.add( m.map(arg) );
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43 if( m instanceof Handler )
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44 lastHandler = (Handler)m;
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45 }
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46 return lastHandler==null ? Collections.emptyList() : lastHandler.reduce(lists);
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47 }
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48 }