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annotate src/goodjava/util/MapReduce.java @ 2172:75c45f1a743e default tip

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MapReduce cleanup
author Franklin Schmidt <fschmidt@gmail.com>
date Sun, 22 Mar 2026 20:58:17 -0600
parents 8b77bd42864d
children
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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 Handler {
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15 public List map(Object arg);
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16 public List reduce(List<List> lists);
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17 }
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19 private static Map<String,MapReduce> instances = new HashMap<String,MapReduce>();
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21 // an active handler will be arbitrarily chosen for reduce
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22 public static synchronized MapReduce register(String key,Handler handler) {
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23 MapReduce mr = instances.get(key);
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24 if( mr == null ) {
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25 mr = new MapReduce();
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26 instances.put(key,mr);
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27 }
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28 mr.handlers.add(handler);
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29 return mr;
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30 }
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31
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32 private Set<Handler> handlers = Collections.newSetFromMap(new WeakHashMap<Handler, Boolean>());
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34 public List run(Object arg) {
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35 List<List> lists = new ArrayList<List>();
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36 Handler lastHandler = null;
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37 Handler[] snapshot = handlers.toArray(new Handler[0]);
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38 for( Handler h : snapshot ) {
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39 lists.add( h.map(arg) );
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40 lastHandler = h;
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41 }
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42 return lastHandler==null ? Collections.emptyList() : lastHandler.reduce(lists);
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43 }
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44 }