1 | /** |
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2 | * Licensed to the Apache Software Foundation (ASF) under one |
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3 | * or more contributor license agreements. See the NOTICE file |
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4 | * distributed with this work for additional information |
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5 | * regarding copyright ownership. The ASF licenses this file |
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6 | * to you under the Apache License, Version 2.0 (the |
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7 | * "License"); you may not use this file except in compliance |
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8 | * with the License. You may obtain a copy of the License at |
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9 | * |
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10 | * http://www.apache.org/licenses/LICENSE-2.0 |
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11 | * |
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12 | * Unless required by applicable law or agreed to in writing, software |
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13 | * distributed under the License is distributed on an "AS IS" BASIS, |
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14 | * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. |
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15 | * See the License for the specific language governing permissions and |
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16 | * limitations under the License. |
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17 | */ |
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18 | package org.apache.hadoop.util; |
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19 | |
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20 | import java.util.Iterator; |
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21 | import java.util.Map; |
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22 | import java.util.NavigableMap; |
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23 | import java.util.NoSuchElementException; |
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24 | |
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25 | /** Provide an cyclic {@link Iterator} for a {@link NavigableMap}. |
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26 | * The {@link Iterator} navigates the entries of the map |
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27 | * according to the map's ordering. |
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28 | * If the {@link Iterator} hits the last entry of the map, |
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29 | * it will then continue from the first entry. |
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30 | */ |
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31 | public class CyclicIteration<K, V> implements Iterable<Map.Entry<K, V>> { |
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32 | private final NavigableMap<K, V> navigablemap; |
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33 | private final NavigableMap<K, V> tailmap; |
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34 | |
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35 | /** Construct an {@link Iterable} object, |
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36 | * so that an {@link Iterator} can be created |
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37 | * for iterating the given {@link NavigableMap}. |
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38 | * The iteration begins from the starting key exclusively. |
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39 | */ |
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40 | public CyclicIteration(NavigableMap<K, V> navigablemap, K startingkey) { |
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41 | if (navigablemap == null || navigablemap.isEmpty()) { |
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42 | this.navigablemap = null; |
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43 | this.tailmap = null; |
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44 | } |
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45 | else { |
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46 | this.navigablemap = navigablemap; |
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47 | this.tailmap = navigablemap.tailMap(startingkey, false); |
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48 | } |
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49 | } |
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50 | |
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51 | /** {@inheritDoc} */ |
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52 | public Iterator<Map.Entry<K, V>> iterator() { |
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53 | return new CyclicIterator(); |
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54 | } |
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55 | |
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56 | /** An {@link Iterator} for {@link CyclicIteration}. */ |
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57 | private class CyclicIterator implements Iterator<Map.Entry<K, V>> { |
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58 | private boolean hasnext; |
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59 | private Iterator<Map.Entry<K, V>> i; |
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60 | /** The first entry to begin. */ |
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61 | private final Map.Entry<K, V> first; |
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62 | /** The next entry. */ |
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63 | private Map.Entry<K, V> next; |
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64 | |
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65 | private CyclicIterator() { |
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66 | hasnext = navigablemap != null; |
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67 | if (hasnext) { |
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68 | i = tailmap.entrySet().iterator(); |
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69 | first = nextEntry(); |
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70 | next = first; |
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71 | } |
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72 | else { |
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73 | i = null; |
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74 | first = null; |
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75 | next = null; |
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76 | } |
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77 | } |
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78 | |
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79 | private Map.Entry<K, V> nextEntry() { |
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80 | if (!i.hasNext()) { |
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81 | i = navigablemap.entrySet().iterator(); |
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82 | } |
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83 | return i.next(); |
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84 | } |
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85 | |
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86 | /** {@inheritDoc} */ |
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87 | public boolean hasNext() { |
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88 | return hasnext; |
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89 | } |
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90 | |
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91 | /** {@inheritDoc} */ |
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92 | public Map.Entry<K, V> next() { |
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93 | if (!hasnext) { |
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94 | throw new NoSuchElementException(); |
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95 | } |
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96 | |
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97 | final Map.Entry<K, V> curr = next; |
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98 | next = nextEntry(); |
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99 | hasnext = !next.equals(first); |
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100 | return curr; |
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101 | } |
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102 | |
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103 | /** Not supported */ |
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104 | public void remove() { |
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105 | throw new UnsupportedOperationException("Not supported"); |
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106 | } |
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107 | } |
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108 | } |
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