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| 1 | +package com.lewdev.probabilitylib; |
| 2 | + |
| 3 | +import java.util.Comparator; |
| 4 | +import java.util.Iterator; |
| 5 | +import java.util.TreeSet; |
| 6 | +import java.util.concurrent.ThreadLocalRandom; |
| 7 | + |
| 8 | +/** |
| 9 | + * ProbabilityCollection for retrieving random elements based on probability. |
| 10 | + * <br> |
| 11 | + * <br> |
| 12 | + * <b>Selection Algorithm Implementation</b>: |
| 13 | + * <p> |
| 14 | + * <ul> |
| 15 | + * <li>Elements have a "block" of space, sized based on their probability share |
| 16 | + * <li>"Blocks" start from index 1 and end at the total probability of all elements |
| 17 | + * <li>A random number is selected between 1 and the total probability |
| 18 | + * <li>Which "block" the random number falls in is the element that is selected |
| 19 | + * <li>Therefore "block"s with larger probability have a greater chance of being |
| 20 | + * selected than those with smaller probability. |
| 21 | + * </p> |
| 22 | + * </ul> |
| 23 | + * |
| 24 | + * @author Lewys Davies |
| 25 | + * |
| 26 | + * @param <E> Type of elements |
| 27 | + */ |
| 28 | +public class ProbabilityCollection<E> { |
| 29 | + |
| 30 | + protected final Comparator<ProbabilitySetElement<E>> comparator = |
| 31 | + (o1, o2)-> Integer.compare(o1.getIndex(), o2.getIndex()); |
| 32 | + |
| 33 | + private final TreeSet<ProbabilitySetElement<E>> collection; |
| 34 | + |
| 35 | + private int totalProbability; |
| 36 | + |
| 37 | + /** |
| 38 | + * Construct a new Probability Collection |
| 39 | + */ |
| 40 | + public ProbabilityCollection() { |
| 41 | + this.collection = new TreeSet<>(this.comparator); |
| 42 | + this.totalProbability = 0; |
| 43 | + } |
| 44 | + |
| 45 | + /** |
| 46 | + * @return Number of objects inside the collection |
| 47 | + */ |
| 48 | + public int size() { |
| 49 | + return this.collection.size(); |
| 50 | + } |
| 51 | + |
| 52 | + /** |
| 53 | + * @return Collection contains no elements |
| 54 | + */ |
| 55 | + public boolean isEmpty() { |
| 56 | + return this.collection.isEmpty(); |
| 57 | + } |
| 58 | + |
| 59 | + /** |
| 60 | + * @param object |
| 61 | + * @return True if the collection contains the object, else False |
| 62 | + */ |
| 63 | + public boolean contains(E object) { |
| 64 | + return this.collection.stream() |
| 65 | + .filter(entry -> entry.getObject().equals(object)) |
| 66 | + .findFirst() |
| 67 | + .isPresent(); |
| 68 | + } |
| 69 | + |
| 70 | + /** |
| 71 | + * @return Iterator over collection |
| 72 | + */ |
| 73 | + public Iterator<ProbabilitySetElement<E>> iterator() { |
| 74 | + return this.collection.iterator(); |
| 75 | + } |
| 76 | + |
| 77 | + /** |
| 78 | + * Add an object to this collection |
| 79 | + * |
| 80 | + * @param object |
| 81 | + * @param probability share |
| 82 | + */ |
| 83 | + public void add(E object, int probability) { |
| 84 | + this.collection.add(new ProbabilitySetElement<E>(object, probability)); |
| 85 | + this.totalProbability += probability; |
| 86 | + this.updateIndexes(); |
| 87 | + } |
| 88 | + |
| 89 | + /** |
| 90 | + * Remove a object from this collection |
| 91 | + * |
| 92 | + * @param object |
| 93 | + * @return True if object was removed, else False. |
| 94 | + */ |
| 95 | + public boolean remove(E object) { |
| 96 | + Iterator<ProbabilitySetElement<E>> it = this.iterator(); |
| 97 | + boolean removed = false; |
| 98 | + |
| 99 | + while(it.hasNext()) { |
| 100 | + ProbabilitySetElement<E> element = it.next(); |
| 101 | + if(element.getObject().equals(object)) { |
| 102 | + removed = true; |
| 103 | + this.totalProbability -= element.getProbability(); |
| 104 | + it.remove(); |
| 105 | + } |
| 106 | + } |
| 107 | + |
| 108 | + this.updateIndexes(); |
| 109 | + return removed; |
| 110 | + } |
| 111 | + |
| 112 | + /** |
| 113 | + * @return Random object based on probability |
| 114 | + */ |
| 115 | + public E get() { |
| 116 | + ProbabilitySetElement<E> toFind = new ProbabilitySetElement<>(null, 0); |
| 117 | + toFind.setIndex(ThreadLocalRandom.current().nextInt(1, this.totalProbability + 1)); |
| 118 | + |
| 119 | + return this.collection.floor(toFind).getObject(); |
| 120 | + } |
| 121 | + |
| 122 | + /** |
| 123 | + * @return Sum of all element's probability |
| 124 | + */ |
| 125 | + public final int getTotalProbability() { |
| 126 | + return this.totalProbability; |
| 127 | + } |
| 128 | + |
| 129 | + /* |
| 130 | + * Calculate the size of all element's "block" of space: |
| 131 | + * i.e 1-5, 6-10, 11-14, 15, 16 |
| 132 | + * |
| 133 | + * We then only need to store the start index of each element |
| 134 | + */ |
| 135 | + private void updateIndexes() { |
| 136 | + int previousIndex = 0; |
| 137 | + |
| 138 | + for(ProbabilitySetElement<E> entry : this.collection) { |
| 139 | + previousIndex = entry.setIndex(previousIndex + 1) + (entry.getProbability() - 1); |
| 140 | + } |
| 141 | + } |
| 142 | + |
| 143 | + /** |
| 144 | + * Used internally to store information about a object's |
| 145 | + * state in a collection. Specifically, the probability |
| 146 | + * and index within the collection. |
| 147 | + * |
| 148 | + * Indexes refer to the start position of this element's "block" of space. |
| 149 | + * The space between element "block"s represents their probability of being selected |
| 150 | + * |
| 151 | + * @author Lewys Davies |
| 152 | + * |
| 153 | + * @param <T> Type of element |
| 154 | + */ |
| 155 | + final static class ProbabilitySetElement<T> { |
| 156 | + private final T object; |
| 157 | + private final int probability; |
| 158 | + private int index; |
| 159 | + |
| 160 | + /** |
| 161 | + * @param object |
| 162 | + * @param probability |
| 163 | + */ |
| 164 | + protected ProbabilitySetElement(T object, int probability) { |
| 165 | + this.object = object; |
| 166 | + this.probability = probability; |
| 167 | + } |
| 168 | + |
| 169 | + /** |
| 170 | + * @return The actual object |
| 171 | + */ |
| 172 | + public final T getObject() { |
| 173 | + return this.object; |
| 174 | + } |
| 175 | + |
| 176 | + /** |
| 177 | + * @return Probability share in this collection |
| 178 | + */ |
| 179 | + public final int getProbability() { |
| 180 | + return this.probability; |
| 181 | + } |
| 182 | + |
| 183 | + // Used internally, see this class's documentation |
| 184 | + protected final int getIndex() { |
| 185 | + return this.index; |
| 186 | + } |
| 187 | + |
| 188 | + // Used Internally, see this class's documentation |
| 189 | + protected final int setIndex(int index) { |
| 190 | + this.index = index; |
| 191 | + return this.index; |
| 192 | + } |
| 193 | + } |
| 194 | +} |
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