Item[] items = …; // Compute the total weight of all items together double totalWeight = 0.0d; for (Item i : items) { totalWeight += i.getWeight(); } // Now choose a random item int randomIndex = -1; double random = Math.random() * totalWeight; for (int i = 0; i < items.length; ++i) { random -= items[i].getWeight(); if (random <= 0.0d) { randomIndex = i; break; } } Item myRandomItem = items[randomIndex];
TreeMap已经为你做了所有的工作。
创build一个TreeMap。 根据您select的方法创build权重。 添加从0.0开始的权重，同时将最后一个元素的权重添加到运行中的权重计数器中。
即(斯卡拉)：
var count = 0.0 for { object
那么你只需要生成rand = new Random(); num = rand.nextDouble() * count rand = new Random(); num = rand.nextDouble() * count得到一个有效的数字。
map.to(num).last // Scala map.floorKey(num) // Java
会给你随机的加权项目。
对于较小数量的桶也是可能的：创build一个数组，即100000个Int，并基于域上的权重分配桶的数量。 然后你创build一个0到100,000-1的随机整数，然后你立即得到桶号。
在Java中，这看起来像这样：
public static E getWeightedRandom(Map weights, Random random) { E result = null; double bestValue = Double.MAX_VALUE; for (E element : weights.keySet()) { double value = -Math.log(random.nextDouble()) / weights.get(element); if (value < bestValue) { bestValue = value; result = element; } } return result; }
我不确定这是否比其他方法更有效，但如果执行时间不是问题，那么这是一个很好的解决scheme。
这和使用Java 8和Streams的想法是一样的：
public static E getWeightedRandomJava8(Stream> weights, Random random) { return weights .map(e -> new SimpleEntry(e.getKey(),-Math.log(random.nextDouble()) / e.getValue())) .min((e0,e1)-> e0.getValue().compareTo(e1.getValue())) .orElseThrow(IllegalArgumentException::new).getKey(); }
您可以通过使用.entrySet().stream()进行转换来获取input权重stream。
给项目…(i1，i2，…，in)中的任意sorting…使用权重w1，w2，…，wn。
select一个介于0和1之间的随机数(具有足够的粒度，使用任意随机函数和适当的缩放比例)。 调用这个r0。
令j = 1
从你的r(j-1)中减去wj得到rj。 如果rj <= 0，则select项目ij。 否则，增加j并重复。
我想我以前就是这样做的……但是可能有更有效的方法来做到这一点。
如果您希望运行时select效率，那么在安装时多花一点时间可能是最好的。 这是一个可能的解决scheme。 它有更多的代码，但保证日志(n)的select。
WeightedItemSelector实现从加权对象集合中select一个随机对象。 select保证在log(n)时间运行。
public class WeightedItemSelector { private final Random rnd = new Random(); private final TreeMap> ranges = new TreeMap>(); private int rangeSize; // Lowest integer higher than the top of the highest range. public WeightedItemSelector(List> weightedItems) { int bottom = 0; // Increments by size of non zero range added as ranges grows. for(WeightedItem wi : weightedItems) { int weight = wi.getWeight(); if(weight > 0) { int top = bottom + weight – 1; Range r = new Range(bottom, top, wi); if(ranges.containsKey(r)) { Range other = ranges.get(r); throw new IllegalArgumentException(String.format(“Range %s conflicts with range %s”, r, other)); } ranges.put(r, r); bottom = top + 1; } } rangeSize = bottom; } public WeightedItem select() { Integer key = rnd.nextInt(rangeSize); Range r = ranges.get(key); if(r == null) return null; return r.weightedItem; } }
范围实现范围select以利用TreeMapselect。
class Range implements Comparable{ final int bottom; final int top; final WeightedItem weightedItem; public Range(int bottom, int top, WeightedItem wi) { this.bottom = bottom; this.top = top; this.weightedItem = wi; } public WeightedItem getWeightedItem() { return weightedItem; } @Override public int compareTo(Object arg0) { if(arg0 instanceof Range>) { Range> other = (Range>) arg0; if(this.bottom > other.top) return 1; if(this.top < other.bottom) return -1; return 0; // overlapping ranges are considered equal. } else if (arg0 instanceof Integer) { Integer other = (Integer) arg0; if(this.bottom > other.intValue()) return 1; if(this.top < other.intValue()) return -1; return 0; } throw new IllegalArgumentException(String.format(“Cannot compare Range objects to %s objects.”, arg0.getClass().getName())); } /* (non-Javadoc) * @see java.lang.Object#toString() */ @Override public String toString() { StringBuilder builder = new StringBuilder(); builder.append(“{\”_class\”: Range {\”bottom\”:\””).append(bottom).append(“\”, \”top\”:\””).append(top) .append(“\”, \”weightedItem\”:\””).append(weightedItem).append(“}”); return builder.toString(); } }
WeightedItem简单地封装一个项目来select。
public class WeightedItem{ private final int weight; private final T item; public WeightedItem(int weight, T item) { this.item = item; this.weight = weight; } public T getItem() { return item; } public int getWeight() { return weight; } /* (non-Javadoc) * @see java.lang.Object#toString() */ @Override public String toString() { StringBuilder builder = new StringBuilder(); builder.append(“{\”_class\”: WeightedItem {\”weight\”:\””).append(weight).append(“\”, \”item\”:\””) .append(item).append(“}”); return builder.toString(); } }
下面是一个保持比例精度的随机函数：
public class WeightedRandomizer { private final Random randomizer; public WeightedRandomizer(Random randomizer) { this.randomizer = randomizer; } public IWeighable getRandomWeighable(List weighables) { double totalWeight = 0.0; long totalSelections = 1; List openWeighables = new ArrayList<>(); for (IWeighable weighable : weighables) { totalWeight += weighable.getAllocation(); totalSelections += weighable.getNumSelections(); } for(IWeighable weighable : weighables) { double allocation = weighable.getAllocation() / totalWeight; long numSelections = weighable.getNumSelections(); double proportion = (double) numSelections / (double) totalSelections; if(proportion < allocation) { openWeighables.add(weighable); } } IWeighable selection = openWeighables.get(this.randomizer.nextInt(openWeighables.size())); selection.setNumSelections(selection.getNumSelections() + 1); return selection; } }
使用包含getWeight()方法的Item类(如您的问题)：
/** * Gets a random-weighted object. * @param items list with weighted items * @return a random item from items with a chance equal to its weight. * @assume total weight == 1 */ public static Item getRandomWeighted(List items) { double value = Math.random(), weight = 0; for (Item item : items) { weight += item.getWeight(); if (value < weight) return item; } return null; // Never will reach this point if assumption is true }
使用Map和更通用的方法：
/** * Gets a random-weighted object. * @param balancedObjects the map with objects and their weights. * @return a random key-object from the map with a chance equal to its weight/totalWeight. * @throws IllegalArgumentException if total weight is not positive. */ public static E getRandomWeighted(Map balancedObjects) throws IllegalArgumentException { double totalWeight = balancedObjects.values().stream().mapToInt(Number::intValue).sum(); // Java 8 if (totalWeight <= 0) throw new IllegalArgumentException(“Total weight must be positive.”); double value = Math.random()*totalWeight, weight = 0; for (Entry e : balancedObjects.entrySet()) { weight += e.getValue().doubleValue(); if (value < weight) return e.getKey(); } return null; // Never will reach this point }