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package org.usc.usc.lottery;

public class Gift {
private int index;
private String gitfId;
private String giftName;
private double probability;

public Gift(int index, String gitfId, String giftName, double probability) {
this.index = index;
this.gitfId = gitfId;
this.giftName = giftName;
this.probability = probability;
}

public int getIndex() {
return index;
}

public void setIndex(int index) {
this.index = index;
}

public String getGitfId() {
return gitfId;
}

public void setGitfId(String gitfId) {
this.gitfId = gitfId;
}

public String getGiftName() {
return giftName;
}

public void setGiftName(String giftName) {
this.giftName = giftName;
}

public double getProbability() {
return probability;
}

public void setProbability(double probability) {
this.probability = probability;
}

@Override
public String toString() {
return "Gift [index=" + index + ", gitfId=" + gitfId + ", giftName=" + giftName + ", probability=" + probability + "]";
}

}

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package org.usc.usc.lottery;

import java.util.ArrayList;
import java.util.Collections;
import java.util.List;

/**
* 不同概率抽奖工具包
*
* @author Shunli
*/
public class LotteryUtil {
/**
* 抽奖
*
* @param orignalRates
*            原始的概率列表，保证顺序和实际物品对应
* @return
*         物品的索引
*/
public static int lottery(List<Double> orignalRates) {
if (orignalRates == null || orignalRates.isEmpty()) {
return -1;
}

int size = orignalRates.size();

// 计算总概率，这样可以保证不一定总概率是1
double sumRate = 0d;
for (double rate : orignalRates) {
sumRate += rate;
}

// 计算每个物品在总概率的基础下的概率情况
List<Double> sortOrignalRates = new ArrayList<Double>(size);
Double tempSumRate = 0d;
for (double rate : orignalRates) {
tempSumRate += rate;
sortOrignalRates.add(tempSumRate / sumRate);
}

// 根据区块值来获取抽取到的物品索引
double nextDouble = Math.random();
sortOrignalRates.add(nextDouble);
Collections.sort(sortOrignalRates);

return sortOrignalRates.indexOf(nextDouble);
}
}

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package org.usc.usc.lottery;

import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
import java.util.Map.Entry;

/**
* 不同概率抽奖
*
* @author ShunLi
*/
public class LotteryTest {
public static void main(String[] args) {
List<Gift> gifts = new ArrayList<Gift>();
// 序号==物品Id==物品名称==概率
gifts.add(new Gift(1, "P1", "物品1", 0.2d));
gifts.add(new Gift(2, "P2", "物品2", 0.2d));
gifts.add(new Gift(3, "P3", "物品3", 0.4d));
gifts.add(new Gift(4, "P4", "物品4", 0.3d));
gifts.add(new Gift(5, "P5", "物品5", 0d));
gifts.add(new Gift(6, "P6", "物品6", -0.1d));
gifts.add(new Gift(7, "P7", "物品7", 0.008d));

List<Double> orignalRates = new ArrayList<Double>(gifts.size());
for (Gift gift : gifts) {
double probability = gift.getProbability();
if (probability < 0) {
probability = 0;
}
orignalRates.add(probability);
}

// // test
// for (int i = 0; i < 10000; i++) {
// try {
// Gift tuple = gifts.get(LotteryUtil.lottery(orignalRates));
// System.out.println(tuple);
// } catch (Exception e) {
// System.out.println("lottery failed, please check it!");
// }
// }

// statistics
Map<Integer, Integer> count = new HashMap<Integer, Integer>();
double num = 1000000;
for (int i = 0; i < num; i++) {
int orignalIndex = LotteryUtil.lottery(orignalRates);

Integer value = count.get(orignalIndex);
count.put(orignalIndex, value == null ? 1 : value + 1);
}

for (Entry<Integer, Integer> entry : count.entrySet()) {
System.out.println(gifts.get(entry.getKey()) + ", count=" + entry.getValue() + ", probability=" + entry.getValue() / num);
}
}

}

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Gift [index=1, gitfId=P1, giftName=物品1, probability=0.2], count=199139, probability=0.199139
Gift [index=2, gitfId=P2, giftName=物品2, probability=0.1], count=99328, probability=0.099328
Gift [index=3, gitfId=P3, giftName=物品3, probability=0.4], count=396575, probability=0.396575
Gift [index=4, gitfId=P4, giftName=物品4, probability=0.3], count=296997, probability=0.296997
Gift [index=7, gitfId=P7, giftName=物品7, probability=0.0080], count=7961, probability=0.007961