1. Story background

A random red packet is generated. The value of the red packet varies from 0.1 yuan to 100 yuan. The specific probability is: 0.1 yuan is the minimum unit, the probability of 0.1 yuan to 0.5 yuan is 40%, the probability of 0.5 yuan to 1 yuan is 50%, the probability of 1-2 yuan is 5%, the probability of 2-3 yuan is 3%, the probability of 3-4 yuan is 1%, the probability of 4-5 yuan is 0.99%, and the probability of 5-100 yuan is 0.01%.

2. Thinking process

In fact, the problem is very simple. Put the probability on a line segment. Let's simplify the problem first. Suppose that the probability of randomly generating 0 is 40%, the probability of generating 1 is 50%, the probability of generating 2 is 5%, and the probability of generating 3 is 3%. 0.4/0.9/0.95 on the line segment is the sum of the two probabilities, If we generate a random number n at this time, its random interval is 0 to 1. If we generate 0.2, it will fall into the probability interval of 0-0.4. If it is 0.7, it will fall into the probability interval of 0.4-0.9, which is in line with the problem logic.

3. Implementation mode

Back to the beginning of the story, I took the probability logic of the first layer, and then generated a random number for the amount of red packets generated in the specific interval. So I designed a CommonRandom class to store the minimum and maximum values of the red packet interval, as well as the probability of the interval,

CommonRandom class

 8 /**
 9  * @author wangjun
10  * @Date 2018/3/6
11  */
12 public class CommonRandom {
13     private Double min;
14     private Double max;
15     private Double chance;
16     private Double calculateRate;
17 
18     private transient Integer reality = 0;
19     private transient Double realityRate;
20 
21     public static CommonRandom getInstance(Double min, Double max, Double rate) {
22         CommonRandom commonRandom = new CommonRandom();
23         commonRandom.setMin(min);
24         commonRandom.setMax(max);
25         commonRandom.setChance(rate);
26         return commonRandom;
27     }
28 
29     public static String getDefault() {
30         return JsonUtils.toJson(getCommonRandom());
31     }
32 
33     public Double getMin() {
34         return min;
35     }
36 
37     private void setMin(Double min) {
38         this.min = min;
39     }
40 
41     public Double getMax() {
42         return max;
43     }
44 
45     private void setMax(Double max) {
46         this.max = max;
47     }
48 
49     public Double getChance() {
50         return chance;
51     }
52 
53     private void setChance(Double chance) {
54         this.chance = chance;
55     }
56 
57     public Double getCalculateRate() {
58         return calculateRate;
59     }
60 
61     public void setCalculateRate(Double calculateRate) {
62         this.calculateRate = calculateRate;
63     }
64 
65     public Integer getReality() {
66         return reality;
67     }
68 
69     public void setReality(Integer reality) {
70         this.reality = reality;
71     }
72 
73     public Double getRealityRate() {
74         return realityRate;
75     }
76 
77     public void setRealityRate(Double realityRate) {
78         this.realityRate = realityRate;
79     }
80 
81     private static List<CommonRandom> getCommonRandom() {
82         List<CommonRandom> commonRandomList = Lists.newArrayList();
83         commonRandomList.add(CommonRandom.getInstance(0.1, 0.5, 0.4));
84         commonRandomList.add(CommonRandom.getInstance(0.5, 1.0, 0.5));
85         commonRandomList.add(CommonRandom.getInstance(1.0, 2.0, 0.05));
86         commonRandomList.add(CommonRandom.getInstance(2.0, 3.0, 0.03));
87         commonRandomList.add(CommonRandom.getInstance(3.0, 4.0, 0.01));
88         commonRandomList.add(CommonRandom.getInstance(4.0, 5.0, 0.0099));
89         commonRandomList.add(CommonRandom.getInstance(5.0, 100.0, 0.0001));
90         return commonRandomList;
91     }
92 }
93 
94 CommonRandom

Generate randomUtil

 8 /**
 9  * @author wangjun
10  * @Date 2018/3/6
11  */
12 public class RandomUtil {
13     /**
14      * Keep decimal places
15      */
16     private static final int POW = (int) Math.pow(10, 3);
17     private static final double ERROR_RANDOM_RESULT = -1D;
18 
19     public static Double randomRedPackMoney() {
20         List<CommonRandom> commonRandoms = CommonRandom.getCommonRandom();
21         Double result = random(commonRandoms);
22         if (result == ERROR_RANDOM_RESULT) {
23             return 1;
24         }
25         return result
26     }
27 
28     /**
29      * Returns the random number generated by a specified probability
30      * The sum of chance probabilities in the incoming set should be 1
31      * @param commonRandomList Probability and interval of random number generated by configuration
32      * @return random number
33      */
34     public static Double random(List<CommonRandom> commonRandomList) {
35         if (CollectionUtils.isEmpty(commonRandomList)) {
36             return ERROR_RANDOM_RESULT;
37         }
38         double randomNumber = Math.random() * getLastRate(commonRandomList);
39         for (CommonRandom item : commonRandomList) {
40             if (randomNumber < item.getCalculateRate()) {
41                 return getRandomNumber(item.getMax(), item.getMin());
42             }
43         }
44         return ERROR_RANDOM_RESULT;
45     }
46 
47     private static double getLastRate(List<CommonRandom> commonRandomList) {
48         return commonRandomList.get(commonRandomList.size() - 1).getCalculateRate();
49     }
50 
51     private static Double getRandomNumber(Double max, Double min) {
52         return Math.floor((Math.random() * (max - min) + min) * POW) / POW;
53     }
54 }