微信红包算法遐想

private final AtomicLong seed;

private static final long multiplier = 0x5DEECE66DL;
private static final long addend = 0xBL;
private static final long mask = (1L << 48) - 1;

/**
* Creates a new random number generator. This constructor sets
* the seed of the random number generator to a value very likely
* to be distinct from any other invocation of this constructor.
*/
public Random() {
this(seedUniquifier() ^ System.nanoTime());
}

闲来无事，想如果自己设计微信红包，怎么设计呢？ 

因为我不是微信红包的开发人员，所以以下内容纯属虚构 

先说下我的算法，每人随机一个数，然后把所有人的加起来成为分母，每个人的数是分子，分子除以分母，就是这个人占用红包的额度。

直接看代码

public class Haobao {

public static List<Integer> buildFen(int fen,int i){
if(fen<=i){
return null;
}
List<Integer> ins=new ArrayList<Integer>(i);
List<Integer> cns=new ArrayList<Integer>(i);
//		Random r=ThreadLocalRandom.current();
Random r=new Random();
int total=0;
for(int j=0;j<i;j++){
int c=r.nextInt(i*10)+1;
total=total+c;
ins.add(c);
}
BigDecimal t=new BigDecimal(total);
BigDecimal f=new BigDecimal(fen);
BigDecimal s=f.divide(t,4,BigDecimal.ROUND_DOWN);
for(int j=0;j<i-1;j++){
int c=s.multiply(new BigDecimal(ins.get(j))).intValue();
if(c==0||(fen-c)<(i-j)){
c=1;
}
fen=fen-c;
cns.add(c);
}
cns.add(fen);
return cns;
}

public static void main(String[] args){
for(int k=0;k<10;k++){
long begin=System.currentTimeMillis();
for(int i=0;i<10*10000;i++){
List<Integer> s=buildFen(2245, 100);
//				System.out.println(s.toString());
}
System.out.println("time:"+(System.currentTimeMillis()-begin));
}

}
}

Random VS  ThreadLocalRandom 

有人说ThreadLocalRandom 性能比Random高，有人说Random线程不安全。

今天就看下源码，本人看的是JDK 1.7的

先看Random的，关键是seed因子 

private final AtomicLong seed;

private static final long multiplier = 0x5DEECE66DL;
private static final long addend = 0xBL;
private static final long mask = (1L << 48) - 1;

/**
* Creates a new random number generator. This constructor sets
* the seed of the random number generator to a value very likely
* to be distinct from any other invocation of this constructor.
*/
public Random() {
this(seedUniquifier() ^ System.nanoTime());
}

再看下ThreadLocalRandom ,首先ThreadLocalRandom 继承了Random，仔细看接口，一些方法都是重用父类的。

用TheadLocal 实现的线程安全。

private static final ThreadLocal<ThreadLocalRandom> localRandom =
new ThreadLocal<ThreadLocalRandom>() {
protected ThreadLocalRandom initialValue() {
return new ThreadLocalRandom();
}
};