微信紅包算法遐想

 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());
    }

閒來無事，想如果自己設計微信紅包，怎麼設計呢？ 

因爲我不是微信紅包的開發人員，所以以下內容純屬虛構 

先說下我的算法，每人隨機一個數，然後把所有人的加起來成爲分母，每個人的數是分子，分子除以分母，就是這個人佔用紅包的額度。

直接看代碼

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 

有人說ThreadLocalRandom 性能比Random高，有人說Random線程不安全。

今天就看下源碼，本人看的是JDK 1.7的

先看Random的，關鍵是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 繼承了Random，仔細看接口，一些方法都是重用父類的。

用TheadLocal 實現的線程安全。

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