From 31e5b003cd5a0af4199e633a39319df94fdc5379 Mon Sep 17 00:00:00 2001
From: finlab <finlab@esin.io>
Date: Mon, 28 Dec 2020 05:11:31 +0800
Subject: [PATCH] add example2

---
 example2/main.go | 89 ++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 89 insertions(+)
 create mode 100644 example2/main.go

diff --git a/example2/main.go b/example2/main.go
new file mode 100644
index 0000000..7f1eba2
--- /dev/null
+++ b/example2/main.go
@@ -0,0 +1,89 @@
+/*
+* 薄雾算法
+*
+* 1      2                                                     48         56       64
+* +------+-----------------------------------------------------+----------+----------+
+* retain | increas                                             | salt     | sequence |
+* +------+-----------------------------------------------------+----------+----------+
+* 0      | 0000000000 0000000000 0000000000 0000000000 0000000 | 00000000 | 00000000 |
+* +------+-----------------------------------------------------+------------+--------+
+*
+* 0. 最高位，占 1 位，保持为 0，使得值永远为正数；
+* 1. 自增数，占 47 位，自增数在高位能保证结果值呈递增态势，遂低位可以为所欲为；
+* 2. 随机因子一，占 8 位，上限数值 255，使结果值不可预测；
+* 3. 随机因子二，占 8 位，上限数值 255，使结果值不可预测；
+*
+* 编号上限为百万亿级，上限值计算为 140737488355327 即 int64(1 << 47 - 1)，假设每天取值 10 亿，能使用 385+ 年
+ */
+
+package main
+
+import (
+	"crypto/rand"
+	"fmt"
+	"math/big"
+	"strconv"
+	"sync"
+	"time"
+)
+
+const startYear = 2019
+const saltBit = uint(8)                  // 随机因子二进制位数
+const saltShift = uint(8)                // 随机因子移位数
+const increasShift = saltBit + saltShift // 自增数移位数
+
+func NewIncreaserFromTime(t time.Time) int64 {
+	year := t.Year() - startYear
+	day := t.YearDay()
+	secs := 60*60*t.Hour() + 60*t.Minute() + t.Second()
+	s := fmt.Sprintf("%d%d%05d", year, day, secs)
+	v, _ := strconv.ParseInt(s, 10, 64)
+	return v
+}
+
+type Mist struct {
+	sync.Mutex       // 互斥锁
+	increas    int64 // 自增数
+	saltA      int64 // 随机因子一
+	saltB      int64 // 随机因子二
+}
+
+/* 初始化 Mist 结构体*/
+func NewMist(c int64) *Mist {
+	mist := Mist{increas: c}
+	return &mist
+}
+
+/* 生成唯一编号 */
+func (c *Mist) Generate() int64 {
+	c.Lock()
+	defer c.Unlock()
+	c.increas++
+	// 获取随机因子数值 ｜ 使用真随机函数提高性能
+	randA, _ := c.newRand()
+	c.saltA = randA.Int64()
+	randB, _ := c.newRand()
+	c.saltB = randB.Int64()
+	// 通过位运算实现自动占位
+	mist := int64((c.increas << increasShift) | (c.saltA << saltShift) | c.saltB)
+	return mist
+}
+
+func (c Mist) newRand() (*big.Int, error) {
+	return rand.Int(rand.Reader, big.NewInt(255))
+}
+
+func main() {
+	// 使用方法
+	// mist := NewMist(NewIncreaserFromTime(time.Now()))
+	mist := NewMist(31175572)
+	m := make(map[int64]int)
+	for i := 0; i < 10; i++ {
+		v := mist.Generate()
+		if _, ok := m[v]; ok {
+			fmt.Println(v, i)
+		}
+		m[v] = i
+		fmt.Println(v)
+	}
+}