86 lines
2.1 KiB
Go
86 lines
2.1 KiB
Go
//go:build windows
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package strutil
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import (
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"math"
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"math/rand"
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"time"
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"unsafe"
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)
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const MaximumCapacity = math.MaxInt>>1 + 1
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var rn = rand.NewSource(time.Now().UnixNano())
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// nearestPowerOfTwo 返回一个大于等于cap的最近的2的整数次幂，参考java8的hashmap的tableSizeFor函数
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// - cap 输入参数
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//
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// 返回一个大于等于cap的最近的2的整数次幂
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func nearestPowerOfTwo(cap int) int {
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n := cap - 1
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n |= n >> 1
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n |= n >> 2
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n |= n >> 4
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n |= n >> 8
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n |= n >> 16
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if n < 0 {
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return 1
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}
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if n >= MaximumCapacity {
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return MaximumCapacity
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}
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return n + 1
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}
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// buildRandomString 生成随机字符串
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// - letters 字符串模板
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// - length 生成长度
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//
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// 返回一个指定长度的随机字符串
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func buildRandomString(letters string, length int) string {
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// 仿照strings.Builder
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// 创建一个长度为 length 的字节切片
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bytes := make([]byte, length)
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strLength := len(letters)
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if strLength <= 0 {
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return ""
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}
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if strLength == 1 {
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for i := 0; i < length; i++ {
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bytes[i] = letters[0]
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}
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return *(*string)(unsafe.Pointer(&bytes))
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}
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// letters的字符需要使用多少个比特位数才能表示完
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// letterIdBits := int(math.Ceil(math.Log2(strLength))),下面比上面的代码快
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letterIdBits := int(math.Log2(float64(nearestPowerOfTwo(strLength))))
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// 最大的字母id掩码
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var letterIdMask int64 = 1<<letterIdBits - 1
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// 可用次数的最大值
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letterIdMax := 63 / letterIdBits
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// UnixNano: 1607400451937462000
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// 循环生成随机字符串
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for i, cache, remain := length-1, rn.Int63(), letterIdMax; i >= 0; {
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// 检查随机数生成器是否用尽所有随机数
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if remain == 0 {
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cache, remain = rn.Int63(), letterIdMax
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}
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// 从可用字符的字符串中随机选择一个字符
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if idx := int(cache & letterIdMask); idx < strLength {
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bytes[i] = letters[idx]
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i--
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}
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// 右移比特位数，为下次选择字符做准备
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cache >>= letterIdBits
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remain--
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}
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// 仿照strings.Builder用unsafe包返回一个字符串，避免拷贝
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// 将字节切片转换为字符串并返回
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return *(*string)(unsafe.Pointer(&bytes))
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}
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