From 778a092257e9b078c719b287f43a774675b90c77 Mon Sep 17 00:00:00 2001
From: pigwantacat <79680042+pigwantacat@users.noreply.github.com>
Date: Wed, 31 Jan 2024 09:57:19 +0800
Subject: [PATCH] =?UTF-8?q?feat:=20=E4=BC=98=E5=8C=96=E7=94=9F=E6=88=90?=
 =?UTF-8?q?=E9=9A=8F=E6=9C=BA=E5=AD=97=E7=AC=A6=E4=B8=B2=20(#154)?=
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see issues #121
---
 strutil/random.go | 115 +++++++++++++++++++++++++++++-----------------
 1 file changed, 73 insertions(+), 42 deletions(-)

diff --git a/strutil/random.go b/strutil/random.go
index 7a00141ad..68914ea76 100644
--- a/strutil/random.go
+++ b/strutil/random.go
@@ -1,8 +1,10 @@
 package strutil
 
 import (
-	mRand "math/rand"
+	"math"
+	"math/rand"
 	"time"
+	"unsafe"
 
 	"github.com/gookit/goutil/byteutil"
 	"github.com/gookit/goutil/encodes"
@@ -10,8 +12,9 @@ import (
 
 // some consts string chars
 const (
-	Numbers  = "0123456789"
-	HexChars = "0123456789abcdef" // base16
+	MaximumCapacity = 1 << 31
+	Numbers         = "0123456789"
+	HexChars        = "0123456789abcdef" // base16
 
 	AlphaBet  = "abcdefghijklmnopqrstuvwxyz"
 	AlphaBet1 = "AaBbCcDdEeFfGgHhIiJjKkLlMmNnOoPpQqRrSsTtUuVvWwXxYyZz"
@@ -21,47 +24,84 @@ const (
 	AlphaNum3 = "0123456789AaBbCcDdEeFfGgHhIiJjKkLlMmNnOoPpQqRrSsTtUuVvWwXxYyZz"
 )
 
-func newRand() *mRand.Rand {
-	return mRand.New(mRand.NewSource(time.Now().UnixNano()))
+var rn = rand.NewSource(time.Now().UnixNano())
+
+// 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
+// cap 输入参数
+// 返回一个大于等于cap的最近的2的整数次幂
+func nearestPowerOfTwo(cap int) int {
+	n := cap - 1
+	n |= n >> 1
+	n |= n >> 2
+	n |= n >> 4
+	n |= n >> 8
+	n |= n >> 16
+	if n < 0 {
+		return 1
+	} else if n >= MaximumCapacity {
+		return MaximumCapacity
+	}
+	return n + 1
 }
 
-// RandomChars generate give length random chars at `a-z`
-func RandomChars(ln int) string {
-	cs := make([]byte, ln)
+// buildRandomString 生成随机字符串
+// letters 字符串模板
+// length 生成长度
+// 返回一个指定长度的随机字符串
+func buildRandomString(letters string, length int) string {
+	// 仿照strings.Builder
+	// 创建一个长度为 length 的字节切片
+	bytes := make([]byte, length)
+	strLength := len(letters)
+	if strLength <= 0 {
+		return ""
+	} else if strLength == 1 {
+		for i := 0; i < length; i++ {
+			bytes[i] = letters[0]
+		}
+		return *(*string)(unsafe.Pointer(&bytes))
+	}
+	// letters的字符需要使用多少个比特位数才能表示完
+	// letterIdBits := int(math.Ceil(math.Log2(strLength))),下面比上面的代码快
+	letterIdBits := int(math.Log2(float64(nearestPowerOfTwo(strLength))))
+	// 最大的字母id掩码
+	var letterIdMask int64 = 1<<letterIdBits - 1
+	// 可用次数的最大值
+	letterIdMax := 63 / letterIdBits
 	// UnixNano: 1607400451937462000
-	rn := newRand()
-
-	for i := 0; i < ln; i++ {
-		// rand in 0 - 25
-		cs[i] = AlphaBet[rn.Intn(25)]
+	// 循环生成随机字符串
+	for i, cache, remain := length-1, rn.Int63(), letterIdMax; i >= 0; {
+		// 检查随机数生成器是否用尽所有随机数
+		if remain == 0 {
+			cache, remain = rn.Int63(), letterIdMax
+		}
+		// 从可用字符的字符串中随机选择一个字符
+		if idx := int(cache & letterIdMask); idx < len(letters) {
+			bytes[i] = letters[idx]
+			i--
+		}
+		// 右移比特位数，为下次选择字符做准备
+		cache >>= letterIdBits
+		remain--
 	}
-	return string(cs)
+	// 仿照strings.Builder用unsafe包返回一个字符串，避免拷贝
+	// 将字节切片转换为字符串并返回
+	return *(*string)(unsafe.Pointer(&bytes))
+}
+
+// RandomChars generate give length random chars at `a-z`
+func RandomChars(ln int) string {
+	return buildRandomString(AlphaBet, ln)
 }
 
 // RandomCharsV2 generate give length random chars in `0-9a-z`
 func RandomCharsV2(ln int) string {
-	cs := make([]byte, ln)
-	// UnixNano: 1607400451937462000
-	rn := newRand()
-
-	for i := 0; i < ln; i++ {
-		// rand in 0 - 35
-		cs[i] = AlphaNum[rn.Intn(35)]
-	}
-	return string(cs)
+	return buildRandomString(AlphaNum, ln)
 }
 
 // RandomCharsV3 generate give length random chars in `0-9a-zA-Z`
 func RandomCharsV3(ln int) string {
-	cs := make([]byte, ln)
-	// UnixNano: 1607400451937462000
-	rn := newRand()
-
-	for i := 0; i < ln; i++ {
-		// rand in 0 - 61
-		cs[i] = AlphaNum2[rn.Intn(61)]
-	}
-	return string(cs)
+	return buildRandomString(AlphaNum2, ln)
 }
 
 // RandWithTpl generate random string with give template
@@ -69,16 +109,7 @@ func RandWithTpl(n int, letters string) string {
 	if len(letters) == 0 {
 		letters = AlphaNum2
 	}
-
-	ln := len(letters)
-	cs := make([]byte, n)
-	rn := newRand()
-
-	for i := 0; i < n; i++ {
-		// rand in 0 - ln
-		cs[i] = letters[rn.Intn(ln)]
-	}
-	return byteutil.String(cs)
+	return buildRandomString(letters, n)
 }
 
 // RandomString generate.