package buildin

import (
	"fmt"
	"math/rand"
	"time"
	"unicode/utf8"
	"unsafe"
)

const (
	// 6 bits to represent a letter index
	letterIdBits = 6
	// All 1-bits as many as letterIdBits
	letterIdMask = 1<<letterIdBits - 1
	letterIdMax  = 63 / letterIdBits
)

// SplitStringByLength 将字符串按长度划分为数组，汉字2个长度英文1个长度
func SplitStringByLength(str string, maxLength int) []string {
	var arr []string
	index := 0
	s := ""
	for _, v := range str {
		s = s + string(v)
		if utf8.RuneLen(v) == 1 {
			index += 1
		} else {
			index += 2
		}
		if index >= maxLength {
			arr = append(arr, s)
			index = 0
			s = ""
		}
	}
	if s != "" {
		arr = append(arr, s)
	}
	return arr
}

// RandString 随机生成字符串 最强性能！
func RandString(n int) string {
	letters := "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ"
	var src = rand.NewSource(time.Now().UnixNano())

	b := make([]byte, n)
	// A rand.Int63() generates 63 random bits, enough for letterIdMax letters!
	for i, cache, remain := n-1, src.Int63(), letterIdMax; i >= 0; {
		if remain == 0 {
			cache, remain = src.Int63(), letterIdMax
		}
		if idx := int(cache & letterIdMask); idx < len(letters) {
			b[i] = letters[idx]
			i--
		}
		cache >>= letterIdBits
		remain--
	}
	return *(*string)(unsafe.Pointer(&b))
}

// RandNumberString 返回特定长度的数据随机数字字符串
func RandNumberString(bit uint8) string {
	rand.Seed(time.Now().UnixNano())
	s := ""
	for i := 0; uint8(i) < bit; i++ {
		r := rand.Intn(9)
		s += fmt.Sprintf("%d", r)
	}
	return s
}