package random

import (
	"math/rand"
	"time"
	"unsafe"
)

type Mode int // 随机数模式

const (
	Letter Mode = iota
	Number
	LetterNumber
	LetterHex
)

const (
	letters       = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ"
	numbers       = "0123456789"
	lettersHex    = "0123456789abcdef"
	letterIdBit   = 6
	letterIdxMask = 1<<letterIdBit - 1
	letterIdxMax  = 63 / letterIdBit
)

func StrRandom(n int64) string {
	return Random(n, Letter)
}

func NumRandom(n int64) string {
	return Random(n, Number)
}

func Random(n int64, mode ...Mode) string {
	var baseStr string
	mode = append(mode, LetterNumber)
	switch mode[0] {
	case LetterHex:
		baseStr = lettersHex
	case Letter:
		baseStr = letters
	case Number:
		baseStr = numbers
	case LetterNumber:
		fallthrough
	default:
		baseStr = letters + numbers
	}
	return random(n, baseStr)
}

func random(n int64, baseStr string) string {

	var src = rand.NewSource(time.Now().UnixNano())
	b := make([]byte, n)
	for i, cache, remain := n-1, src.Int63(), letterIdxMax; i >= 0; {
		if remain == 0 {
			cache, remain = src.Int63(), letterIdxMax
		}
		if idx := int(cache & letterIdxMask); idx < len(baseStr) {
			b[i] = baseStr[idx]
			i--
		}
		cache >>= letterIdBit
		remain--
	}
	return *(*string)(unsafe.Pointer(&b))
}