package nrandom

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

// Random generate a random string based on given string range.
func Random(letters string, length int) string {
	b := make([]byte, length)
	r := rand.NewSource(time.Now().UnixNano())
	for i, cache, remain := length-1, r.Int63(), letterIdMax; i >= 0; {
		if remain == 0 {
			cache, remain = r.Int63(), letterIdMax
		}
		if idx := int(cache & letterIdMask); idx < len(letters) {
			b[i] = letters[idx]
			i--
		}
		cache >>= letterIdBits
		remain--
	}
	return *(*string)(unsafe.Pointer(&b))
}

// RandString generate random string of specified length.
func RandString(length int) string {
	return Random(Letters, length)
}

// RandStrUpper generate a random upper case string.
func RandStrUpper(length int) string {
	return Random(UpperLetters, length)
}

// RandStrLower generate a random lower case string.
func RandStrLower(length int) string {
	return Random(LowerLetters, length)
}

// RandNumeralStr generate a random numeral string of specified length.
func RandNumeralStr(length int) string {
	return Random(Numeral, length)
}

// RandNumeralOrLetter generate a random numeral or letter string.
func RandNumeralOrLetter(length int) string {
	return Random(Numeral+Letters, length)
}