counting up with characters based on index

My current code:

var basicChars = []rune("abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789")
func GetFromIndex(index int) string {
	length := len(basicChars)

	size := 0 // size is the amount of characters in the final string
	for {
		if float64(index) > math.Pow(float64(length), float64(size))-2 {
			size++
		} else {
			break
		}
	}

	str := make([]rune, size)

	for i := 0; i < size; i++ {
		str[i] = basicChars[index%length]
	}

	return string(str)
}

I am trying to count up with letters instead of numbers.

I know I can use for loops but there is no good way of saving state or going up indefinitely

based on your description GetFromIndex(122) should return a8, not bb

either I didn't get the correct scenario or we are missing informations

-eddit-

I'm not sure it's the best way to do it

• the first character will moove every time
• the 2nd -> 3 times
• the 3rd -> 9 times ...

I substract the useless cycles

• the 2nd -> each 3 * len(array)
• the 3rd -> each 9 * len(array)

and get their position in the array

func TestA(t *testing.T) {
	for i:=0; i<=30; i++ {
		fmt.Printf("%s i:{%d} \n",GetFromIndexBis(i),i)
	}

}
func GetFromIndex(index int) string {
	var basicChars = []rune("abc")
	len := len(basicChars)
	pow := 1
	sumPow := 0
	res :=""	
	
	// the first is a simple modulo
	res += string(basicChars[index%len])
	
	for {
		
		pow = pow * len
		// is the index big enought ?
		if index < pow+sumPow{
			break
		}

		// remove the first cycles where nothing pushed the wheels
		start := index - sumPow

		// number of cycle we need to make a full turn
		fullCycles := pow * len

		if start>=fullCycles {
			nbrOfUselessCycles := start / fullCycles
			start = start - fullCycles * nbrOfUselessCycles
		}

		index := (start / pow) -1

		// it's the last one
		if (index == -1) {
			res += string(basicChars[len-1])	
		}else {
			res += string(basicChars[index])
		}
		
		sumPow += pow
	}
	
	return res
}