A DESCRIPTION OF THE PROBLEM :
In the current implementation, Long.toUnsignedLong(long, int) sometimes creates a BigInteger and calls its toString(int) method. However, we don't have to create the BigInteger. The string can be calculated by the following algorithm:

long leadingDigits = divideUnsigned(i, radix); // always positive
int lastDigit = (int)remainderUnsigned(i, radix);
yield toString(leadingDigits, radix) + Character.forDigit(lastDigit, radix);

This implementation prevents from creating an additional object and is typically faster than BigInteger.toString(int).