Faster Alternative to Java's BigInteger Class

BigInt.java

public class BigInt {
	
	protected long[] digits;
	protected int length;
	protected boolean negative;
	
	public BigInt(int value) {
		this((long)value);
	}
	
	public BigInt(long value) {
		if(value < 0) {
			value = -value;
			negative = true;
		} else
			negative = false;
		length = (int)Math.ceil((Math.log10(value) + 1) / 18);
		digits = new long[length];
		int i = 0;
		for(; value != 0; value /= 1000000000000000000L)
			digits[i++] = value % 1000000000000000000L;
	}
	
	public BigInt(String s) {
		negative = s.startsWith("-");
		s = s.replaceFirst("-", "");
		length = (int)Math.ceil(s.length() / 18.0);
		digits = new long[length];
		int j = 0;
		for(int i = s.length() - 18; i > -1; i -= 18)
			digits[j++] = Long.parseLong(s.substring(i, i + 18));
		if(s.length() % 18 != 0)
			digits[j] = Long.parseLong(s.substring(0, s.length() % 18));
	}
	
	public String toString() {
		StringBuilder str = new StringBuilder();
		for(int i = length - 1; i > -1; i--) {
			if(i != length - 1)
				str.append(String.format("%018d", digits[i]));
			else
				str.append(digits[i]);
		}
		str.insert(0, negative ? "-" : "");
		return str.toString();
	}
	
	public void add(BigInt other) {
		if(negative == other.negative)
			addUnsigned(other);
	}
	
	private void addUnsigned(BigInt other) {
		if(length < other.length)
			if(digits.length < (other.length + 1))
				realloc(other.length + 1);
		if(digits.length < length + 1)
			realloc(length + 1);
		length++;
		long carry = 0;
		for(int i = 0; i < Math.min(length, other.length); i++) {
			long nval = digits[i] + other.digits[i] + carry;
			carry = nval / 1000000000000000000L;
			digits[i] = nval % 1000000000000000000L;
		}
		for(int i = Math.min(length, other.length); i < Math.max(length, other.length); i++) {
			digits[i] += carry;
			carry = digits[i] / 1000000000000000000L;
			if(carry == 0)
				break;
		}
		if(digits[length - 1] == 0)
			length--;
	}
	
	private void realloc(int newSize) {
		if(length > newSize)
			return;
		long[] newArr = new long[newSize];
		System.arraycopy(digits, 0, newArr, 0, length);
		digits = newArr;
	}

}