Java Mutable BigInteger Class

Question

I am doing calculations with BigIntegers that uses a loop that calls multiply() about 100 billion times, and the new object creation from the BigInteger is making it very slow.

Answer 1

I copied MutableBigInteger, then commented out some methods' bodies I dind't need, adding a nice

throw new UnsupportedOperationException("...");

when invoked.

here is how it looks.

In Revisions you can see what's changed from the original java.math.MutableBigInteger.

I also added some convenience methods,

public void init(long val) {};
public MutableBigInteger(long val) {};
// To save previous value before modifying.
public void addAndBackup(MutableBigInteger addend) {}
// To restore previous value after modifying.  
public void restoreBackup() {}

Here is how I used it:

private BigInteger traverseToFactor(BigInteger offset, BigInteger toFactorize, boolean forward) {
    MutableBigInteger mbiOffset = new  MutableBigInteger(offset);
    MutableBigInteger mbiToFactorize = new MutableBigInteger(toFactorize);
    MutableBigInteger blockSize = new MutableBigInteger(list.size);

    if (! MutableBigInteger.ZERO.equals(mbiOffset.remainder(blockSize))) {
        throw new ArithmeticException("Offset not multiple of blockSize");
    }

    LongBigArrayBigList pattern = (LongBigArrayBigList) list.getPattern();

    while (true) {
        MutableBigInteger divisor = new MutableBigInteger(mbiOffset);
        for (long i = 0; i < pattern.size64(); i++) {
            long testOperand = pattern.getLong(i);
            MutableBigInteger.UNSAFE_AUX_VALUE.init(testOperand);
            divisor.addAndBackup(MutableBigInteger.UNSAFE_AUX_VALUE);
            if (MutableBigInteger.ZERO.equals(mbiToFactorize.remainder(divisor))) {
                return divisor.toBigInteger();
            }
            divisor.restoreBackup();
        }

        if (forward) {
            mbiOffset.add(blockSize);
        } else {
            mbiOffset.subtract(blockSize);
        }
        System.out.println(mbiOffset);
    }
}