CODEWITHSUNDEEP
javaarraysbiginteger
andandandand
andandandand

Reputation: 22270

Implementing BigInteger's multiply...from scratch (and making sure it's O(n^2))

As homework, I'm implementing Karatsuba's algorithm and benchmarking it against a primary-school-style O(n^2) multiplication algorithm on large integers.

I guessed my only choice here was to bring the numbers to their byte array representations and then work them from there.

Well, I'm stuck here... when using the * operator, I don't know how would I detect/correct if the number overflows a byte multiplication or adds a carry. Any ideas? 

public static BigInteger simpleMultiply(BigInteger x, BigInteger y){

        //BigInteger result = x.multiply(y);

        byte [] xByteArray = x.toByteArray();
        byte [] yByteArray = y.toByteArray();

        int resultSize = xByteArray.length*yByteArray.length;

        byte [][] rowsAndColumns = new byte[resultSize][resultSize];

        for (int i =0; i<xByteArray.length;i++)
           for (int j=0; j<yByteArray.length;j++){


               rowsAndColumns[i][j] = (byte )(xByteArray[i] * yByteArray[j]); 
               // how would I detect/handle carry or overflow here?               
           }

        return null;
    }

Upvotes: 5

Views: 3932

Answers (2)

hugomg
hugomg

Reputation: 69964

The best way to avoid overflow is not to have it happen in the first place. Make all your calculations with a higher width numbers to avoid problems.

For example, lets say we have base 256 numbers and each digit is stored as a single unsigned byte.

d1 = (int) digits[i] //convert to a higher-width number
d2 = (int) digits[j]
product = d1*d2  //ints can handle up to around 2^32. Shouldn't overflow w/ 256*256
result = product % 256
carry  = product / 256

You could be fancy and convert the divisions by powers of two into bit operations, but it isn't really necessary.

Upvotes: 1

Gabe
Gabe

Reputation: 86768

The result of a byte multiplication is 2 bytes. You have to use the low order byte as the result and the high order byte as the carry (overflow).

I would also advise you to be careful of the sign of your bytes. Since bytes in Java are signed, you'll have to either use only the low 7 bits of them or convert them to ints and correct the sign before multiplying them.

You'll want a loop like:

        for (int i =0; i<xByteArray.length;i++)
           for (int j=0; j<yByteArray.length;j++){
               // convert bytes to ints
               int xDigit = xByteArray[i], yDigit = yByteArray[j];
               // convert signed to unsigned
               if (xDigit < 0)
                   xDigit += 256;
               if (yDigit < 0)
                   yDigit += 256;
               // compute result of multiplication
               int result = xDigit * yDigit;
               // capture low order byte
               rowsAndColumns[i][j] = (byte)(result & 0xFF);
               // get overflow (high order byte)
               int overflow = result >> 8;
               // handle overflow here
               // ...
           }

Upvotes: 2

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