Name: mgC56079 Date: 05/12/99



The spec for BigInteger.modPow says:
    Returns a BigInteger whose value is
    <tt>(this^exponent mod m)</tt>.
    (Unlike <tt>pow</tt>, this method permits negative exponents.)

0^-1 is a division by zero and should result in an ArithmeticException.
JDK 1.2.x works correctly. JDK 1.3 build C returns 0 which is incorrect.

Here is a minimized test:
---------- BigIntegerTest.java ----------
import java.math.*;
class BigIntegerTest {

    public static void main(String[] args) {
        BigInteger b = new BigInteger("0");
        BigInteger p = new BigInteger("-1");
        BigInteger m = new BigInteger("2");
        System.out.println(b.modPow(p, m));
    }

}
---------- Sample run (JDK1.2.1 FCS, correct) ----------
% java BigIntegerTest
Exception in thread "main" java.lang.ArithmeticException: BigInteger not invertible
at java.math.BigInteger.modInverse(Compiled Code)
at java.math.BigInteger.modPow(Compiled Code)
at BigIntegerTest.main(Compiled Code)
---------- Sample run (JDK1.3S, wrong) ----------
% java BigIntegerTest
0

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