在java中计算模逆

import java.math.BigInteger;

public class ModularInverse {
    public static BigInteger calculateModularInverse(BigInteger a, BigInteger m) {
        BigInteger[] result = extendedEuclideanAlgorithm(a, m);
        
        if (result[0].compareTo(BigInteger.ONE) == 0) {
            return positiveModulus(result[1], m);
        } else {
            throw new ArithmeticException("Modular inverse does not exist.");
        }
    }
    
    private static BigInteger[] extendedEuclideanAlgorithm(BigInteger a, BigInteger b) {
        if (b.compareTo(BigInteger.ZERO) == 0) {
            return new BigInteger[] {a, BigInteger.ONE, BigInteger.ZERO};
        } else {
            BigInteger[] prev = extendedEuclideanAlgorithm(b, a.mod(b));
            BigInteger d = prev[0];
            BigInteger x = prev[2];
            BigInteger y = prev[1].subtract(a.divide(b).multiply(prev[2]));
            return new BigInteger[] {d, x, y};
        }
    }
    
    private static BigInteger positiveModulus(BigInteger a, BigInteger m) {
        BigInteger result = a.mod(m);
        return result.compareTo(BigInteger.ZERO) >= 0 ? result : result.add(m);
    }
}