Java Integer Mod modInverse(int n, int mod)

Description

mod Inverse

License

Open Source License

Declaration

public static int modInverse(int n, int mod) 

Method Source Code

//package com.java2s;
//License from project: Open Source License 

public class Main {

    /* source : http://www.liafa.univ-paris-diderot.fr/~carton/Enseignement/CalculFormel/Style/ModArith/Gcd.java*/
    public static int modInverse(int n, int mod) {
        int[] g = extgcd(mod, n);
        if (g[0] != 1) {
            return -1; // n et mod ne sont pas premiers entre eux
        } else {//w w w  . java2s.  c  o  m
            return reduce(g[2], mod);
        }
    }

    /* source : http://www.liafa.univ-paris-diderot.fr/~carton/Enseignement/CalculFormel/Style/ModArith/Gcd.java*/
    public static int[] extgcd(int m, int n) {
        // Both arrays ma and na are arrays of 3 integers such that
        // ma[0] = m ma[1] + n ma[2] and na[0] = m na[1] + n na[2]
        int[] ma = new int[] { m, 1, 0 };
        int[] na = new int[] { n, 0, 1 };
        int[] ta; // Temporary variable 
        int i; // Loop index
        int q; // Quotient
        int r; // Rest

        // Exchange ma and na if m < n
        if (m < n) {
            ta = na;
            na = ma;
            ma = ta;
        }

        // It can be assumed that m >= n
        while (na[0] > 0) {
            q = ma[0] / na[0]; // Quotient
            for (i = 0; i < 3; i++) {
                r = ma[i] - q * na[i];
                ma[i] = na[i];
                na[i] = r;
            }
        }
        return ma;
    }

    /* source : http://www.liafa.univ-paris-diderot.fr/~carton/Enseignement/CalculFormel/Style/ModArith/Gcd.java*/
    public static int reduce(int n, int mod) {
        int m = n % mod; // -mod < m < mod

        if (m >= 0) {
            return m;
        } else {
            return m + mod;
        }
    }
}

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