C# ModInverse Function

Question

Is there a built in function that would allow me to calculate the modular inverse of a(mod n)? e.g. 19^-1 = 11 (mod 30), in this case the 19^-1 == -11==19;

Answer 1

There is no library for getting inverse mod, but the following code can be used to get it.

// Given a and b->ax+by=d
long[] u = { a, 1, 0 };
long[] v = { b, 0, 1 };
long[] w = { 0, 0, 0 };
long temp = 0;
while (v[0] > 0)
{
    double t = (u[0] / v[0]);
    for (int i = 0; i < 3; i++)
    {
        w[i] = u[i] - ((int)(Math.Floor(t)) * v[i]);
        u[i] = v[i];
        v[i] = w[i];
    }
}
// u[0] is gcd while u[1] gives x and u[2] gives y. 
// if u[1] gives the inverse mod value and if it is negative then the following gives the first positive value
if (u[1] < 0)
{
        while (u[1] < 0)
        {
            temp = u[1] + b;
            u[1] = temp;
        }
}