public static long GetFibonacciNumber(long n, long mod = 0)
{
long[,] matrix = new long[, ] {
{ 0, 1 }, { 1, 1 }
};
MatrixInt result = MatrixBinPower(new MatrixInt(matrix, mod), n);
return(result[1, 1]);
}
public static MatrixInt MatrixBinPower(MatrixInt a, long n)
{
MatrixInt result = MatrixInt.IdentityMatrix(a.Size, a.Mod);
while (n > 0)
{
if ((n & 1) != 0)
{
result *= a;
}
a *= a;
n >>= 1;
}
return(result);
}