/// <summary>
/// Compute the factorial of natural number num
/// </summary>
/// <param name="num">Natural number represented as string (bigNum)</param>
/// <returns></returns>
/// <exception cref="IllegalNumberException"></exception>
public static BigInteger Factofial(string num)
{
num = StringNumber.RemoveSpaces(num);
num = StringNumber.RemoveUnnecessaryZeros(num);
if (num.Equals("0") || num.Equals("1"))
{
return(BigInteger.One);
}
if (num.Contains('.'))
{
throw new IllegalNumberException($"Illegal number. Expected Natural number, but found Rational: {num}");
}
else if (num.Contains('-'))
{
throw new IllegalNumberException($"Illegal number. Expected Natural number, but found negaive Integer: {num}");
}
BigInteger n = BigInteger.Parse(num);
BigInteger res = BigInteger.Parse(num);
while (n > BigInteger.One)
{
n = BigInteger.Subtract(n, BigInteger.One);
res = BigInteger.Multiply(res, n);
}
return(res);
}
/// <summary>
/// Computes ith Fibonachi number.
/// </summary>
/// <param name="i">The number of position at Fibonachi numbers series (i-th)</param>
/// <returns></returns>
/// <exception cref="IllegalNumberException"></exception>
public static BigInteger GetFibboNumber(string i)
{
i = StringNumber.RemoveSpaces(i);
i = StringNumber.RemoveUnnecessaryZeros(i);
if (i.Equals("0"))
{
return(BigInteger.Zero);
}
else if (i.Equals("1"))
{
return(BigInteger.One);
}
if (i.Contains('.'))
{
throw new IllegalNumberException($"Illegal number. Expected Natural number, but found Rational: {i}");
}
else if (i.Contains('-'))
{
throw new IllegalNumberException($"Illegal number. Expected Natural number, but found negaive Integer: {i}");
}
/* Compute ith Fibonachi number */
BigInteger f1 = BigInteger.Zero;
BigInteger f2 = BigInteger.One;
BigInteger res = BigInteger.Zero;
BigInteger n = BigInteger.Parse(i);
BigInteger n_i = BigInteger.One;
while (n_i < n)
{
res = BigInteger.Add(f1, f2);
f1 = f2;
f2 = res;
n_i = BigInteger.Add(n_i, BigInteger.One);
}
return(res);
}