package calculator.model.binaryOperations;

import calculator.model.*;

public class Permutation extends BinaryOperation
{
    public double operateBinary(double n, double r)
    {
        // n and r should be integral values
        double rfloor = Math.floor(r);
        double nfloor = Math.floor(n);

        // Check for all invalid values of n and r.
        if ((n < 1) || (r < 0) || (r > n) || (r != rfloor) || (n != nfloor))
        {
            throw new OperationParameterException("Invalid Input to Permutation Function");
        }

        // Limit r to 175 as r >= 175 will result in value > Double.MAX_VALUE
        // i.e. infinity. This is done to prevent going into big for loop.
        if (r >= 175.0)
        {
            throw new OperationParameterException("Infinity");
        }

        double result = 1;

        // Loop on r and not n-r as n-r can be huge resulting in rounding
        // off error while doing ++ operation and possible infinite loop.
        for (double i = 0; i < r; i++)
        {
            result *= (n - i);
        }

        return result;
    }

    public Precedence getPrecedence()
    {
        return Precedence.Permutation;
    }
}