Euler.java

import java.math.BigDecimal;
import java.math.RoundingMode;


public class Euler {
    private Euler()
    {

    }

    private static BigDecimal factorial(long n)
    {
        BigDecimal ret = new BigDecimal(1);
        long i;
        if(n > 0) for(i = 1L; i <= n; i++) ret = ret.multiply(new BigDecimal(i));
        return ret;
    }

    public static BigDecimal e(int precision)
    {
        return e(0.0, precision);
    }

    public static BigDecimal e(double epsilon, int precision)
    {
        BigDecimal ret = new BigDecimal(0), add, big_one = new BigDecimal(1), big_epsilon = new BigDecimal(epsilon);
        long i = 0;
        if(epsilon < 0) epsilon = -epsilon;
        if(precision < 0) precision = -precision;
        do
        {
            add = big_one.divide(factorial(i++), precision, RoundingMode.HALF_UP);
            ret = ret.add(add);
        }
        while(add.compareTo(big_epsilon) > 0);
        return ret;
    }
}