IsPrime.java

/*
    BeepBeep, an event stream processor
    Copyright (C) 2008-2016 Sylvain Hallé

    This program is free software: you can redistribute it and/or modify
    it under the terms of the GNU Lesser General Public License as published
    by the Free Software Foundation, either version 3 of the License, or
    (at your option) any later version.

    This program is distributed in the hope that it will be useful,
    but WITHOUT ANY WARRANTY; without even the implied warranty of
    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
    GNU Lesser General Public License for more details.

    You should have received a copy of the GNU Lesser General Public License
    along with this program.  If not, see <http://www.gnu.org/licenses/>.
 */
package functions;

import ca.uqac.lif.cep.functions.FunctionException;
import ca.uqac.lif.cep.functions.UnaryFunction;

/**
 * Create a custom unary function that checks if a number is prime.
 * <p>
 * Instead of inheriting from the top-level {@link Function} class,
 * it is easier in most cases to descend from lower classes, which provide a
 * few more functionalities. For a function that takes a single argument,
 * it is wise to extend {@link UnaryFunction}.
 * 
 * @author Sylvain Hallé
 */
public class IsPrime extends UnaryFunction<Number,Boolean>
{
    /**
     * This is optional. Since a Function object is stateless, the
     * same instance can be used everywhere it is needed. Therefore,
     * it is recommended to enforce the existence of a single instance
     * by making the function's constructor private, and by making
     * public a static field pointing to an instance of the function.
     * By convention, you are encouraged to use the name "instance" for
     * this field.
     */
    public static final transient IsPrime instance = new IsPrime();

    private IsPrime()
    {
        /* The constructor should call super(), and pass the type of
         * the input and output. */
        super(Number.class, Boolean.class);
    }

    /*
     * Method getValue() is where the output of the function is
     * computed for the input. For the sake of our example, the
     * actual way to check if x is prime does not matter;
     * we'll simply enumerate all numbers up to sqrt(x) until we
     * find one that divides x, and otherwise return true.
     */
    @Override
    public Boolean getValue(Number x)
    {
        int k = x.intValue(); // Convert x to an int
        int max = (int) Math.sqrt(k);
        for (int n = 2; n <= max; n++)
        {
            if (k % n == 0)
                return false; // Found a divisor: x is not prime
        }
        return true;
    }

    /*
     * A small main method to illustrate the function
     */
    public static void main(String[] args) throws FunctionException
    {
        IsPrime ip = IsPrime.instance;
        Object[] value = new Object[1];
        // A function is always called on an array of objects; this array
        // corresponds to the arguments. Here the function is unary, hence
        // the array is of size 1
        ip.evaluate(new Integer[]{3}, value);
        // Likewise, a function always returns an array of objects. Most
        // functions (like this one) return a single object, so the output
        // array is also of size 1
        System.out.printf("Return value of the function: %b\n", value[0]);
        ip.evaluate(new Integer[]{8}, value);
        System.out.printf("Return value of the function: %b\n", value[0]);
    }
}