Main.java Source code

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Here is the source code for Main.java

Source

import java.io.File;

/* 
 * Licensed to the Apache Software Foundation (ASF) under one or more
 *  contributor license agreements.  See the NOTICE file distributed with
 *  this work for additional information regarding copyright ownership.
 *  The ASF licenses this file to You under the Apache License, Version 2.0
 *  (the "License"); you may not use this file except in compliance with
 *  the License.  You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 *  Unless required by applicable law or agreed to in writing, software
 *  distributed under the License is distributed on an "AS IS" BASIS,
 *  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 *  See the License for the specific language governing permissions and
 *  limitations under the License.
 *
 *
 */
public class Main {

    /**
     * Returns n!. Shorthand for <code>n</code> <a
     * href="http://mathworld.wolfram.com/Factorial.html"> Factorial</a>, the
     * product of the numbers <code>1,...,n</code>.
     * <p>
     * <Strong>Preconditions</strong>:
     * <ul>
     * <li> <code>n >= 0</code> (otherwise
     * <code>IllegalArgumentException</code> is thrown)</li>
     * <li> The result is small enough to fit into a <code>long</code>. The
     * largest value of <code>n</code> for which <code>n!</code> <
     * Long.MAX_VALUE</code> is 20. If the computed value exceeds <code>Long.MAX_VALUE</code>
     * an <code>ArithMeticException </code> is thrown.</li>
     * </ul>
     * </p>
     * 
     * @param n argument
     * @return <code>n!</code>
     * @throws ArithmeticException if the result is too large to be represented
     *         by a long integer.
     * @throws IllegalArgumentException if n < 0
     */
    public static long factorial(final int n) {
        long result = Math.round(factorialDouble(n));
        if (result == Long.MAX_VALUE) {
            throw new ArithmeticException("result too large to represent in a long integer");
        }
        return result;
    }

    /**
     * Returns n!. Shorthand for <code>n</code> <a
     * href="http://mathworld.wolfram.com/Factorial.html"> Factorial</a>, the
     * product of the numbers <code>1,...,n</code> as a <code>double</code>.
     * <p>
     * <Strong>Preconditions</strong>:
     * <ul>
     * <li> <code>n >= 0</code> (otherwise
     * <code>IllegalArgumentException</code> is thrown)</li>
     * <li> The result is small enough to fit into a <code>double</code>. The
     * largest value of <code>n</code> for which <code>n!</code> <
     * Double.MAX_VALUE</code> is 170. If the computed value exceeds
     * Double.MAX_VALUE, Double.POSITIVE_INFINITY is returned</li>
     * </ul>
     * </p>
     * 
     * @param n argument
     * @return <code>n!</code>
     * @throws IllegalArgumentException if n < 0
     */
    public static double factorialDouble(final int n) {
        if (n < 0) {
            throw new IllegalArgumentException("must have n >= 0 for n!");
        }
        return Math.floor(Math.exp(factorialLog(n)) + 0.5);
    }

    /**
     * Returns the natural logarithm of n!.
     * <p>
     * <Strong>Preconditions</strong>:
     * <ul>
     * <li> <code>n >= 0</code> (otherwise
     * <code>IllegalArgumentException</code> is thrown)</li>
     * </ul></p>
     * 
     * @param n argument
     * @return <code>n!</code>
     * @throws IllegalArgumentException if preconditions are not met.
     */
    public static double factorialLog(final int n) {
        if (n < 0) {
            throw new IllegalArgumentException("must have n > 0 for n!");
        }
        double logSum = 0;
        for (int i = 2; i <= n; i++) {
            logSum += Math.log((double) i);
        }
        return logSum;
    }

}