commons-numbers-combinatorics/src/main/java/org/apache/commons/numbers/combinatorics/Factorial.java

/*
 * 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.
 */

package org.apache.commons.numbers.combinatorics;

/**
 * <a href="http://mathworld.wolfram.com/Factorial.html">Factorial of a number</a>.
 */
public final class Factorial {

    /** All long-representable factorials. */
    static final long[] FACTORIALS = new long[] {
                       1L,                  1L,                   2L,
                       6L,                 24L,                 120L,
                     720L,               5040L,               40320L,
                  362880L,            3628800L,            39916800L,
               479001600L,         6227020800L,         87178291200L,
           1307674368000L,     20922789888000L,     355687428096000L,
        6402373705728000L, 121645100408832000L, 2432902008176640000L
    };

    /** Private constructor. */
    private Factorial() {
        // intentionally empty.
    }

    /**
     * Computes the factorial of {@code n}.
     *
     * @param n Argument.
     * @return {@code n!}
     * @throws IllegalArgumentException if {@code n < 0}.
     * @throws IllegalArgumentException if {@code n > 20} (the factorial
     * value is too large to fit in a {@code long}).
     */
    public static long value(int n) {
        if (n < 0 ||
            n > 20) {
            throw new CombinatoricsException(CombinatoricsException.OUT_OF_RANGE,
                                             n, 0, 20);
        }

        return FACTORIALS[n];
    }
}