/*
 * Copyright (c) 2004 David Flanagan.  All rights reserved.
 * This code is from the book Java Examples in a Nutshell, 3nd Edition.
 * It is provided AS-IS, WITHOUT ANY WARRANTY either expressed or implied.
 * You may study, use, and modify it for any non-commercial purpose,
 * including teaching and use in open-source projects.
 * You may distribute it non-commercially as long as you retain this notice.
 * For a commercial use license, or to purchase the book, 
 * please visit http://www.davidflanagan.com/javaexamples3.
 */
package je3.basics;

/**
 * This class computes factorials and caches the results in a table for reuse.
 * 20! is as high as we can go using the long data type, so check the argument
 * passed and "throw an exception" if it is too big or too small.
 **/
public class Factorial3 {
    // Create an array to cache values 0! through 20!.
    static long[] table = new long[21];
    // A "static initializer": initialize the first value in the array
    static { table[0] = 1; }  // factorial of 0 is 1.
    // Remember the highest initialized value in the array
    static int last = 0;

    public static long factorial(int x) throws IllegalArgumentException {
        // Check if x is too big or too small.  Throw an exception if so.
        if (x >= table.length)   // ".length" returns length of any array
            throw new IllegalArgumentException("Overflow; x is too large.");
        if (x<0) throw new IllegalArgumentException("x must be non-negative.");

        // Compute and cache any values that are not yet cached.
        while(last < x) {
            table[last + 1] = table[last] * (last + 1);
            last++;
        }
        // Now return the cached factorial of x.
        return table[x];
    }
}