Permuter.java

/*
 * Copyright (c) 2007, 2016, Oracle and/or its affiliates. All rights reserved.
 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
 *
 * This code is free software; you can redistribute it and/or modify it
 * under the terms of the GNU General Public License version 2 only, as
 * published by the Free Software Foundation.
 *
 * This code 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 General Public License
 * version 2 for more details (a copy is included in the LICENSE file that
 * accompanied this code).
 *
 * You should have received a copy of the GNU General Public License version
 * 2 along with this work; if not, write to the Free Software Foundation,
 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
 *
 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
 * or visit www.oracle.com if you need additional information or have any
 * questions.
 */
package com.sun.swingset3.demos.list;

/**
 * An object that implements a cheesy pseudorandom permutation of the integers
 * from zero to some user-specified value. (The permutation is a linear
 * function.)
 *
 * @version 1.9 11/17/05
 * @author Josh Bloch
 */
class Permuter {

    /**
     * The size of the permutation.
     */
    private int modulus;

    /**
     * Nonnegative integer less than n that is relatively prime to m.
     */
    private int multiplier;

    /**
     * Pseudorandom nonnegative integer less than n.
     */
    private static final int ADDEND = 22;

    public Permuter(int n) {
        if (n < 0) {
            throw new IllegalArgumentException();
        }
        modulus = n;
        if (n == 1) {
            return;
        }

        // Initialize the multiplier and offset
        multiplier = (int) Math.sqrt(n);
        while (gcd(multiplier, n) != 1) {
            if (++multiplier == n) {
                multiplier = 1;
            }
        }
    }

    /**
     * Returns the integer to which this permuter maps the specified integer.
     * The specified integer must be between 0 and n-1, and the returned integer
     * will be as well.
     */
    public int map(int i) {
        return (multiplier * i + ADDEND) % modulus;
    }

    /**
     * Calculate GCD of a and b, which are assumed to be non-negative.
     */
    private static int gcd(int a, int b) {
        while (b != 0) {
            int tmp = a % b;
            a = b;
            b = tmp;
        }
        return a;
    }

    /**
     * Simple test. Takes modulus on command line and prints out permutation.
     */
    public static void main(String[] args) {
        int modulus = Integer.parseInt(args[0]);
        Permuter p = new Permuter(modulus);
        for (int i = 0; i < modulus; i++) {
            System.out.print(p.map(i) + " ");
        }
        System.out.println();
    }
}