Permuter.java revision 13978:1993af50385d 
1/*
2 * Copyright (c) 2007, 2016, Oracle and/or its affiliates. All rights reserved.
3 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
4 *
5 * This code is free software; you can redistribute it and/or modify it
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7 * published by the Free Software Foundation.
8 *
9 * This code is distributed in the hope that it will be useful, but WITHOUT
10 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
11 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
12 * version 2 for more details (a copy is included in the LICENSE file that
13 * accompanied this code).
14 *
15 * You should have received a copy of the GNU General Public License version
16 * 2 along with this work; if not, write to the Free Software Foundation,
17 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
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22 */
23package com.sun.swingset3.demos.list;
24
25/**
26 * An object that implements a cheesy pseudorandom permutation of the integers
27 * from zero to some user-specified value. (The permutation is a linear
28 * function.)
29 *
30 * @version 1.9 11/17/05
31 * @author Josh Bloch
32 */
33class Permuter {
34
35    /**
36     * The size of the permutation.
37     */
38    private int modulus;
39
40    /**
41     * Nonnegative integer less than n that is relatively prime to m.
42     */
43    private int multiplier;
44
45    /**
46     * Pseudorandom nonnegative integer less than n.
47     */
48    private static final int ADDEND = 22;
49
50    public Permuter(int n) {
51        if (n < 0) {
52            throw new IllegalArgumentException();
53        }
54        modulus = n;
55        if (n == 1) {
56            return;
57        }
58
59        // Initialize the multiplier and offset
60        multiplier = (int) Math.sqrt(n);
61        while (gcd(multiplier, n) != 1) {
62            if (++multiplier == n) {
63                multiplier = 1;
64            }
65        }
66    }
67
68    /**
69     * Returns the integer to which this permuter maps the specified integer.
70     * The specified integer must be between 0 and n-1, and the returned integer
71     * will be as well.
72     */
73    public int map(int i) {
74        return (multiplier * i + ADDEND) % modulus;
75    }
76
77    /**
78     * Calculate GCD of a and b, which are assumed to be non-negative.
79     */
80    private static int gcd(int a, int b) {
81        while (b != 0) {
82            int tmp = a % b;
83            a = b;
84            b = tmp;
85        }
86        return a;
87    }
88
89    /**
90     * Simple test. Takes modulus on command line and prints out permutation.
91     */
92    public static void main(String[] args) {
93        int modulus = Integer.parseInt(args[0]);
94        Permuter p = new Permuter(modulus);
95        for (int i = 0; i < modulus; i++) {
96            System.out.print(p.map(i) + " ");
97        }
98        System.out.println();
99    }
100}
101