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Datei: Permuter.java Sprache: JAVA

/*
*
* Copyright (c) 2007, Oracle and/or its affiliates. All rights reserved.
*
* Redistribution and use in source and binary forms, with or without
* modification, are permitted provided that the following conditions
* are met:
*
*   - Redistributions of source code must retain the above copyright
*     notice, this list of conditions and the following disclaimer.
*
*   - Redistributions in binary form must reproduce the above copyright
*     notice, this list of conditions and the following disclaimer in the
*     documentation and/or other materials provided with the distribution.
*
*   - Neither the name of Oracle nor the names of its
*     contributors may be used to endorse or promote products derived
*     from this software without specific prior written permission.
*
* THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS
* IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO,
* THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
* PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL THE COPYRIGHT OWNER OR
* CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,
* EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
* PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR
* PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF
* LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING
* NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
* SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
*/

import java.util.Random;

/**
* An object that implements a cheesy pseudorandom permutation of the integers
* from zero to some user-specified value. (The permutation is a linear
* function.)
*
* @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 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();
    }
}

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