com.sun.swingset3.demos.list.Permuter Maven / Gradle / Ivy
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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();
}
}