Permutations of an Array in Java

Permutations.java

package net.example.math;

import java.io.PrintStream;
import java.util.Arrays;

/**
 * <blockquote>
 * <p>The number of permutation increases fast with <em>n</em>. While it takes
 * only a few seconds to generate all permutations of ten elements, it will
 * take two weeks to generate all permutations of 15 elements:</p>
 * <table border="1">
 * <tr><th> N </th><th> Number of permutations </th>
 *   <th colspan="2"> Time (assuming 1000000 permutations / second) </th></tr>
 * <tr><td> 10 </td><td> 3628800 </td><td> 4 </td><td> Seconds </td></tr>
 * <tr><td> 11 </td><td> 39916800 </td><td> 40 </td><td> Seconds </td></tr>
 * <tr><td> 12 </td><td> 479001600 </td><td> 8 </td><td> Minutes </td></tr>
 * <tr><td> 13 </td><td> 6227020800 </td><td> 104 </td><td> Minutes </td></tr>
 * <tr><td> 14 </td><td> 87178291200 </td><td> 1 </td><td> Day </td></tr>
 * <tr><td> 15 </td><td> 1307674368000 </td><td> 15 </td><td> Days </td></tr>
 * <tr><td> 16 </td><td> 20922789888000 </td><td> 242 </td><td> Days </td></tr>
 * <tr><td> 17 </td><td> 355687428096000 </td><td> 11 </td><td> Years </td></tr>
 * <tr><td> 18 </td><td> 6402373705728000 </td><td> 203 </td><td> Years </td></tr>
 * <tr><td> 19 </td><td> 121645100408832000 </td><td> 3857 </td><td> Years </td></tr>
 * <tr><td> 20 </td><td> 2432902008176640000 </td><td> 77147 </td><td> Years </td></tr>
 * </table>
 * </blockquote>
 * 
 * @see  <a href="https://www.baeldung.com/java-array-permutations">Permutations of an Array in Java</a>
 * @see  <a href="https://en.wikipedia.org/wiki/Heap%27s_algorithm">Heap's algorithm</a>
 */
public class Permutations {

    public static <T> void printAll(T[] elements) {
        final int n = elements.length;
        int[] indexes = new int[n];
        for (int i = 0; i < n; i++) {
            indexes[i] = 0;
        }
    
        printArray(elements);
    
        int i = 0;
        boolean even = (i % 2 == 0);
        while (i < n) {
            if (indexes[i] < i) {
                swap(elements, even ? 0 : indexes[i], i);
                printArray(elements);
                indexes[i]++;
                i = 0;
            } else {
                indexes[i] = 0;
                i++;
                even = (i % 2 == 0);
            }
        }
    }

    public static <T> void printAllRecursive(T[] elements) {
        printAllRecursive(elements.length, elements);
    }

    static <T> void printAllRecursive(final int n, T[] elements) {
        if (n == 1) {
            printArray(elements);
            return;
        }

        final int nMinus1 = n - 1;
        final boolean even = (n % 2 == 0);
        for (int i = 0; i < nMinus1; i++) {
            printAllRecursive(nMinus1, elements);
            swap(elements, even ? i : 0, nMinus1);
        }
        printAllRecursive(nMinus1, elements);
    }

    private static <T> void swap(final T[] input, final int a, final int b) {
        T tmp = input[a];
        input[a] = input[b];
        input[b] = tmp;
    }

    static <T> void printArray(T[] input) {
        count += 1;
        if (out == null) {
            return;
        }

        buf.append(input[0]);
        for(int i = 1, len = input.length; i < len; i++) {
            buf.append(',').append(input[i]);
        }
        buf.append(lineSeparator);
        if (buf.length() >= BUF_CAP) {
            out.print(buf);
            buf.setLength(0);
        }
    }

    private static long count = 0;
    private static PrintStream out;
    private static final int BUF_CAP = 1_000_000;
    private static StringBuilder buf = new StringBuilder(BUF_CAP + 100);
    private static String lineSeparator = System.getProperty("line.separator");

    public static void main(String[] args) throws Exception {
        String[] elements = new String[10];
        for (int i = 0, len = elements.length; i < len; i++) {
            elements[i] = String.valueOf(i + 1);
        }
        System.out.println("Permutations of " + Arrays.asList(elements));
        //out = System.out;
        //out = new PrintStream("permutations.txt");
        //out = new PrintStream(new OutputStream() {
        //    @Override public void write(int b) { }
        //    @Override public void write(byte[] b, int off, int len) { }
        //});

        long startTime = System.currentTimeMillis(), elapsedTime;
        printAll(elements);
        //printAllRecursive(elements);
        if (buf.length() > 0) {
            out.print(buf);
            out.flush();
        }
        elapsedTime = System.currentTimeMillis() - startTime;

        System.out.println("Count: " + count);
        System.out.println("Time: " + (elapsedTime / 1e3) + " sec");
    }

}