TennisTournament.java

package dev.bemoty.Test;

import java.util.Arrays;
import java.util.stream.IntStream;

public class TennisTournament {
    public static void main(String[] args) {
        // 32 indices: [0, 31, 15, 16, 8, 23, 9, 24]
        // 16 indices:  [0, 15, 7, 8]
        int[] input16 = IntStream.range(1, 5).toArray();
        int[] input32 = IntStream.range(1, 9).toArray();
        int[] input64 = IntStream.range(1, 17).toArray();
        int[] input128 = IntStream.range(1, 33).toArray();
        System.out.println(Arrays.toString(setSeeded(128, input128)));
        System.out.println(Arrays.toString(setSeeded(64, input64)));
        System.out.println(Arrays.toString(setSeeded(32, input32)));
        System.out.println(Arrays.toString(setSeeded(16, input16)));
    }

    private static int[] setSeeded(int size, int[] input) {
        if (input.length != size / 4) throw new IllegalArgumentException();
        int[] output = new int[size];
        output[0] = input[0];
        output[size - 1] = input[1];
        int[] rest = Arrays.copyOfRange(input, 2, input.length);
        setCenter(output, rest);
        return output;
    }

    private static int[] setCenter(int[] output, int[] input) {
        if (input.length == 0) return output;
        int index = output.length / 2;
        output[index - 1] = input[0];
        output[index] = input.length > 1 ? input[1] : 0;
        if (input.length > 2) {
            int[] rest = Arrays.copyOfRange(input, 2, input.length);
            set(output, setCenter(Arrays.copyOfRange(output, 0, output.length / 2),
                    IntStream.range(0, rest.length).filter((i) -> i % 2 != 0).map(i -> rest[i]).toArray()), 0);
            set(output, setCenter(Arrays.copyOfRange(output, output.length / 2 + 1, output.length),
                    IntStream.range(0, rest.length).filter(i -> i % 2 == 0).map(i -> rest[i]).toArray()), index + 1);
        }
        return output;
    }

    private static void set(int[] output, int[] input, int offset) {
        for (int i = offset; i < input.length + offset; i++) {
            if (i >= output.length) break;
            output[i] = input[i - offset];
        }
    }
}