UUIDUtil.java

import java.util.ArrayList;
import java.util.List;

import org.apache.commons.lang3.StringUtils;
import org.apache.commons.lang3.tuple.Pair;

public class UUIDUtil {

    public static List<Pair<String, String>> uuidSplit(int n) {
        if (n < 2 || n > 256) {
            throw new IllegalArgumentException("The number of divisions n supports between 2 and 256");
        }
        final var acm = new ArrayList<Pair<String, String>>();
        recursiveSplit(0, log2(n), 0, 0, acm);
        return acm;
    }

    private static int log2(double a) {
        return (int) Math.ceil(Math.log(a) / Math.log(2));
    }

    private static void recursiveSplit(
            int i, int upper, long _start, long _end,
            List<Pair<String, String>> acm
    ) {
        if (i >= upper) {
            final int digits = (int) Math.ceil((double) upper / 4);
            final var startHex = Long.toHexString(_start);
            final var endHexArr = Long.toHexString(_end).toCharArray();
            endHexArr[endHexArr.length - 1] += 1;
            acm.add(Pair.of(
                    StringUtils.repeat('0', digits - startHex.length()) + startHex,
                    StringUtils.repeat('0', digits - endHexArr.length) + new String(endHexArr)));
            return;
        }

        long start = _start;
        long end = _end;
        if (i % 4 == 0) {
            start <<= 4;
            end = (end << 4) | 0xf;
        }
        final long mid = start + ((end & 0xf) - (start & 0xf)) / 2;

        recursiveSplit(i + 1, upper, start, mid, acm);
        recursiveSplit(i + 1, upper, mid + 1, end, acm);
    }

}