private static long[][] getVals(int size) {
    int num = 10_000_000;
    return IntStream.range(0, size)
      .mapToObj(i -> random
        .longs(0, 1_000_000_000)
        .limit(num / size)
        .sorted()
        .toArray()
      ).toArray(long[][]::new);
  }
Performance tests for {@link LongMinHeap} implementations.

Times how long it takes to merge N sorted lists of random elements.
planetiler-benchmarks/src/main/java/com/onthegomap/planetiler/collection/BenchmarkKWayMerge.java