code: public static Permutation fromSortIndicesUnchecked(final int[] sortIndices,
        final int startIndex) {
      final int[] sources = sortIndices.clone();
      for (int i = 0; i < sources.length; ++i) {
        sources[i] -= startIndex;
      }
      return new Permutation(sources);
    }
docs: Creates a Permutation from sort indices returned by HPPC's IndirectSort.  No validity checking
  is done, so this *must* be called with the sort indices from an HPPC indirect sort using the
  same start index passed to that sort.  If you do otherwise, you will end up with a invalid
  permutation with undefined behavior.