reference code, won't compile 'as is' contains a (hopefully) easy-to-follow implementation of Adaptive (octree-based) Dual Contouring, but without the QEF simplification part. "dual_contouring2.cpp" also implements "An Efficient Parametric Algorithm for Octree Traversal" [2000].
It relies on QEF/SVD code written by /u/unzret (on reddit.com) which can be obtained from: https://github.com/nickgildea/DualContouringSample