[Users] (possibly) inconsistent handling of ioffsetdenom in CarpetIOHDF5

[Roland Haas]

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Hello all,

due to some issues we have with recovering checkpoints I find myself
reading CarpetIOHDF5's Output.cc and Input.cc files.

In particular lines 849--860 of Output.cc:
 849   ivect const ioffsetdenom = bbox.stride();
 850   HDF5_ERROR (attr = H5Acreate (dataset, "ioffsetdenom", \
                                     H5T_NATIVE_INT,
 851                                 dataspace, H5P_DEFAULT));
 852   HDF5_ERROR (H5Awrite (attr, H5T_NATIVE_INT, &ioffsetdenom[0]));
 853   HDF5_ERROR (H5Aclose (attr));
 854   ivect const ioffset =
 855     ((bbox.lower() % bbox.stride()) + bbox.stride()) % \
         bbox.stride();
 856   HDF5_ERROR (attr = H5Acreate (dataset, "ioffset", H5T_NATIVE_INT,
 857                                 dataspace, H5P_DEFAULT));
 858   HDF5_ERROR (H5Awrite (attr, H5T_NATIVE_INT, &ioffset[0]));
 859   HDF5_ERROR (H5Aclose (attr));
 860   ivect const iorigin = (bbox.lower() - ioffset) / bbox.stride();
which as far as I can tell always just sets ioffsetdenom to the stride.

On the other hand, Input.cc in line 1399-1400:
1399 assert (all (stride % patch->ioffsetdenom == 0));
1400 ivect lower = patch->iorigin * stride + patch->ioffset * stride / \
                   patch->ioffsetdenom;
only requires stride to be a multiple of ioffsetdenom (rather than
being identical).

Finally if I read the code correctly then if actually stride !=
ioffsetdenom then the code produces incorrect results since Input.cc
would compute lower as:

lower = bbox.lower() - ioffset + patch->ioffset * stride / \
                                 patch->ioffsetdenom

(substituting for patch->iorigin) which only ever evaluates to the
(presumably correct) bbox.lower() if stride == ioffsetdenom.

Any thoughts?

Yours,
Roland

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