Hi all,

I just finish the GAMER frontend for YT, and below I want to summarize some issues and things that might be confusing in the current skeleton frontend. I appreciate if you could help validate these issues and/or clarify some points in the section "Things to be clarified". Hopefully it will make frontend implementation easier in the future. I'll collect all comments and issue a PR for the revised skeleton. It will be complementary to the recent PR #2130 by Jonah Miller, which provides an extremely helpful example frontend.

Things to be clarified:
============================
1. Difference between FieldInfoContainer.known_other_fields and GridIndex._detect_output_field:
=> known_other_fields contains fields that *might* be in an output, and lists their units and aliases. _detect_output_fields finds the fields that are defined in the currently loaded dataset.

2. Dataset.unique_identifier:

=> ??

3. Set AMRGridPatch.Parent = None for grids without parents, and AMRGridPatch.Children = [] for grids without children

4. Field array returned by BaseIOHandler._read_fluid_selection should be contiguous along the z instead of x direction. Therefore, for C-like array with the dimension [x][y][z] and for Fortran-like array with the the dimension (z,y,x), a matrix transpose is required (e.g., using np_array.transpose() or np_array.swapaxes(0,2))

5. start_index, stop_index, and ActiveDimensions in the AMRGridPatch objects

=> ??
(1) It looks like stop_index is not used anymore
(2) ActiveDimensions will be set by AMRGridPatch._prepare_grid. So perhaps frontend does not need to set it explicitly anymore

(3) It seems that start_index is also calculated automatically

6. chunk, selector, and _read_chunk_data in io.py.

=> ??

Also the following comment about caching is confusing:
def _read_chunk_data(self, chunk, fields):
# This reads the data from a single chunk, and is only used for caching.

7. float type:
(1) GridIndex.float_type is the float type for left and right simulation edges and must be float64 now
(2) BaseIOHandler._read_fluid_selection should always return float64 now even if the on-disk data are stored in single precision

8. Difference between add_output_field and add_field used in FieldInfoContainer.setup_fluid_fields:

9. Dataset._parse_parameter_file: what does the following comment mean: "Note that these are all assumed to be in code units; domain_left_edge and domain_right_edge will be updated to be in code units at a later time"?
=> Perhaps it means that "domain_left_edge and domain_right_edge will be converted to YTArray automatically at a later time"?

Missing procedure
============================
1. Add the new frontend in frontends/api.py
2. Set GridIndex.max_level
3. Set Dataset.refine_by

Bugin the current skeleton frontend

============================
1. data_structures.py: self.dataset is not set in GridIndex.__init__ when calling self.index_filename = self.dataset.parameter_filename
2. data_structures.py: replace self.Parent=[] by self.Parent = None
3. fields.py: the field_list argument is missing in FieldInfoContainer.__init__. This issue has been fixed in Jonah's PR.
def __init__(self, ds, field_list):
super(SkeletonFieldInfo, self).__init__(ds, field_list)

Sincerely,

Hsi-Yu (Justin)