# [ICON4Py] Hannes' fixes 2: Fix skip values

- Shaped by: Enrique
- Appetite (FTEs, weeks): 1 week
- Developers:

## Problem

[Hannes' handover document](https://hackmd.io/@gridtools/BJN-Fn3Jee) contains a list of changes used in ICON4Py ([PR #731](https://github.com/C2SM/icon4py/pull/731)) done in his experiments to get better performance. We need to clean up these changes and merge them cleanly in ICON4Py.

This project deals with fixing the skip values in the connectivities.

## Solution

The solution implemented by Hannes consists in replacing the `-1` values in the connectivities by `0` assuming they will never be actually used in the neighbor reductions. This is a hacky solution (similar to what is actually used in ICON Fortran) but works for now and it is safe as long as we make sure the skip values are never really use.

The alternative would be to implement proper backwards domain inference in gt4py, but this is a large feature and it won't be ready in time for the MCH production deadline. We should add TODOs and notes in the code to make sure this hacks gets removed as soon as gt4py domain inference is fully implemented.

Check PR#731: https://github.com/C2SM/icon4py/pull/731

## Rabbit holes
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## Progress

- [x] Cleanups ([PR 755](https://github.com/C2SM/icon4py/pull/755))
- [x] cleanup `skip_value` configuration for individual connectivities
- [x] determine `limited_area` from grid file
- [x] renaming: connectivity vs offset_provider vs neighbor_tables in IconGrid
- [x] skip value substition ([PR 761](https://github.com/C2SM/icon4py/pull/761))
- [x] Cleanup from [PR 731](PR#731: https://github.com/C2SM/icon4py/pull/731)
- [x] Allow configuration