Hi all, there is a PR to add a faster path to `np.isin`, that uses a look-up- table for all the elements that are included in the haystack (`test_elements`):
https://github.com/numpy/numpy/pull/12065/files
Such a table means that the memory overhead can be very significant,
but the speedup as well, so there was the idea of adding an option to
pick which version is used.
The current documentation for this new `method` keyword argument would
be. So the main questions are:
* Is there any concern about adding such a new kwarg?
* Is `method` the best name? Sorts uses `kind` which may also be good
There is also the smaller question of what heuristic 'auto' would use,
but that can be tweaked at any time.
```
method : {'auto', 'sort', 'dictionary'}, optional
The algorithm to use. This will not affect the final result,
but will affect the speed. Default is 'auto'.
- If 'sort', will use a mergesort-based approach. This will have
a memory usage of roughly 6 times the sum of the sizes of
`ar1` and `ar2`, not accounting for size of dtypes.
- If 'dictionary', will use a key-dictionary approach similar
to a counting sort. This is only available for boolean and
integer arrays. This will have a memory usage of the
size of `ar1` plus the max-min value of `ar2`. This tends
to be the faster method if the following formula is true:
`log10(len(ar2)) > (log10(max(ar2)-min(ar2)) - 2.27) / 0.927`,
but may use greater memory.
- If 'auto', will automatically choose the method which is
expected to perform the fastest, using the above
formula. For larger sizes or smaller range,
'dictionary' is chosen. For larger range or smaller
sizes, 'sort' is chosen.`
```
Cheers,
Sebastian
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