WIP: ENH: improve _count_paths_outside_method via pythran

[charlotte12l]

Could speed up stats.ks_2samp(rvs1, rvs2, alternative='less', mode='exact') >20x times on my computer if went well with Pythran master

from scipy import stats
import numpy as np
from timeit import timeit

print(stats.__file__)
rng = np.random.default_rng(1234)
n1 = 200
n2 = 300

rvs1 = stats.norm.rvs(size=n1, loc=0., scale=1, random_state=rng)
rvs2 = stats.norm.rvs(size=n2, loc=0.5, scale=1.5, random_state=rng)
stats.ks_2samp(rvs1, rvs2, alternative='less', mode='exact')

[rgommers]

CI failures can be ignore, they're due to Pythran master being needed (0.9.12 will be released soon, and it has support for scipy.special.binom that's needed here).

Right now this PR is also blocked by the macOS crash in gh-14315.

[mdhaber]

Wow, I'm seeing 90x speedup. Once gh-13265 merges, I'll merge main into this PR.
After that, if CI passes, is there anything else holding up this one @rgommers? Looks like the things you mentioned above have been resolved.

[rgommers]

Wow, I'm seeing 90x speedup. Once gh-13265 merges, I'll merge main into this PR.

It'd be great to see this move forward indeed.

After that, if CI passes, is there anything else holding up this one @rgommers? Looks like the things you mentioned above have been resolved.

The crash report on macOS in gh-14315 was based on this PR, it may still be a problem - not sure, requires testing. The other thing is the Pythran annotation; it uses plain int's, which is likely not correct. The int/int32/int64/intp/intc issues in gh-16646 are probably relevant. Ideally this code uses fixed dtypes, perhaps only int64.

[mdhaber]

This is closely related to gh-13957 so let's continue this work there.

[charlotte12l]

@rgommers @mdhaber Thank you so much!