regex jumpscare

It seems that the issue here is that "last" is a reference, but the result of ".collect<Vec>()" is a temporary, so in the next invocation of the closure last would be referencing a Vec from a previous closure's stack-frame which would probably blow-up, and Rust is protecting you from the creating the corruption. Happy coding!

Unspecified AssertionError

The assert libary has problem?

please change to chai

Yes, correct

Which language ? Python ?

Updated JS version with randomized tests

updated in this fork

rust version with (pseudo-randomized tests

I should have learnt Math properly in school.

Flabergastingly not efficient yet beautiful.
Take my neverending admiration

Ikr, hard math == easy programming;

flatMap is supported by Node 11 onwards.

Any idea why I can't use Array.flatMap() in js ?

I think it may have more to do with this:

Attempting to invoke an infinitely recursive pattern of any form (including non-recursive patterns which call on infinitely recursive patterns in their pattern body, etc.) in the global scope. Extreme cases (e.g. 500 levels of non-infinite recursion) will not be tested in the test cases so a sensible recursion limit will do.