Using Test.expect without any error message provided is unacceptable.

Could use tag probability

warning:

STDERR
assertSimilar is deprecated, use assertDeepEquals

• JS Node 14. should be enabled

• Python new test framework should be used (Refer this & this for more detail)

• Ruby 3.0 should be enabled (Refer this & this for more detail)

1. So the set of characters to retain has been changed suddenly after 300 kata completions, so many best practices solutions aren't best practices anymore, but they are still there...
2. The tests should contain different kinds of characters: probably all ASCII, min non-ASCII codepoint, non-ASCII alphabetic...

needs random tests

(interesting concept, btw, thx)

Note: might be good to not rely on the external links, about the description.

needs random tests

The random tests are so weak there are practically no random tests.

Not to mention that the function used are fixed and has a very easy explicit inverse function.

f will be a continuous function with exactly one inverse for y between the lower and upper bounds provided.

This sentence looks ambiguous to me. It says that the given y has exactly one inverse in the range, but other y values may be associated to two or more x.

Looking at the given functions, it could be better to say that "f has one-to-one correspondence within the given region".

@buttonupbub: do tell ..