i didn't expect an extended test input "fo99obar100". The given test cases look like:
do_test("foobar000", "foobar001");
do_test("foo", "foo1");
do_test("foobar001", "foobar002");
do_test("foobar99", "foobar100");
do_test("foobar099", "foobar100");
do_test("", "1");
do_test("1", "2");

no way one can infere such a test case after seeing these Examples.

Hi @jora_ipoteka - there are many users on Codewars who are intereted in improving descriptions of katas; since you are the first person who has commented about the description of this kata, could you maybe explain specifically what you think is missing from it?

Or what condition did you not understand clearly from the given description? If this is a good observation, then a moderator can update the kata to include it, if applicable.