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Collections are a way for you to organize kata so that you can create your own training routines. Every collection you create is public and automatically sharable with other warriors. After you have added a few kata to a collection you and others can train on the kata contained within the collection.
Get started now by creating a new collection.
Whoever wrote the tests might have been lazy and chose to use random integers as keys to not implement a random string generator. If you are parsing the string manually, this makes the task harder, but it can be solved in a trivial way if you use a rather notorious language feature instead.
The error message is unclear, what it means is that your output is wrong. Print the input string and your hash before returning it to see whether they're the same.